Role of Ash Content in Biochar for Copper Immobilization

Abstract

Abstract

Sorption of various contaminants on biochar has been investigated extensively in the literature. However, ash content was generally removed through weak acid or deionized water washing. The objective of this study was to investigate the role of biochar ash content in Cu(II) removal. Twenty-four biochars were prepared from pine chips (PCs) and peanut shells (PSs) at 200–500°C. All the biochars were washed with deionized water or HCl/HF to remove ash contents. Treated and untreated biochar particles were then compared for their physical–chemical properties as well as sorption characteristics for Cu(II). Cation exchange was an important mechanism controlling Cu(II) sorption on biochars. Precipitation was not important for most biochars, but contributed up to 20% for biochars produced from PSs at 500°C. Removal of ash content increased Cu(II) sorption on PC biochars. A significantly positive relationship was observed between ash content and sorption coefficients for Cu(II) for PS biochars. Thus, the impact of ash content on Cu(II) sorption is biochar property dependent. For biochars dominated with anions of PO₄³⁻, CO₃²⁻, OH⁻, and SO₄²⁻, ash content will play a positive role in Cu(II) removal from aqueous phase. However, for biochars dominated with competitive cations, ash content will play a negative role. Therefore, for the purpose of heavy metal immobilization, removal of ash content is dependent on biochar properties and the composition of ash content.