Na 4 P 2 O 7 -Modified Biochar Derived from Sewage Sludge: Effective Cu(II)-Adsorption Removal from Aqueous Solution

Abstract

With the rapid development of industrialization, the amount of copper-containing wastewater is increasing, thereby posing a threat to the aquatic ecological environment and human health. Sludge biochar has received extensive concern in recent years due to its advantages of low cost and sustainability for the treatment of heavy-metal-containing wastewater. However, the heavy-metal-adsorption capacity of sludge biochar is limited. This study prepared a sodium pyrophosphate- (Na₄P₂O₇-) modified municipal sludge-based biochar (SP-SBC) and evaluated its adsorption performance for Cu(II). Results showed that SP-SBC had higher yield, ash content, pH, Na and P content, and surface roughness than original sewage sludge biochar (SBC). The Cu(II)-adsorption capacity of SP-SBC was 4.55 times than that of SBC at room temperature. For Cu(II) adsorption by SP-SBC, the kinetics and isotherms conformed to the pseudo-second-order model and the Langmuir–Freundlich model, respectively. The maximum adsorption capacity of SP-SBC was 38.49 mg·g⁻¹ at 35°C. Cu(II) adsorption by SP-SBC primarily involved ion exchange, electrostatic attraction, and precipitation. The desired adsorption performance for Cu(II) in the fixed-bed column experiment indicated that SP-SBC can be reused and had good application potential to treat copper-containing wastewater. Overall, this study provided a desirable sorbent (SP-SBC) for Cu(II) removal, as well as a new simple chemical-modification method for SBC to enhance Cu(II)-adsorption capacity.

1. Introduction

With the rapid development of industrialization, the heavy metals that enter the water environment through human activities inevitably increase [1–3]. Due to the persistence, bioaccumulation, pathogenicity, and carcinogenicity of heavy metals, the water-environment pollution they cause is attracting extensive attention [1, 4]. Copper, as an important heavy metal material, is widely used in the fields of electronics, energy, communications, and machinery production, resulting in a large amount of copper-containing wastewater. The concentration of copper in the wastewater varies from tens to thousands of milligrams per liter [5]. Once the wastewater is directly discharged into natural water bodies without proper treatment, it harms aquatic organisms and leads to a series of health problems to humans, such as nausea, diarrhea, liver, and kidney damage [1, 6]. Thus, the removal of copper from wastewater has become an important issue for heavy-metal pollution control. Several methods are presently used to deal with copper-containing wastewater, primarily including chemical precipitation, bioremediation, membrane separation, and adsorption [7]. Among these methods, adsorption has elicited extensive attention in view of its advantages of convenient operation, considerable efficiency, low cost, and strong anticontamination ability [8, 9]. For adsorption, sorbent development is an important research task [10–14].

Biochar is a porous and carbon-rich solid material obtained by pyrolyzing biomass under anoxic or anaerobic conditions [15, 16]. Due to the high porosity, large surface area, rich surface functional groups, and high pH value of biochar, it receives extensive attention in the field of heavy-metal-containing wastewater treatment [17]. In previous years, a large number of biochar materials such as discarded mushroom-stick biochar [18], corn straw biochar [19], date seed biochar [20], tobacco stem biochar [21], industrial alkali lignin biochar [22], and Ascophyllum nodosum seaweed biochar [23] have been successfully prepared to remove heavy metal from wastewater. These studies show that biochar is an environmentally friendly and cost-effective adsorption material for heavy-metal adsorption. With the deepening of research, some scholars have performed studies on biochar modification, particularly chemical modification, to improve the heavy-metal-adsorption capacity by raw biochar [24–31]. These studies have indicated that the heavy-metal-adsorption capacity of biochar can be significantly improved by an effective modifier. Such a modifier adds adsorption sites and enhances electrostatic attraction, surface complexation, or surface precipitation on biochar.

In China, the growing sewage sludge produced from sewage treatment has become a serious burden on ecology and society [32]. Accordingly, China has the urgent need for environment-friendly methods to realize the effective treatment of sewage sludge. However, the commonly used methods of incineration, sanitary landfilling, and land application [32] have some shortcomings that could lead to secondary pollution. For example, NO_x, SO₂, and volatile heavy metals produced from incineration can cause serious air pollution [33]. Sanitary landfilling can occupy land. Direct agricultural application may harm living organisms [34]. Many researchers have reported the related studies on converting sewage sludge to biochar for the treatment of heavy-metal-containing wastewater [35–41]. They found that preparing biochar from sewage sludge can realize the harmless and reductive treatments of sewage sludge and obtain a low-cost and sustainable sorbent for the treatment of heavy-metal-containing wastewater. However, the heavy-metal-adsorption capacity of sludge-based biochar is unsatisfactory, which limits its promotion and application to a certain extent [42, 43]. Some modifiers such as Fe₃O₄ [44], nanostructured CaCO₃ [45], trithiocyanuric acid trisodium salt [46], α-Fe₂O₃ and α-FeOOH [47], K₂FeO₄ [48], and hydroxyapatite [42] have been successfully applied to prepare modified sewage sludge biochar, and satisfactory heavy-metal-adsorption capacity has been achieved. At present, chemical modification is a noteworthy research direction for improving the adsorption capacity of heavy metals by sewage sludge biochar [45, 49]. In the present study, we used sodium pyrophosphate (Na₄P₂O₇), an additive widely used in the food industry, water industry, and daily chemical industry, as a modifier to prepare Na₄P₂O₇-modified sewage sludge-based biochar (SP-SBC). The desirable Cu(II)-adsorption capacity of SP-SBC was hoped to be achieved through the precipitation between pyrophosphate or phosphate and Cu(II). Although some similar reports exist on sewage sludge biochar modified with chemical modification for Cu(II) adsorption [42, 50, 51], to our knowledge, SP-SBC and its Cu(II)-adsorption performance are reported herein for the first time. Moreover, our one-pot preparation method of SP-SBC was simpler than those of biochars reported by Chen et al. [42], Phoungthong and Suwunwong [50], and Tang et al. [51]. This study could provide an alternative path for modifying sewage sludge biochar, which is beneficial to promote the practical application of sludge biochar.

The aims of this study were as follows: (i) to prepare SP-SBC and explore its properties, (ii) to investigate the adsorption behaviors of SP-SBC for Cu(II), and (iii) to understand the application potential of SP-SBC for Cu(II)-adsorption removal. Herein, SP-SBC was prepared. The properties of SP-SBC and original sewage sludge biochar (SBC) were characterized by the yield; ash content; pH, K, Ca, Na, Mg, and P contents; SEM; and FTIR. Then, the effects of sorbent dosage, initial pH, ionic strength, contact time, and temperature on the Cu(II) adsorption by SP-SBC were determined by batch-adsorption experiments. For Cu(II) adsorption by SP-SBC, the kinetics and isotherms were explored. Lastly, a fixed-bed column experiment was performed.

2. Materials and Methods

2.1. Materials

Sodium pyrophosphate (Na₄P₂O₇; AR, 99%), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O; AR, 99%), sodium nitrate (NaNO₃; AR, 99%), silica sand, nitric acid (HNO₃; AR, 65–68 wt%), and sodium hydroxide (NaOH, AR, 98%) were all supplied by Kelong Chemical Co., Ltd. (Chengdu, China) without further purification. Ultrapure water (18.25 MΩ) was used to prepare the used solutions. Sewage sludge was acquired from a local municipal wastewater-treatment plant in Chengdu city, China.

2.2. Preparation of Biochar

SP-SBC was prepared with a one-pot method (Figure 1). The specific procedures were as follows. Sewage sludge was ground and sieved to pass a 10-mesh sieve after natural drying. A part of sieved sludge was impregnated with Na₄P₂O₇ (60 g·L⁻¹) at a ratio of 1/20 (mass/volume) for 12 h in a constant-temperature shaker under the conditions of 25°C and 130 rpm. Afterwards, the impregnated sludge was dried at 60°C until the weight was stable. The dried impregnated sludge and the other part of the sieved sludge were loaded into different ceramic crucibles, sealed, and capped. Then, they were pyrolyzed for 2 h at 400°C in a muffle furnace. After grinding the pyrolyzed product of the original sludge and sieving (60 mesh), the acquired particles under the sieve were recorded as SBC. The pyrolyzed product of the impregnated sludge was washed several times with 55–65°C deionized water and dried at 60°C until the weight was stable. The dried product was passed through a sieve (60 mesh) after grinding. The obtained particles under the sieve were recorded as SP-SBC.

Figure 1

Flow chart of SP-SBC preparation.

2.3. Properties of Biochar

The analysis methods for the properties of SBC or SP-SBC were as follows. Equation (1) was used to calculate the yield ( $Y$ , %): $\begin{matrix} (1) & Y = (\frac{m_{0}}{m_{t}}) \times 100 %, \end{matrix}$ where $m_{0}$ and $m_{t}$ denote the weight of SBC (or SP-SBC) and sewage sludge (g), respectively. Ash content was detected by burning 1.00 g of samples in a muffle furnace at 700°C for 2 h and calculated by the following equation: $\begin{matrix} (2) & Ash (%) = (\frac{M_{0}}{M_{t}}) \times 100 %, \end{matrix}$ where $M_{0}$ and $M_{t}$ represent the weight of SBC (or SP-SBC) after and before burning (g), respectively. The pH; contents of Na, K, Ca, Mg, and P; and the surface structural morphology (i.e., SEM images) were determined as in our previous methods [52]. The surface functional groups on SBC, SP-SBC, and SP-SBC after Cu(II) adsorption (SP-SBC+Cu) were recorded with a Nicolet iS50-type FTIR spectroscope (Thermo Fisher Scientific, Inc., Waltham, USA). At different solution pH values, the zeta potentials of SP-SBC were measured based on our previous methods to calculate the isoelectric point (pH_IEP) [52].

2.4. Batch Experiments

The influences of sorbent dosage, initial solution pH, ionic strength, contact time, and temperature on Cu(II) adsorption by SP-SBC were determined with batch experiments. For all batch experiments, the Cu(II) solution with a preset concentration was prepared by diluting 1000 mg·L⁻¹ of Cu(II) stock solution, which was in turn prepared with Cu(NO₃)₂·3H₂O and ultrapure water (18.25 MΩ). In a typical procedure, a certain amount of SP-SBC was mixed with 50 mL of Cu(II) solution with a designed concentration, followed by adjusting the pH to a predesigned value. Then, the mixture was shaken for a certain time period at a preset temperature and at 130 rpm in a constant-temperature shaker. The specific conditions for each batch experiment are shown in Table 1. Furthermore, SP-SBC was replaced by SBC and subjected to the same experiment processes in the batch experiment of the effect of contact time to compare the difference in adsorption capacity between SP-SBC and SBC.

Table 1

Specific conditions for each batch experiment.

Batch experiment	Experimental conditions
Batch experiment	Sorbent dosage (g)	Initial solution pH	Ionic strength(NaNO₃, M)	Contact time (h)	Temperature (°C)	Cu(II) concentration (mg·L^-1)
Sorbent dosage	0.025, 0.05, 0.075, 0.1, 0.2, 0.3, 0.5	5	0	12	25	50
Initial solution pH	0.1	3, 3.5, 4, 4.5, 5, 6	0	12	25	50
Ionic strength	0.1	5	0, 0.005, 0.01, 0.03, 0.05, 0.08, 0.10, 0.50	12	25	50
Contact time	0.1	5	0	0.5, 1, 2, 3, 4, 6, 8, 10, 12, 14	25	50
Temperature	0.1	5	0	12	15, 25, 35	50, 150, 200, 300, 400, 600

Once a batch experiment was completed, the mixture was taken out and the suspension was passed through a 0.45 μm microfiltration membrane. In the filtrate, the Cu(II) concentration was tested with a PinAAcle900T-type flame atomic adsorption spectrophotometer (FAAS) (PerkinElmer Instrument Co., Ltd., Akron, USA). Then, Equation (3) was used to calculate the Cu(II)-adsorption capacity ( $q_{e}$ ): $\begin{matrix} (3) & q_{e} = \frac{0.05 (C_{0} - C_{e})}{W}, \end{matrix}$ where $C_{0}$ refers to the initial Cu(II) concentration (mg·L⁻¹) at the beginning of experiment, $C_{e}$ refers to the detected Cu(II) concentration (mg·L⁻¹) at the end of experiment, and $W$ is the amount of biochar (g).

2.5. Adsorption Kinetics and Isotherms

For Cu(II) adsorption by SP-SBC, the acquired data in the batch experiment of the effect of contact time were simulated with the pseudo-first-order (PFO) model, the pseudo-second-order (PSO) model, and the intraparticle diffusion (IPD) model (Equations (4)–(6)) to analyze the kinetic characteristics [12]. $\begin{matrix} (4) & Q_{t} = Q_{et} (1 - e^{- k_{1} t}), \\ (5) & Q_{t} = \frac{t {Q_{et}}^{2} k_{2}}{1 + t Q_{et} k_{2}}, \\ (6) & Q_{t} = k_{i} t^{1 / 2} + C, \end{matrix}$ where $Q_{et}$ is the adsorption capacity (mg·g⁻¹) at equilibrium and $Q_{t}$ is the adsorption capacity (mg·g⁻¹) at time $t$ . $k_{1}$ , $k_{2}$ , and $k_{i}$ denote the constants of the PFO model (h⁻¹), the PSO model (g·mg⁻¹·h⁻¹), and the rate constant of the IPD diffusion (mg·(g·min^0.5)⁻¹), respectively. $C$ indicates the boundary-layer thickness (mg·g⁻¹). The acquired data in the batch experiment of the effect of temperature was matched with the Langmuir (L) model, the Freundlich (F) model, the Langmuir–Freundlich (L-F) model, and the Dubinin–Radushkevich (D-R) model (Equations (7)–(12)) [16, 53–56] to explain the adsorption isotherms, respectively. $\begin{matrix} (7) & Q_{e} = \frac{Q_{m} K_{L} C_{e}}{1 + K_{L} C_{e}}, \\ (8) & Q_{e} = K_{f} {C_{e}}^{1 / n}, \\ (9) & Q_{e} = \frac{Q_{m} {(K_{a} C_{e})}^{n^{'}}}{1 + {(K_{a} C_{e})}^{n^{'}}}, \\ (10) & ε = RTln (1 + \frac{1}{C_{eD}}), \\ (11) & Q_{eD} = Q_{mD} \exp (- β ε^{2}), \\ (12) & E = \frac{1}{\sqrt{2 β}}, \end{matrix}$ where $K_{L}$ is the constant of the L model (L·mg⁻¹). $K_{f}$ is the constant of the F model (mg⁽¹⁻ⁿ⁾·L ⁿ ·g⁻¹). $1 / n$ is an empirical constant representing adsorption intensity. $K_{a}$ is the constant of the L-F model (L·mg⁻¹), which can be used to reflect the adsorption affinity. $n^{'}$ is the heterogeneity factor. $Q_{e}$ is the equilibrium adsorption capacity at the end of the experiment (mg·g⁻¹). $Q_{m}$ is the maximum adsorption capacity (mg·g⁻¹). $ε$ is the Polanyi potential constant (J·mol⁻¹). $C_{ed}$ is the detected Cu(II) concentrations (mol·L⁻¹) at the equilibrium. $Q_{eD}$ and $Q_{mD}$ are the equilibrium adsorption capacity (mol·g⁻¹) and the maximum adsorption capacity (mol·g⁻¹), respectively. $R$ is the gas constant (8.314 J·(mol·K)⁻¹). $T$ is the absolute temperature (K).

2.6. Fixed-Bed Column Experiment

A fixed-bed column experiment was conducted to understand the application potential of SP-SBC for Cu(II) adsorption. The sketch map of the fixed-bed column device is shown in Figure 2. Four cycles were performed in the fixed-bed column experiment. Each cycle included two processes, i.e., adsorption process and desorption process. For the adsorption process, the Cu(II) solution ( $C_{in} = 50 mg \cdot L^{- 1}$ , pH 5) was pumped into the top of the column (flow rate $(Q) = 15 mL \cdot \min^{- 1}$ ). The effluent samples were collected every 15 min, and the Cu(II) concentrations of samples ( $C_{en}$ , mg·L⁻¹) were measured by the FAAS to draw the breakthrough curve. Based on the acquired breakthrough curve, the total inputted Cu(II) amount ( $M$ , mg), total Cu(II)-adsorption amount ( $M^{'}$ , mg), volume of treated Cu(II) solution ( $V_{eff}$ , mL), and equilibrium adsorption capacity of SP-SBC ( $q_{ec}$ , mg·g⁻¹) were calculated by Equations (13)–(16), respectively. For the desorption process, a certain volume of 0.5 M NaOH solution was added to the fixed-bed column until the solution level was kept above the top quartz sand-packed layer. After soaking for 12 h, the fixed-bed column was rinsed with ultrapure water several times until the eluent pH was about neutral. Then, the regenerated fixed-bed column reactor was obtained. The regenerated fixed-bed column was reused for the next cycle. $\begin{matrix} (13) & M = \frac{Q t_{e} C_{in}}{1000}, \\ (14) & M^{'} = \frac{Q}{1000} \int_{0}^{t_{e}} (C_{in} - C_{en}) dt, \\ (15) & V_{eff} = Q t_{e}, \\ (16) & q_{ec} = \frac{M^{'}}{m}, \end{matrix}$ where $t_{e}$ is the adsorption saturation time (min), which is the corresponding time when $C_{t} / C_{0} = 0.9$ [57, 58]. $m$ is the SP-SBC mass packed in the experimental device (g).

Figure 2

Sketch of fixed-bed column device (①: 2 cm thick quartz sand-packed layer; ②: 9 cm thick SP-SBC-packed layer).

2.7. Statistical Analysis

To ensure the reliability of experimental data, all experiments were repeated three times in this study. Origin 8.0 was used for plotting. SPSS Statistics 23.0 was used to analyze the experimental data.

3. Results and Discussion

3.1. Properties of SBC and SP-SBC

The property parameters of SBC and SP-SBC are shown in Table 2. The yields of SBC and SP-SBC were 60.57% and 68.34%, respectively, which were close to those of other sludge biochars prepared at the same pyrolysis temperature [59, 60]. Compared with those of SBC, the Na and P contents of SP-SBC significantly increased ( $p < 0.05$ ), indicating that pyrophosphate was successfully introduced onto SP-SBC. However, the K, Ca, and Mg contents of SP-SBC decreased, which may be related to the dilution effect resulting from the increase in Na and P contents after modification. The ash content of SP-SBC was higher ( $p < 0.05$ ) than that of SBC, which was due to the introduction of Na and P elements after modification. The alkalinity of SP-SBC was higher than that of SBC, which could be attributed to the increase in ash content and the alkalinity of the sodium pyrophosphate solution.

Table 2

Some properties of SBC and SP-SBC.

Items	SBC	SP-SBC
Yield (%)	$60.57 \pm 0.17$	$68.34 \pm 0.21$
Ash content (%)	$66.77 \pm 0.39$	$75.23 \pm 0.33$
pH value	$8.13 \pm 0.03$	$10.54 \pm 0.05$
Na (mg·g^-1)	$1.76 \pm 0.41$	$31.72 \pm 1.97$
K (mg·g^-1)	$6.81 \pm 0.36$	$3.68 \pm 0.28$
Ca (mg·g^-1)	$27.48 \pm 0.57$	$20.63 \pm 2.57$
Mg (mg·g^-1)	$7.87 \pm 0.18$	$5.34 \pm 0.38$
P (mg·g^-1)	$24.94 \pm 1.46$	$40.86 \pm 3.32$

Figure 3 shows the scanning electron microscopy (SEM) images of SBC and SP-SBC, which reflected the surface structural morphology. The SP-SBC surface (Figure 3(b)) was rougher than that of SBC (Figure 3(a)) and filled with many microparticles, which was caused by the introduction of sodium pyrophosphate. The results of FTIR spectra of SBC and SP-SBC (Figure 4) revealed peaks at around 790 and 1061 cm⁻¹, which were due to the aromatic C-H out-of-plane vibrations [61] and C-O stretching vibration, respectively [15, 62, 63]. The peak at around 1433 cm⁻¹ represented the C=C stretching vibration of aromatic hydrocarbons [64]. The peak at 1629 cm⁻¹ was associated with the –COOH stretching vibration [65]. The observed peak at around 2924 cm⁻¹ represented the C-H stretching vibration [66]. The intense peak at around 3405 cm⁻¹ was due to the -OH stretching vibration [67]. Overall, the peaks of the two biochars were consistent, suggesting that the categories of functional groups did not change before and after modification. However, for SP-SBC, the intensities of these peaks weakened. These results suggested that the amounts of the functional groups on the SP-BC surface (especially for –OH and –COOH) decreased. This finding was consistent with the above result that the alkalinity of SP-SBC was higher than that of SBC.

Figure 3

SEM image of biochar: (a) SBC and (b) SP-SBC.

(b)

Figure 4

FTIR spectra of SBC, SP-SBC, and SP-SBC+Cu.

3.2. Adsorption Capacities of SBC and SP-SBC for Cu(II)

The adsorption capacities of SBC and SP-SBC for Cu(II) varied with increased contact time (Figure 5). For SBC, the Cu(II)-adsorption capacity tended to be stable after 10 h. For SP-SBC, the Cu(II)-adsorption capacity tended to reach equilibrium at 12 h. At adsorption equilibrium, SP-SBC and SBC achieved the Cu(II)-adsorption capacities of 20.01 and 4.40 mg·g⁻¹, respectively. Results showed that SP-SBC had a higher Cu(II)-adsorption capacity (4.55 times) than SBC. First, the surface of SP-SBC was rougher than that of SBC (Figure 3), which helped in increasing the contact of Cu(II) with SP-SBC [68]. Second, for SP-SBC, the increased equivalent of sodium was greater than the sum of the decreased equivalents of potassium, calcium, and magnesium (Table 2), which enhanced the ion exchange with Cu(II) [52]. Third, the significant increase in the phosphorus and alkalinity of SP-SBC (Table 2) led to increased copper precipitation [69]. The above three reasons caused the significant improvement in the Cu(II)-adsorption capacity of SP-SBC.

Figure 5

Adsorption capacities of SBC and SP-SBC for Cu(II) at different contact times.

For Cu(II) adsorption by biochar, the complexation and π electron coordination of surface functional groups may be involved [6, 70, 71]. In the Section 3.1, compared with SBC, SP-SBC had fewer surface functional groups. However, the Cu(II)-adsorption capacity of SP-SBC was 4.55 times higher than that of SBC. This finding indicated that the increased Cu(II)-adsorption capacity of SP-SBC was independent of surface functional groups. Thus, the complexion and π electron coordination of surface functional groups may not be the main mechanism for the Cu(II) adsorption by SP-SBC. Additionally, the SP-SBC after Cu(II) adsorption (SP-SBC+Cu) was characterized by FTIR (Figure 4). We found inconspicuous changes in the positions and intensities of peaks between the FTIR spectra of SP-SBC and SP-SBC+Cu. These results suggested that surface functional groups had little effect on Cu(II) adsorption, which strongly supported the above discussion of surface functional group effects for Cu(II) adsorption.

Overall, modifying SBC with sodium pyrophosphate can effectively improve its Cu(II)-adsorption capacity.

3.3. Effect of SP-SBC Dosage

For the Cu(II)-adsorption capacity, the influence of SP-SBC dosage is shown in Figure 6. With increased SP-SBC dosage (from 0.5 g·L⁻¹ to 10 g·L⁻¹), the Cu(II)-adsorption capacity decreased ( $p < 0.05$ ) from 26.06 mg·g⁻¹ to 4.983 mg·g⁻¹, in accordance with those of other reported biochars [71, 72]. This decrease in adsorption capacity for Cu(II) was associated with the following reasons: (i) the nonsaturated Cu(II) adsorption on SP-SBC caused by the excessive amount of SP-SBC [72] and (ii) the agglomeration and polymerization of SP-SBC at a high dosage [73].

Figure 6

Effect of SP-SBC dosage on the adsorption capacity of SP-SBC for Cu(II).

3.4. Effect of Initial pH

To avoid the experimental interference caused by the extra precipitation of Cu(II) at the initial pH of >6 [6, 69], the initial pH was adjusted to 3–6. Figure 7(a) shows the effect of initial pH on the Cu(II) adsorption by SP-SBC. As shown in Figure 7, with increased initial pH from 3 to 5, the Cu(II)-adsorption capacity rapidly increased from 4.52 mg·g⁻¹ to 20.63 mg·g⁻¹ ( $p < 0.05$ ). pH importantly influences the adsorption of metal ions because it usually determines the surface charge of the sorbent [3, 6, 12, 23]. Figure 7 also displays the zeta potentials of SP-SBC at different pH values. We found that the zeta potential gradually decreased with increased initial pH ( $p < 0.05$ ), and the isoelectric point (pH_IEP) of SP-SBC was 3.58. These results suggested that the negative charges on the surface of SP-SBC increased with increased pH to >3.58. It enhanced the electrostatic attraction between Cu(II) and SP-SBC, leading to the increasing trend of Cu(II)-adsorption capacity from the initial pH of 3 to 5 [4, 15]. Moreover, the adsorption competition between Cu(II) and H⁺ gradually weakened with increased initial pH within the range of 3–5, also resulting in increased Cu(II)-adsorption capacity of SP-SBC. These findings suggested that electrostatic attraction participated in the adsorption of Cu(II) by SP-SBC (Figure 7(b)). When the initial pH was above 5, the Cu(II)-adsorption capacity remained unchanged ( $p > 0.05$ ), consistent with the results of Deng et al. [74].

Figure 7

(a) Effect of initial pH on the adsorption capacity of SP-SBC for Cu(II), and (b) an illustration of the effect of initial pH on Cu(II) adsorption by SP-SBC.

(b)

3.5. Effect of Ionic Strength

Ionic strength usually has a significant effect on adsorption [75]. The effect of ionic strength (Na⁺) on Cu(II) adsorption by SP-SBC is displayed in Figure 8. With increased ionic strength (0.005–0.5 M), the Cu(II)-adsorption capacity decreased significantly ( $p < 0.05$ ). The reasons for this trend were as follows: (i) Na⁺ competed with Cu²⁺ in the adsorption [6]; (ii) a dense hydrating shell formed on the surface of SP-SBC once Na⁺ was adsorbed [76], which prevented Cu(II) from making contact with the surface of SP-SBC; and (iii) the increase in ionic strength greatly reduced the activity coefficient of Cu(II), making it difficult for SP-SBC to capture Cu(II) [77]. In conclusion, ionic strength can significantly influence the Cu(II)-adsorption capacity of SP-SBC, especially under the high-ionic-strength condition. These results suggested that attention should be paid to the salinity control for the application of SP-SBC in high-salinity copper-containing wastewater.

Figure 8

Effect of ionic strength on the adsorption capacity of SP-SBC for Cu(II).

3.6. Effect of Contact Time and Adsorption Kinetics

Figure 9 shows the effect of contact time on Cu(II) adsorption by SP-SBC. With increased contact time, the Cu(II)-adsorption capacity increased ( $p < 0.05$ ) within the initial 12 h and then reached equilibrium ( $p > 0.05$ ) (Figure 9). The Cu(II) adsorption by SP-SBC was fast within the initial 0.5 h, and the adsorption capacity at 0.5 h reached 58% of the adsorption capacity at equilibrium time. Then, the Cu(II)-adsorption rate gradually slowed down between 0.5 and 12 h, finally reaching stability after 12 h. The high initial Cu(II)-adsorption rate was due to the considerable amount of unsaturated adsorption sites on SP-SBC, whereas the decelerating adsorption rate was due to the adsorption sites being gradually saturated as adsorption proceeded [23].

Figure 9

Effect of contact time on the adsorption capacity of SP-SBC for Cu(II) and fitting results of the PFO and PSO models (a) and the IPD model (b).

(b)

Figure 9 exhibits the fitting curves of the PFO, PSO, and IPD models for Cu(II) adsorption by SP-SBC. The fitting curve of the PSO model showed a better approximation to the experimental data than those of the PFO and IPD models. The fitting parameters, i.e., $Q_{et}$ , $k_{1}$ , $k_{2}$ , $k_{i}$ , $C$ , and correlation coefficient ( $R^{2}$ ), are displayed in Table 3. Compared with the PFO and IPD models, the PSO model had the highest $R^{2}$ (0.980). Additionally, the simulated $Q_{et}$ (20.02 mg·g⁻¹) obtained by the PSO model better agreed with the $q_{e}$ (20.01 mg·g⁻¹) obtained in the experiment. Therefore, Cu(II) adsorption by SP-SBC was better depicted by the PSO model. The result implied that the number of active sites of the SP-SBC surface restricted the Cu(II)-adsorption rate of SP-SBC; i.e., chemical adsorption was the rate-limiting step for Cu(II) adsorption by SP-SBC [15, 78, 79]. Thus, chemical adsorption may be a key mechanism in this study [11, 13, 71], consistent with the discussion in Section 3.2. Meanwhile, Figure 9(b) shows that the adsorption process of Cu(II) by SP-SBC can be divided two stages: boundary-layer diffusion with fast diffusion speed and intraparticle diffusion with relatively slow diffusion speed. Notably, the fitting curves of the above two stages did not pass through the original point, indicating that intraparticle diffusion was not the only rate-limiting step to control the Cu(II)-adsorption process of SP-SBC [11, 13].

Table 3

Fitting parameters of three kinetic models.

Pseudo-first order			Pseudo-second order
$Q_{et}$ (mg·g^-1)	$k_{1}$ (h^-1)	$R^{2}$	$Q_{et}$ (mg·g^-1)	$k_{2}$ (g·mg^-1·h^-1)	$R^{2}$
18.50	1.348	0.931	20.02	0.103	0.980

Intraparticle diffusion
Step 1			Step 2
$C$ (mg·g^-1)	$k_{i}$ (mg·(g·min^0.5)^-1)	$R^{2}$	$C$ (mg·g^-1)	$k_{i}$ (mg·(g·min^0.5)^-1)	$R^{2}$
8.91	4.12	0.98	14.91	1.43	0.95

3.7. Effect of Temperature and Adsorption Isotherms

The Cu(II) equilibrium adsorption capacities of SP-SBC ( $Q_{e}$ ) at different initial Cu(II) concentrations and temperatures are shown in Figure 10. At the same initial concentration, $Q_{e}$ increased with increased temperature (15–35°C). This result was due to the increased contact between Cu(II) and SP-SBC at a high temperature. At the same temperature, $Q_{e}$ increased sharply within the range of low initial Cu(II) concentration (i.e., <200 mg·L⁻¹) ( $p < 0.05$ ) and then gradually reached equilibrium.

Figure 10

Effect of temperature on the adsorption capacity of SP-SBC for Cu(II), and fitting results of the F and L (a), L-F (b), and D-R (c) models.

(b)

(c)

To further ascertain the Cu(II)-adsorption behavior of SP-SBC, the experimental data were simulated by the L, F, L-F, and D-R models. Table 4 lists the fitting values of $K_{L}$ , $Q_{m}$ , $K_{f}$ , $1 / n$ , $K_{a}$ , $n^{'}$ , $Q_{mD}$ , $E$ , and correlation coefficient obtained using the four models. For the L model, $K_{L}$ can be used to calculate the separation parameter ( $R_{L}$ ) and reflect the type of isotherm [4, 12], as shown in the following equation. $\begin{matrix} (17) & R_{L} = \frac{1}{1 + C_{0} K_{L}} . \end{matrix}$

Table 4

Fitting parameters of four isotherm models.

Temperature	Langmuir		Freundlich
Temperature	$K_{L}$ (L·mg^-1)	$Q_{m}$ (mg·g^-1)	$R^{2}$	$K_{f}$ (mg^(1-n)·L ⁿ ·g^-1)	$1 / n$	$R^{2}$
15°C	0.17	22.59	0.996	13.63	0.085	0.995
25°C	0.28	26.44	0.994	16.80	0.081	0.996
35°C	0.45	30.52	0.992	19.53	0.080	0.998

Temperature	Langmuir-Freundlich				Dubinin-Radushkevich
Temperature	$K_{a}$ (L·mg^-1)	$Q_{m}$ (mg·g^-1)	$n^{'}$	$R^{2}$	$E$ (kJ·mol^-1)	$Q_{mD}$ (mol·g^-1)	$R^{2}$
15°C	0.21	23.22	0.74	0.998	17.51	4.48	0.91
25°C	0.48	31.50	0.33	0.998	21.13	4.89	0.74
35°C	0.52	38.49	0.27	0.999	23.17	5.73	0.91

Figure 11 shows the $R_{L}$ values at different temperatures and initial Cu(II) concentrations. The values of $R_{L}$ were all less than 1, implying that Cu(II) adsorption by SP-SBC was a favorable process [3, 4, 13]. Moreover, with increased the temperature and Cu(II) initial concentration, the $R_{L}$ value continuously decreased. This result suggested that the Cu(II) adsorption was more favorable at a higher concentration and temperature [80]. For the F model, $1 / n$ can reflect the adsorption intensity [56]. If $1 / n < 0.5$ , the adsorption of the adsorbate is easy, whereas if $1 / n > 2$ , the adsorption of the adsorbate is difficult [80]. As shown in Table 4, for the different temperatures, the calculated $1 / n$ was always less than 0.5, demonstrating that Cu(II) was easily absorbed by SP-SBC. With increased temperature, the value of $K_{f}$ also increased. A larger value of $K_{f}$ implies a higher adsorption capacity [79]. Thus, a high temperature was favorable for Cu(II) adsorption; i.e., the Cu(II)-adsorption capacity strengthened with increased the temperature. For the L-F model, the $n^{'}$ values at three temperatures ranged within 0–1, and a higher temperature corresponded with a higher $K_{a}$ value in this work. The results also demonstrated that Cu(II) adsorption by SP-SBC was a favorable process [81], and the adsorption capability of SP-SBC for Cu(II) was enhanced with increased temperature [82]. For the D-R model, the values of $E$ at all experimental temperatures exceeded 16 kJ·mol⁻¹, indicating that Cu(II) adsorption by SP-SBC involved chemical adsorption [83–85].

Figure 11

Separation parameters ( $R_{L}$ ) for Cu(II) adsorption by SP-SBC at different temperatures and initial Cu(II) concentrations.

Figure 10 shows that the experimental data were matched better by the L-F model than the other three models. Furthermore, the L-F model had the highest $R^{2}$ among all models (Table 4). Thus, the L-F model was better adapted to fit the isotherms, suggesting that both processes (i.e., the Langmuir and Freundlich processes) were involved in Cu(II) adsorption by SP-SBC. In other words, Cu(II) adsorption was controlled by multiple mechanisms [86], such as ion exchange, precipitation (in Section 3.2) and electrostatic attraction (in Section 3.4). Additionally, the maximum Cu(II)-adsorption capacity of 38.49 mg·g⁻¹ was achieved for SP-SBC at 35°C by the L-F model. Table 5 summarizes the Cu(II)-adsorption capacities of different biochars. Compared with these sorbents, SP-SBC had a moderate adsorption capacity. The Cu (II)-adsorption equilibrium time of SP-SBC was also less than those of most adsorbents. Notably, the adsorption capacity of SP-SBC exceeded those of reported commercial activated carbons in Table 5. Overall, SP-SBC is a desirable and effective sorbent for Cu(II) removal from aqueous solutions.

Table 5

Cu(II)-adsorption capacities of different biochars.

Biochar	Conditions	Cu(II) maximum adsorption capacity (mg·g^-1)	References
Corn straw biochar	$pH = 5$ , $t = 24 h$ , $T = 22 ° C$	12.52	[87]
Composted swine manure biochar	$pH = 5$ , $t = 24 h$ , $T = 25 ° C$	21.94	[88]
S. hermaphrodita biochar	$pH = 5.5$ , $t = 24 h$ , $T = 22 ° C$	33.33	[89]
Date seed-derived biochar	$pH = 6$ , $t = 24 h$ , $T = 23 ° C$	26.94	[90]
Miscanthus giganteus biochar	$pH = 6$ , $t = 1 h$ , $T = 25 ° C$	19.72	[91]
Sewage sludge biochar	$pH = 5.2$ , $t = 20 h$ , $T = 25 ° C$	7.32	[36]
Municipal sewage sludge biochar	$pH = /$ , $t = 24 h$ , $T = 25 ° C$	5.34	[37]
Steam-activated giant Miscanthus biochar	$pH = 6$ , $t = 48 h$ , $T = 20 ° C$	15.4	[92]
Amino-modified sawdust biochar	$pH = 5$ , $t = 200 \min$ , $T = 30 ° C$	17.01	[25]
KOH-activated brewers draft biochar	$pH = 5$ , $t = 24 h$ , $T = /$	10.30	[93]
KMnO₄-modified loofah biochar	$pH = 5.5$ , $t = 10 h$ , $T = /$	47.64	[71]
Modified date seed biochar with HCl pretreatment	$pH = 6$ , $t = 24 h$ , $T = 23 ° C$	45.12	[24]
NaOH-modified Opuntia ficus-indica-activated biochar	$pH = 6$ , $t = /$ , $T = 30 ° C$	49.36	[94]
Commercial activated carbon	$pH = 5$ , $t = /$ , $T = 25 ° C$	19.21	[95]
Commercial activated carbon	$pH = 4$ , $t = 4 h$ , $T = 25 ° C$	6.9	[96]
Commercial activated carbon	$pH = 6.5$ , $t = 1 h$ , $T = 22 ° C$	13.7	[97]
SP-SBC	$pH = 5$ , $t = 12 h$ , $T = 35 ° C$	38.49	This study

3.8. Fixed-Bed Column Adsorption

Fixed-bed column adsorption can be used to understand the application potential of SP-SBC and offer support for the large-scale treatment of real copper-containing wastewater by SP-SBC [98]. Figure 12 and Table 6 show the breakthrough curves and the corresponding calculated parameters of four adsorption–desorption cycles in the fixed-bed column experiment, respectively. The values of $q_{ec}$ , $M^{'}$ , and $V_{eff}$ in the first cycle were 8.528 mg·g⁻¹, 153.5 mg, and 5175 mL, respectively, indicating that SP-SBC can be used for fixed-bed column adsorption. With the number of cycles increasing, the slope of the breakout curve increased gradually, indicating that the breakthrough ( $C_{t} / C_{0} = 0.1$ ) and saturation ( $C_{t} / C_{0} = 0.9$ ) points came earlier, and the values of $M$ , $M^{'}$ , $V_{eff}$ , and $q_{ec}$ decreased. These changes may be due to the inactivation of adsorption sites on the surface of SP-SBC during the adsorption–desorption cycles. After four adsorption–desorption cycles, $q_{ec}$ was still 2.794 mg·g⁻¹, indicating that SP-SBC can be reused at least four times. Considering the large sewage sludge production in China $(> 3 \times 10^{7} t o n s p e r y e a r)$ , the relatively simple preparation procedures, and the desired adsorption performance in the fixed-bed column adsorption, SP-SBC has a good application potential to treat copper-containing wastewater. Nevertheless, to effectively guide the practical application of SP-SBC, the following works need to be carried out: (i) optimization of adsorption-operating parameters to determine the optimum adsorption conditions; (ii) evaluation of the desorption effect of various desorbents, such as H₂SO₄, HNO₃, and EDTA, to select the effective desorbents; and (iii) design of feasible preparation procedures of SP-SBC from the perspective of industrial production to provide technical guidance for the market promotion of SP-SBC.

Figure 12

Breakthrough curves of four adsorption–desorption cycles in the fixed-bed column experiment.

Table 6

Adsorption parameters acquired by fixed-bed column adsorption in four adsorption–desorption cycles.

Cycle	${t_{b}}^{*}$ (min)	$t_{e}$ (min)	$V_{eff}$ (mL)	$M$ (mg)	$M^{'}$ (mg)	$q_{ec}$ (mg·g^-1)
1	110	345	5175	258.75	153.5	8.528
2	95	310	4650	232.50	133.71	7.428
3	50	195	2925	146.25	82.49	4.583
4	30	125	1875	93.75	50.29	2.794

$^{*} t_{b}$ is the breakthrough time (min), which is the corresponding time when $C_{t} / C_{0} = 0.1$ .

4. Conclusions

Na₄P₂O₇-modified biochar (SP-SBC) was successfully prepared by a simple one-pot method. Compared with SBC, the Cu(II)-adsorption capacity of SP-SBC improved 4.55 times. The Cu(II)-adsorption process of SP-SBC can be better described by the pseudo-second-order and Langmuir-Freundlich models. Cu(II) adsorption by SP-SBC involved ion exchange, electrostatic attraction, and precipitation. For Cu(II) adsorption, SP-SBC gained the maximum adsorption capacity of 38.49 mg·g⁻¹ at 35°C, which was higher than those of some reported commercial activated carbons. The fixed-bed column experiment indicated that SP-SBC can be used at least four times and had a good application potential for the treatment of copper-containing wastewater. Overall, SP-SBC can serve as an alternative sorbent to effectively remove Cu(II) from aqueous solutions. For the practical application of SP-SBC, the optimization of adsorption-operation parameters, evaluation of the desorption effect of various desorbents, and design of a feasible industrial-production scheme are required.

Footnotes

Data Availability

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors have no competing interests to declare that are relevant to the content of this article.

Authors’ Contributions

Liangqian Fan, Xianda Wang, and Jiaxin Miao contributed equally to this work and should be considered as co-first authors.

Acknowledgments

This work was financially supported by the Scientific Research Innovation Team Project of Sichuan Provincial Department of Education (No. 16TD0006).

References

Karri

R. R.

Ravindran

Dehghani

M. H.

Wastewater--sources, toxicity, and their consequences to human health

Soft computing techniques in solid waste and wastewater management 2021

Elsevier

3 33

10.1016/B978-0-12-824463-0.00001-X

Dehghani

M. H.

Omrani

G. A.

Karri

R. R.

Solid waste--sources, toxicity, and their consequences to human health

Soft computing techniques in solid waste and wastewater management 2021

Elsevier

205 213

10.1016/B978-0-12-824463-0.00013-6

Bhowmik

K. L.

Debnath

Nath

R. K.

Saha

Synthesis of MnFe₂O₄ and Mn₃O₄ magnetic nano-composites with enhanced properties for adsorption of Cr(VI): artificial neural network modeling

Water Science and Technology 2017 76

8217910

12 3368 3378

10.2166/wst.2017.501

2-s2.0-85038559439

29236016

Debnath

Bera

Chattopadhyay

K. K.

Saha

Facile additive-free synthesis of hematite nanoparticles for enhanced adsorption of hexavalent chromium from aqueous media: kinetic, isotherm, and thermodynamic study

Inorganic and Nano-Metal Chemistry 2017 47

8217910

12 1605 1613

10.1080/24701556.2017.1357581

2-s2.0-85033374462

Yan

Liang

Shen

Rapid removal of copper from wastewater by Fe-based amorphous alloy

Intermetallics 2020 124, article 106849

8217910

10.1016/j.intermet.2020.106849

Wang

Kong

Cai

Wang

Zhang

Zuo

Tian

Compressible amino-modified carboxymethyl chitosan aerogel for efficient Cu(II) adsorption from wastewater

Separation and Purification Technology 2022 293, article 121146

8217910

10.1016/j.seppur.2022.121146

Zhang

Chen

Bie

Zhi

Sun

A clean process for selective recovery of copper from industrial wastewater by extraction-precipitation with p-tert-octyl phenoxy acetic acid

Journal of Environmental Management 2022 304, article 114164

8217910

10.1016/j.jenvman.2021.114164

34864416

Pan

Miao

Liu

Shi

Single and binary dye adsorption of methylene blue and methyl orange in alcohol aqueous solution via rice husk based activated carbon: kinetics and equilibrium studies

Chemical Research in Chinese Universities 2020 36

8217910

6 1272 1278

10.1007/s40242-020-0063-9

Feng

Wang

Liu

Rapid, high-efficient and selective removal of cationic dyes from wastewater using hollow polydopamine microcapsules: isotherm, kinetics, thermodynamics and mechanism

Applied Surface Science 2021 542, article 148633

8217910

10.1016/j.apsusc.2020.148633

10.

Tala

Chantara

Use of spent coffee ground biochar as ambient PAHs sorbent and novel extraction method for GC-MS analysis

Environmental Science and Pollution Research 2019 26

8217910

13 13025 13040

10.1007/s11356-019-04473-y

2-s2.0-85063221949

30895544

11.

Lingamdinne

L. P.

Karri

R. R.

Khan

M. R.

Chang

Koduru

J. R.

Evaluation of surface phenomena of magnetic biomass for dye removal via surface modeling

Journal of Environmental Chemical Engineering 2021 105953

8217910

5, article 105953

12.

Saha

Debnath

Saha

Fabrication of PANI@Fe–Mn–Zr hybrid material and assessments in sono-assisted adsorption of methyl red dye: uptake performance and response surface optimization

Journal of the Indian Chemical Society 2022 99

8217910

9, article 100635

10.1016/j.jics.2022.100635

13.

Das

Debnath

Fabrication of MgFe₂O₄/polyaniline nanocomposite for amputation of methyl red dye from water: isotherm modeling, kinetic and cost analysis

Journal of Dispersion Science and Technology 2022 43

8217910

1 12

10.1080/01932691.2022.2110110

14.

Saha

Debnath

Saha

Evaluation of Fe–Mn–Zr trimetal oxide/polyaniline nanocomposite as potential adsorbent for abatement of toxic dye from aqueous solution

Polymer Technology in dye-Containing Wastewater 2022

Singapore

Springer Nature Singapore

15 37

10.1007/978-981-19-1516-1_2

15.

Lingamdinne

L. P.

Choi

Angaru

G. K. R.

Karri

R. R.

Yang

Chang

Koduru

J. R.

Magnetic-watermelon rinds biochar for uranium-contaminated water treatment using an electromagnetic semi-batch column with removal mechanistic investigations

Chemosphere 2022 286, article 131776

8217910

Part 2

10.1016/j.chemosphere.2021.131776

34371355

16.

Hairuddin

M. N.

Mubarak

N. M.

Khalid

Abdullah

E. C.

Walvekar

Karri

R. R.

Magnetic palm kernel biochar potential route for phenol removal from wastewater

Environmental Science and Pollution Research 2019 26

8217910

34 35183 35197

10.1007/s11356-019-06524-w

31691169

17.

Meng

Huang

Lin

Effects of competitive adsorption with Ni(II) and Cu(II) on the adsorption of Cd(II) by modified biochar co-aged with acidic soil

Chemosphere 2022 293, article 133621

8217910

10.1016/j.chemosphere.2022.133621

35033512

18.

Wang

Liu

Chen

Liu

Zhao

Mixed heavy metal removal from wastewater by using discarded mushroom-stick biochar: adsorption properties and mechanisms

Environmental Science: Processes & Impacts 2019 21

8217910

3 584 592

10.1039/c8em00457a

2-s2.0-85063356327

30785433

19.

Chi

Zuo

Liu

Performance and mechanism for cadmium and lead adsorption from water and soil by corn straw biochar

Frontiers of Environmental Science & Engineering 2017 11

8217910

2 15

10.1007/s11783-017-0921-y

2-s2.0-85017470324

20.

Mahdi

Q. J.

El Hanandeh

Competitive adsorption of heavy metal ions (Pb²⁺, Cu²⁺, and Ni²⁺) onto date seed biochar: batch and fixed bed experiments

Separation Science and Technology 2019 54

8217910

6 888 901

10.1080/01496395.2018.1523192

2-s2.0-85054414658

21.

Zhou

Feng

Yao

Wang

Liu

Zhou

Zhong

Effect of pyrolysis condition on the adsorption mechanism of lead, cadmium and copper on tobacco stem biochar

Journal of Cleaner Production 2018 187

8217910

996 1005

10.1016/j.jclepro.2018.03.268

2-s2.0-85047474927

22.

Chen

Wang

Deng

Industrial alkali lignin-derived biochar as highly efficient and low-cost adsorption material for Pb(II) from aquatic environment

Bioresource Technology 2021 322, article 124539

8217910

10.1016/j.biortech.2020.124539

33340951

23.

Katiyar

Patel

A. K.

Nguyen

Singhania

R. R.

Chen

Dong

Adsorption of copper (II) in aqueous solution using biochars derived from Ascophyllum nodosum seaweed

Bioresource Technology 2021 328, article 124829

8217910

10.1016/j.biortech.2021.124829

33618185

24.

Mahdi

Hanandeh

A. E.

Q. J.

Preparation, characterization and application of surface modified biochar from date seed for improved lead, copper, and nickel removal from aqueous solutions

Journal of Environmental Chemical Engineering 2019 7

8217910

5, article 103379

10.1016/j.jece.2019.103379

2-s2.0-85072083868

25.

Yang

Jiang

Amino modification of biochar for enhanced adsorption of copper ions from synthetic wastewater

Water Research 2014 48

8217910

396 405

10.1016/j.watres.2013.09.050

2-s2.0-84888849809

24183556

26.

Wang

Gao

Wang

Fang

Xue

Yang

Removal of Pb(II), Cu(II), and Cd(II) from aqueous solutions by biochar derived from KMnO₄ treated hickory wood

Bioresource Technology 2015 197

8217910

356 362

10.1016/j.biortech.2015.08.132

2-s2.0-84940998963

26344243

27.

Wang

Jin

Guo

Min

Zhang

Sun

Zhu

Zhang

Enhanced heavy metals sorption by modified biochars derived from pig manure

Science of the Total Environment 2021 786, article 147595

8217910

10.1016/j.scitotenv.2021.147595

28.

Bashir

Zhu

Comparing the adsorption mechanism of Cd by rice straw pristine and KOH-modified biochar

Environmental Science and Pollution Research 2018 25

8217910

12 11875 11883

10.1007/s11356-018-1292-z

2-s2.0-85042080269

29446023

29.

Xiong

Zhou

Chen

Wang

Tan

Quantitative analysis of Pb adsorption on sulfhydryl-modified biochar

Biochar 2021 3

8217910

1 37 49

10.1007/s42773-020-00077-9

30.

Wang

Liu

H₂O₂ treatment enhanced the heavy metals removal by manure biochar in aqueous solutions

Science of the Total Environment 2018 628-629

8217910

1139 1148

10.1016/j.scitotenv.2018.02.137

2-s2.0-85042213009

31.

Song

Zhang

Yang

Preparation of montmorillonite modified biochar with various temperatures and their mechanism for Zn ion removal

Journal of Hazardous Materials 2020 391, article 121692

8217910

10.1016/j.jhazmat.2019.121692

32062342

32.

Zhou

Yuan

Gong

Selecting sustainable technologies for disposal of municipal sewage sludge using a multi-criterion decision-making method: a case study from China

Resources Conservation and Recycling 2020 161, article 104881

8217910

10.1016/j.resconrec.2020.104881

33.

Zhang

Wang

Mei

Chi

Status and development of sludge incineration in China

Waste and Biomass Valorization 2021 12

8217910

7 3541 3574

10.1007/s12649-020-01217-9

34.

Bondarczuk

Markowicz

Piotrowska-Seget

The urgent need for risk assessment on the antibiotic resistance spread via sewage sludge land application

Environment International 2016 87

8217910

49 55

35.

Chen

Zhang

Wang

Zhou

Zhang

Ren

Influence of pyrolysis temperature on characteristics and heavy metal adsorptive performance of biochar derived from municipal sewage sludge

Bioresource Technology 2014 164

8217910

47 54

10.1016/j.biortech.2014.04.048

2-s2.0-84900466917

24835918

36.

Shen

Tang

Meng

Mechanisms of copper stabilization by mineral constituents in sewage sludge biochar

Journal of Cleaner Production 2018 193

8217910

185 193

10.1016/j.jclepro.2018.05.071

2-s2.0-85048192445

37.

Dan

Fengxiang

Mengke

Xianping

Effects of biochar-derived sewage sludge on heavy metal adsorption and immobilization in soils

International Journal of Environmental Research and Public Health 2017 14

8217910

7 681

10.3390/ijerph14070681

2-s2.0-85021415043

38.

Gao

Deng

Huang

Cai

Liu

Huang

Relative distribution of Cd²⁺ adsorption mechanisms on biochars derived from rice straw and sewage sludge

Bioresource Technology 2019 272

8217910

114 122

10.1016/j.biortech.2018.09.138

2-s2.0-85054435475

30316193

39.

Chen

Zhou

Wang

Adsorption behavior comparison of trivalent and hexavalent chromium on biochar derived from municipal sludge

Bioresource Technology 2015 190

8217910

388 394

10.1016/j.biortech.2015.04.115

2-s2.0-84929157627

25978792

40.

Liu

Influence of pyrolysis temperature on sludge biochar: the ecological risk assessment of heavy metals and the adsorption of Cd(II)

Environmental Science and Pollution Research 2022

10.1007/s11356-022-22827-x

41.

Zhao

Dai

Zhang

Feng

Qin

Zhang

Zhao

Tsang

D. C. W.

Qiu

Mechanisms of Pb and/or Zn adsorption by different biochars: biochar characteristics, stability, and binding energies

Science of the Total Environment 2020 717, article 136894

8217910

10.1016/j.scitotenv.2020.136894

42.

Chen

Liu

Chen

Jiang

Chen

Hydroxyapatite modified sludge-based biochar for the adsorption of Cu²⁺ and Cd²⁺: adsorption behavior and mechanisms

Bioresource Technology 2021 321, article 124413

8217910

10.1016/j.biortech.2020.124413

33285503

43.

Smith

K. M.

Fowler

G. D.

Pullket

Graham

N. J. D.

Sewage sludge-based adsorbents: a review of their production, properties and use in water treatment applications

Water Research 2009 43

8217910

10 2569 2594

10.1016/j.watres.2009.02.038

2-s2.0-65549108823

19375772

44.

Santhosh

Daneshvar

Tripathi

K. M.

Baltrėnas

Kim

Baltrėnaitė

Bhatnagar

Synthesis and characterization of magnetic biochar adsorbents for the removal of Cr(VI) and acid orange 7 dye from aqueous solution

Environmental Science and Pollution Research 2020 27

8217910

26 32874 32887

10.1007/s11356-020-09275-1

32519109

45.

Zuo

Chen

Cui

Enhanced removal of Cd(II) from aqueous solution using CaCO₃ nanoparticle modified sewage sludge biochar

RSC Advances 2017 7

8217910

26 16238 16243

10.1039/C7RA00324B

2-s2.0-85015439803

46.

Zhao

Huang

Zhang

Cao

Modification of kelp and sludge biochar by TMT-102 and NaOH for cadmium adsorption

Journal of the Taiwan Institute of Chemical Engineers 2020 116

8217910

101 111

10.1016/j.jtice.2020.10.036

47.

Yang

Xue

Peng

Zhang

Highly efficient nickel (II) removal by sewage sludge biochar supported α-Fe₂O₃ and α-FeOOH: sorption characteristics and mechanisms

PLoS One 2019 14

8217910

6, article e218114

48.

Wang

Zhu

Zhang

Shi

Chovelon

Wang

Preparation of a novel sludge-derived biochar by K₂FeO₄ conditioning to enhance the removal of Pb²⁺

Colloid and Interface Science Communications 2021 42, article 100417

8217910

10.1016/j.colcom.2021.100417

49.

Shahib

I. I.

Ifthikar

Oyekunle

D. T.

Elkhlifi

Jawad

Wang

Lei

Chen

Influences of chemical treatment on sludge derived biochar; physicochemical properties and potential sorption mechanisms of lead (II) and methylene blue

Journal of Environmental Chemical Engineering 2022 10

8217910

3, article 107725

10.1016/j.jece.2022.107725

50.

Phoungthong

Suwunwong

Magnetic biochar derived from sewage sludge of concentrated natural rubber latex (CNRL) for the removal of Al³⁺ and Cu²⁺ ions from wastewater

Research on Chemical Intermediates 2020 46

8217910

1 385 407

10.1007/s11164-019-03956-4

2-s2.0-85071445054

51.

Tang

Shao

Zheng

Yan

Zhang

Amino-functionalized sewage sludge-derived biochar as sustainable efficient adsorbent for Cu(II) removal

Waste Management 2019 90

8217910

17 28

10.1016/j.wasman.2019.04.042

2-s2.0-85064498195

31088670

52.

Fan

Miao

Yang

Zhao

Shi

Xie

Wang

Chen

Luo

Cheng

Invasive plant-crofton weed as adsorbent for effective removal of copper from aqueous solution

Environmental Technology & Innovation 2022 26, article 102280

8217910

10.1016/j.eti.2022.102280

53.

Delgado

Capparelli

Navarro

Marino

Pharmaceutical emerging pollutants removal from water using powdered activated carbon: study of kinetics and adsorption equilibrium

Journal of Environmental Management 2019 236

8217910

301 308

10.1016/j.jenvman.2019.01.116

2-s2.0-85061326647

30738300

54.

Slyusarenko

Gerasimova

Atamanova

Plotnikov

Slyusareva

Adsorption of eosin Y on polyelectrolyte complexes based on chitosan and arabinogalactan sulfate

Colloids and Surfaces A: Physicochemical and Engineering Aspects 2021 610, article 125731

8217910

10.1016/j.colsurfa.2020.125731

55.

Karri

R. R.

Sahu

J. N.

Meikap

B. C.

Improving efficacy of Cr (VI) adsorption process on sustainable adsorbent derived from waste biomass (sugarcane bagasse) with help of ant colony optimization

Industrial Crops and Products 2020 143, article 111927

8217910

10.1016/j.indcrop.2019.111927

56.

Karri

R. R.

Sahu

J. N.

Jayakumar

N. S.

Optimal isotherm parameters for phenol adsorption from aqueous solutions onto coconut shell based activated carbon: error analysis of linear and non-linear methods

Journal of the Taiwan Institute of Chemical Engineers 2017 80

8217910

472 487

10.1016/j.jtice.2017.08.004

2-s2.0-85027996843

57.

Wang

Zhang

Chi

An amino-functionalized ramie stalk-based adsorbent for highly effective Cu²⁺ removal from water: adsorption performance and mechanism

Process Safety and Environmental Protection 2018 117

8217910

511 522

10.1016/j.psep.2018.05.023

2-s2.0-85047877177

58.

Abdolali

Ngo

H. H.

Guo

Zhou

J. L.

Zhang

Liang

Chang

S. W.

Duc Nguyen

Liu

Application of a breakthrough biosorbent for removing heavy metals from synthetic and real wastewaters in a lab-scale continuous fixed-bed column

Bioresource Technology 2017 229

8217910

78 87

10.1016/j.biortech.2017.01.016

2-s2.0-85009830445

28110128

59.

Jin

Zhang

Cao

Liang

Zhang

Wong

M. H.

Wang

Shan

Christie

Influence of pyrolysis temperature on properties and environmental safety of heavy metals in biochars derived from municipal sewage sludge

Journal of Hazardous Materials 2016 320

8217910

417 426

10.1016/j.jhazmat.2016.08.050

2-s2.0-84989361139

27585274

60.

Chen

Yang

Nagarajan

Chang

Ren

High-efficiency removal of lead from wastewater by biochar derived from anaerobic digestion sludge

Bioresource Technology 2017 246

8217910

142 149

10.1016/j.biortech.2017.08.025

2-s2.0-85028310042

28811160

61.

Dai

Wen

Jia

Wang

Shi

Biochar for asphalt modification: a case of high-temperature properties improvement

Science of the Total Environment 2022 804, article 150194

8217910

10.1016/j.scitotenv.2021.150194

34798737

62.

Yang

Feng

McGrouther

Wang

Chen

Chemical characterization of rice straw-derived biochar for soil amendment

Biomass and Bioenergy 2012 47

8217910

268 276

10.1016/j.biombioe.2012.09.034

2-s2.0-84870688585

63.

Hagemann

Subdiaga

Orsetti

Maria De La Rosa

Knicker

Schmidt

Kappler

Behrens

Effect of biochar amendment on compost organic matter composition following aerobic composting of manure

Science of the Total Environment 2018 613-614

8217910

20 29

10.1016/j.scitotenv.2017.08.161

2-s2.0-85028988021

28892724

64.

Luo

Pei

Jiang

The spectral characteristics of biochar-derived dissolved organic matter at different pyrolysis temperatures

Journal of Environmental Chemical Engineering 2021 9

8217910

5, article 106075

10.1016/j.jece.2021.106075

65.

Chellappan

Nair

Sajith

Aparna

Synthesis, optimization and characterization of biochar based catalyst from sawdust for simultaneous esterification and transesterification

Chinese Journal of Chemical Engineering 2018 26

8217910

12 2654 2663

10.1016/j.cjche.2018.02.034

2-s2.0-85048482159

66.

Hackbarth

F. V.

Girardi

de Souza

S. M. A. G.

Souza

A. D.

Boaventura

Vilar

Marine macroalgae Pelvetia canaliculata (Phaeophyceae) as a natural cation exchanger for cadmium and lead ions separation in aqueous solutions

Chemical Engineering Journal 2014 242

8217910

294 305

10.1016/j.cej.2013.12.043

2-s2.0-84892900392

67.

Zhao

Cao

Masek

Zimmerman

Heterogeneity of biochar properties as a function of feedstock sources and production temperatures

Journal of Hazardous Materials 2013 256-257

8217910

1 9

10.1016/j.jhazmat.2013.04.015

2-s2.0-84877834302

68.

Chen

Liu

Chen

Wang

Peng

Zeng

Novel magnetic pomelo peel biochar for enhancing Pb(II) and Cu(II) adsorption: performance and mechanism

Water, Air, & Soil Pollution 2020 231

8217910

8 404

10.1007/s11270-020-04788-4

69.

Zhao

Wang

Absorption of Cu(II) and Zn(II) from aqueous solutions onto biochars derived from apple tree branches

Energies 2020 13

8217910

13 3498

10.3390/en13133498

70.

Han

H. H.

Kim

Bae

Facile Cu(I) loading for adsorptive C₃H₆/C₃H₈ separation through double Cu(II) salts incorporation within pores with unsaturated Fe(II) sites

Bulletin of the Korean Chemical Society 2021 42

8217910

3 471 476

10.1002/bkcs.12225

71.

Zhao

Shan

Zhang

Yuan

Chen

Magnetically recyclable loofah biochar by KMnO₄ modification for adsorption of Cu(II) from aqueous solutions

ACS Omega 2022 7

8217910

10 8844 8853

10.1021/acsomega.1c07163

35309443

72.

Jung

Lee

S. Y.

Lee

Y. J.

Hydrothermal synthesis of hierarchically structured birnessite-type MnO₂/biochar composites for the adsorptive removal of Cu(II) from aqueous media

Bioresource Technology 2018 260

8217910

204 212

10.1016/j.biortech.2018.03.125

2-s2.0-85044965632

29626779

73.

Zhang

Shahab

Zhang

Zeng

Bacha

Nabi

Ullah

Comparative study on characterization and adsorption properties of phosphoric acid activated biochar and nitrogen-containing modified biochar employing eucalyptus as a precursor

Journal of Cleaner Production 2021 303, article 127046

8217910

10.1016/j.jclepro.2021.127046

74.

Deng

Liu

Zeng

Tan

Huang

Tang

Wang

Hua

Yan

Competitive adsorption of Pb(II), Cd(II) and Cu(II) onto chitosan-pyromellitic dianhydride modified biochar

Journal of Colloid and Interface Science 2017 506

8217910

355 364

10.1016/j.jcis.2017.07.069

2-s2.0-85025166696

28750237

75.

Bernal

Giraldo

Moreno-Piraján

J. C.

Thermodynamic analysis of acetaminophen and salicylic acid adsorption onto granular activated carbon: importance of chemical surface and effect of ionic strength

Thermochimica Acta 2020 683, article 178467

8217910

10.1016/j.tca.2019.178467

76.

Xiao

Xue

Gao

Mosa

Sorption of heavy metal ions onto crayfish shell biochar: effect of pyrolysis temperature, pH and ionic strength

Journal of the Taiwan Institute of Chemical Engineers 2017 80

8217910

114 121

10.1016/j.jtice.2017.08.035

2-s2.0-85028880753

77.

Chen

Wang

Xin

Sun

Adsorption behavior and mechanism of Cr(VI) by modified biochar derived from Enteromorpha prolifera

Ecotoxicology and Environmental Safety 2018 164

8217910

440 447

10.1016/j.ecoenv.2018.08.024

2-s2.0-85051827572

30144704

78.

Tan

Sun

Wang

Sorption of mercury (II) and atrazine by biochar, modified biochars and biochar based activated carbon in aqueous solution

Bioresource Technology 2016 211

8217910

727 735

10.1016/j.biortech.2016.03.147

2-s2.0-84962456233

27061260

79.

Chen

Liu

Tang

Zheng

Wang

Shao

Chen

Zhu

Feng

Characteristics and mechanisms of cadmium adsorption from aqueous solution using lotus seedpod-derived biochar at two pyrolytic temperatures

Environmental Science and Pollution Research 2018 25

8217910

12 11854 11866

10.1007/s11356-018-1460-1

2-s2.0-85042069066

29446021

80.

Fan

Wang

Tang

Removal of methylene blue from aqueous solution by sewage sludge-derived biochar: adsorption kinetics, equilibrium, thermodynamics and mechanism

Journal of Environmental Chemical Engineering 2017 5

8217910

1 601 611

10.1016/j.jece.2016.12.019

2-s2.0-85007436460

81.

Marouf

Khelifa

Marouf-Khelifa

Schott

Khelifa

Removal of pentachlorophenol from aqueous solutions by dolomitic sorbents

Journal of Colloid and Interface Science 2006 297

8217910

1 45 53

10.1016/j.jcis.2005.10.030

2-s2.0-33645227957

16376921

82.

Zhao

Wang

Yang

Competitive adsorption and selectivity of benzene and water vapor on the microporous metal organic frameworks (HKUST-1)

Chemical Engineering Journal 2015 259

8217910

79 89

10.1016/j.cej.2014.08.012

2-s2.0-84906221030

83.

Pan

You

Xie

Wang

Shang

Study of ciprofloxacin removal by biochar obtained from used tea leaves

Journal of Environmental Sciences 2018 73

8217910

20 30

10.1016/j.jes.2017.12.024

2-s2.0-85040542676

30290868

84.

Yan

Zhang

Kan

Dong

Jiang

Yang

Cheng

Sorption of methylene blue by carboxymethyl cellulose and reuse process in a secondary sorption

Colloids and Surfaces A: Physicochemical and Engineering Aspects 2011 380

8217910

1-3 143 151

10.1016/j.colsurfa.2011.02.045

2-s2.0-79954418637

85.

Bayazit

Ş. S.

Kurtulbaş

Bilgin

Şahin

Decontamination of endocrine disruptors from water by graphene nanoplatelet/UiO-66 nanocomposites

Environmental Nanotechnology, Monitoring & Management 2022 18, article 100733

8217910

10.1016/j.enmm.2022.100733

86.

Zhang

Yang

Wang

Xue

Enhanced removal of phosphate from aqueous solution using Mg/Fe modified biochar derived from excess activated sludge: removal mechanism and environmental risk

Environmental Science and Pollution Research 2021 28

8217910

13 16282 16297

10.1007/s11356-020-12180-2

33389575

87.

Chen

Lehmann

McBride

M. B.

Hay

A. G.

Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution

Bioresource Technology 2011 102

8217910

19 8877 8884

10.1016/j.biortech.2011.06.078

2-s2.0-80052352102

21764299

88.

Meng

Feng

Dai

Liu

Adsorption characteristics of Cu(II) from aqueous solution onto biochar derived from swine manure

Environmental Science and Pollution Research 2014 21

8217910

11 7035 7046

10.1007/s11356-014-2627-z

2-s2.0-84901255250

24532283

89.

Bogusz

Oleszczuk

Dobrowolski

Application of laboratory prepared and commercially available biochars to adsorption of cadmium, copper and zinc ions from water

Bioresource Technology 2015 196

8217910

540 549

10.1016/j.biortech.2015.08.006

2-s2.0-84939856982

26295440

90.

Mahdi

Q. J.

Hanandeh

A. E.

Investigation of the kinetics and mechanisms of nickel and copper ions adsorption from aqueous solutions by date seed derived biochar

Journal of Environmental Chemical Engineering 2018 6

8217910

1 1171 1181

10.1016/j.jece.2018.01.021

2-s2.0-85042404674

91.

Cibati

Foereid

Bissessur

Hapca

Assessment of Miscanthus × giganteus derived biochar as copper and zinc adsorbent: study of the effect of pyrolysis temperature, pH and hydrogen peroxide modification

Journal of Cleaner Production 2017 162

8217910

1285 1296

10.1016/j.jclepro.2017.06.114

2-s2.0-85024127880

92.

Shim

Yoo

Ryu

Park

Jung

Effect of steam activation of biochar produced from a giant Miscanthus on copper sorption and toxicity

Bioresource Technology 2015 197

8217910

85 90

10.1016/j.biortech.2015.08.055

2-s2.0-84940052817

26318926

93.

Trakal

Šigut

Šillerová

Faturíková

Komárek

Copper removal from aqueous solution using biochar: effect of chemical activation

Arabian Journal of Chemistry 2014 7

8217910

1 43 52

10.1016/j.arabjc.2013.08.001

2-s2.0-84891373182

94.

Choudhary

Kumar

Neogi

Activated biochar derived from Opuntia ficus-indica for the efficient adsorption of malachite green dye, Cu⁺² and Ni⁺² from water

Journal of Hazardous Materials 2020 392, article 122441

8217910

10.1016/j.jhazmat.2020.122441

32193109

95.

Huang

M. L.

Wang

Liu

S. Q.

Adsorption of Cu and Ni ions from aqueous solutions by commercial activated carbon and the reutilization in glass coloration

Journal of Wuhan University of Technology-Materials Science 2019 34

8217910

1 41 46

10.1007/s11595-019-2012-3

2-s2.0-85061778845

96.

Pehlivan

Cetin

Application of fly ash and activated carbon in the removal of Cu²⁺ and Ni²⁺ ions from aqueous solutions

Energy Sources Part A-Recovery Utilization and Environmental Effects 2008 30

8217910

13 1153 1165

10.1080/15567030701258220

2-s2.0-47749100509

97.

Safaei

Langeroodi

N. S.

Baher

Investigation of removal of Cu(II) ions by commercial activated carbon: equilibrium and thermodynamic studies

Protection of Metals and Physical Chemistry of Surfaces 2019 55

8217910

1 28 33

10.1134/S2070205119010180

2-s2.0-85065862983

98.

Zhang

Ahmad

Al-Wabel

M. I.

Vithanage

Rajapaksha

A. U.

Kim

H. S.

Lee

S. S.

Y. S.

Adsorptive removal of trichloroethylene in water by crop residue biochars pyrolyzed at contrasting temperatures: continuous fixed-bed experiments

Journal of Chemistry 2015 2015

8217910

647072

10.1155/2015/647072

2-s2.0-84944340018