Sorption of heavy metals by natural biopolymers

Abstract

The sorption properties of modified hydrolysis lignin towards Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ ions in the pH range from 2 to 8 at temperature of 298.15 K solution are studied. The analysis of kinetic curves of ions sorption on modified lignin at pH 7 shows that sorption equilibrium in heterogenic system ion–metal–sorbent is attained in 30–60 min. Obtained results indicate good equilibrium kinetic properties of sorbent. Supposed that reaction between sorbate and functional group of sorbent are the second-order reaction and they interact between each other in ratio 1:1. The character of the obtained electrokinetic curve suggests that the zeta potential of modified lignin strongly depends on the pH. The dependencies of cations metals sorption coefficients on the pH value of water phase are S-shaped. Maximal extraction of studied cations is observed at pH values of equilibrium solutions close to neutral ones.

Introduction

Application of heavy metals and their compounds in numerous processes causes serious threat for the environment and health. Therefore, it is important to limit their presence in the natural environment.

The purification of large amounts of wastewater has substantial economic aspects of technical solutions. The literature reports many different methods of water treatment and waste water containing toxic elements to the environment with natural sorbents such as clay minerals, ceolites, waste products of wood industry, lignin, peat, chitin–lignin hybrid biosorbent (Bartczak et al., 2015; Ciesielczyk et al., 2017; Klapiszewski et al., 2015, 2017; Krasnopyorova et al., 2013, 2007, 2008; Tarasevich Yu and Dabrowski, 1999; Wong et al., 2003; Wysokowski et al., 2014).

Application of cheap and non-traditional sorbent in this work – hydrolysis lignin, obtained from wastes of woods processing – allows to solve several problems, such as water purification from toxic components, environmental protection, reduction of net cost of process of water conditioning. The prerequisites for using the natural biopolymer of modified hydrolytic lignin as a sorbent for the recovery of heavy metals are its properties, a highly developed surface and a large number of functional groups.

The aim of this work is the choice of the optimal conditions of sorption of different heavy metal ions (Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ ) by modified hydrolysis lignin.

Experimental

Sorption ability of natural biopolymer, the modified lignin, with respect to Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ ions was studied by method of limited solution volume in the pH range from 2 to 8 at 298.15 K (Krasnopyorova et al., 2013, 2007, 2008). The acidity of solutions was monitored by pH meter and was adjusted by addition of H₂SO₄ or NaOH solutions. To obtain sorption kinetic curves, 0.1 g sorbent samples were put in the number of tubes. Next the samples were flooded by 10 ml of aqueous solution of metal sulphate and kept from 10 min up to 24 h. Initial concentration of metal ions was 1.5 10⁻⁴mol/dm³. At certain time intervals the solution was extracted from the sorbent by filtration, and current concentration of metal ions therein was determined by atom emission spectrometer with inductive connected plasma (TRACE SCAN Advantage produced by ‘Thermo Jarrell Ash’).

The electrokinetic properties of modified lignin at various pH values have been studied. ζ-potential was measured by using dynamic scattering instrument using Zetasizer Nano ZS (‘Malvern’, UK).

Results and discussion

Quantitative characteristics of sorbent interactions with heavy metals were determined according to partition coefficient (K _d , cm³/g), and sorption coefficients (K_s,%), which were calculated according to the formulas (Krasnopyorova et al., 2007)

$$K_d=\frac{C_0-C_{\mathrm{eq}}}{C_{\mathrm{eq}}}\frac{V}{m}$$

(1)

$$K_s=\frac{C_0-C_{\mathrm{eq}}}{C_0}100$$

(2)

where C₀ and C_eq are the initial and equilibria concentration of solutions, respectively (mol/dm³), m is the mass of the sorbent (g) and V is the total volume of solution (cm³).

The analysis of kinetic curves of ions sorption on modified lignin at $\mathrm{pH}\hspace{1em}\sim 7$ shows that sorption equilibrium in heterogenic system ion–metal–sorbent is attained in 30–60 min (Figure 1). Obtained results indicate good equilibrium kinetic properties of sorbent.

Figure 1.

Kinetic curves of heavy metals ions sorption on modified lignin at pH 7.

One of the commonly used models for description of sorption is pseudo-first-order (3) and pseudo-second-order (4) (Bartczak et al., 2015), which can be expressed in linear form

$$\lg (q_e-q_t)={\mathrm{lgq}}_e-\frac{k_{1}t}{2.303}$$

(3)

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{1}{q_e}t$$

(4)

where q_t and q_e are total adsorbed amounts at time t and at equilibrium, respectively (mmol/g); k₁ is the pseudo-first order constant (\min) and k₂ is the pseudo-second order constant (g/mmol min).

The model of pseudo-first-order kinetics cannot well describe the sorption of ions Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ by modified lignin. It is evidenced by the relatively low correlation coefficients obtained in the processing of the kinetic curves in coordinates lg (q_e − q_t) – t: 0.95 to Cu, 0.89 – Zn, 0.82 – Co, 0.67 – Ni.

The kinetic curves have been obtained least squares method in the coordinates t/q_e − t. The corresponding correlation coefficient is 0.999 for all ions.

As the pseudo-first-order model, the pseudo-second-order model considers that the chemical exchange reaction limits sorption process. In this case it is supposed that reaction limits between sorbate and functional group of sorbent are the second-order reaction and they interact between each other in ratio 1:1.

The dependencies of the sorption coefficients of cations of heavy metals on the pH of the aqueous phase is S-shaped. Maximal extraction of studied cations is observed at pH values of equilibrium solutions close to neutral ones (Figure 2). In the pH range 1–7 sulphate metal salts, taken in tracer amounts $(\sim 10-4\text{mol}/\text{dm}3)$ in the aqueous phase are dissociated, so the sorption complex can be associated with the state of sorption sites depending on the pH of the aqueous phase. In the alkaline range at $\mathrm{pH}\ge 7$ in the process of making metal salt solution pH decreased significantly, due to the formation of metal hydroxides which have low solubility product values $K_{\mathrm{spMOH}}\approx 10-13-10-20$ .

Figure 2.

Sorption coefficients K _s values are different for various ions (Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ ) on modified lignin.

Complicated character of the dependence of cations sorption on pH value of aqueous phase could be related to the change of state of sorption centres which also depends on these pH values.

This assumption is confirmed by the results of zeta potential measurements. The character of the obtained electrokinetic curve suggests that the zeta potential of modified lignin strongly depends on the pH. For the analysed modified lignin sample, the zeta potential values were negative over the whole investigated pH range, namely for pH from 2 to 8, the electrokinetic potential varied from −4.9 to −27.2 mV (Figure 3).

Figure 3.

Zeta potential lignin from pH solution.

Based on the data obtained (Table 1), the following sorption series can be proposed for heavy metal cations for different pH values

$$\mathrm{Cu}>\mathrm{Ni}>\mathrm{Zn}>\mathrm{Co}(\mathrm{pH}=2);\mathrm{Cu}>\mathrm{Zn}>\mathrm{Ni}>\mathrm{Co}(\mathrm{pH}=4);\mathrm{Cu}>\mathrm{Ni}>\mathrm{Zn}>\mathrm{Co}(\mathrm{pH}=5);\mathrm{Cu}>\mathrm{Zn}>\mathrm{Ni}>\mathrm{Co}(\mathrm{pH}=8)$$

Table 1.

Partition coefficient K _d (cm³/g) values are different for various Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ ions on modified lignin.

pH	Cu $2+$	Zn $2+$	Ni $2+$	Co $2+$
2	37	31	32	30
4	65	56	46	37
5	864	534	651	325
6	2099	1509	1169	777
8	14086	2975	1570	898

The obtained rows are well corresponded with the data (Kendorf and Schnitzer, 1980) for sorption of metals by humic acids. Supposedly this is related with different solidity of metal complexes, which are formed with fragments of lignin at different pH.

Due to the presence of COOH- groups and OH- groups in lignin molecules, lignin possesses properties of weak acidic cationite (Burba and Willmer, 1983). Probably, at low pH values, carboxyl and hydroxyl functional groups of lignin either become partly dissociated or remain undissociated at all. Therefore, their ion exchange capacity essentially decreases in acidic media and increases in alkaline ones. It could be supposed that the ion exchange in lignin occurs stepwise via mobile ions of carboxyl and phenol groups. It was shown that the mobile hydrogen ion of carboxyl group is exchanged first, followed by the ion of phenol group (Hatakeyama and Hatakeyama, 2010). Due to bipolar character of the structure of modified lignin, its interaction with metals ions is possible via several mechanisms: ion exchange, chelate formation, chemisorption and physical adsorption (Demirbas and Hazard, 2008; Jesus et al., 2010; Suhas, 2007; Wysokowski et al., 2014). The domination of contribution of either mechanism depends on the state of lignin fragment, in particular, on the hydrolysis of zwitterion structure and on the formation of intra- and intermolecular chelates at changes of pH.

Conclusion

Sorption ability of the modified lignin towards Cu $2+$ , Zn $2+$ , Ni $2+$ , Co $2+$ ions in the pH range from 2 to 8 at 298.15 K was studied. Sorption equilibrium in heterogenic system ion–metal–sorbent is attained in 30–60 min. This indicates good equilibrium kinetic properties of modified lignin. Supposed that reaction between sorbate and functional group of sorbent are the second-order reaction and they interact between each other in ratio 1:1. The character of the obtained electrokinetic curve suggests that the zeta potential of modified lignin strongly depends on the pH. The dependencies of cations metals sorption coefficient (K_s) on the pH value of water phase are S-shaped. Maximal extraction of studied cations is observed at pH values of equilibrium solutions close to neutral ones. On the basis of the data obtained we may suggest the following rows of sorption for heavy metals cations on these pH values

$$\mathrm{Cu}>\mathrm{Ni}>\mathrm{Zn}>\mathrm{Co}(\mathrm{pH}=2);\mathrm{Cu}>\mathrm{Zn}>\mathrm{Ni}>\mathrm{Co}(\mathrm{pH}=4);\mathrm{Cu}>\mathrm{Ni}>\mathrm{Zn}>\mathrm{Co}(\mathrm{pH}=5);\mathrm{Cu}>\mathrm{Zn}>\mathrm{Ni}>\mathrm{Co}(\mathrm{pH}=8)$$