Insight into the Adsorption Behavior of Cd(II) and Pb(II) from Mud by HCl-Modified Coconut Shell Biochar: Experimental and DFT Studies

This study investigated the efficiency of biochar in eliminating Cd(II) and Pb(II) ions from slurries generated from construction-derived waste materials. The construction waste slurry samples consisted of genuinely contaminated sludge sediments. To improve the adsorption capacity of biochar for metal ions, coconut shell-derived biochar was subjected to hydrochloric acid treatment. The modified biochar demonstrated an improved porous structure and showed a higher concentration of oxygen-containing functional groups compared to the untreated biochar. After a 48 h contact with the contaminated slurry, the treated biochar attained removal efficiencies of 21.15% for Cd(II) and 19.43% for Pb(II). The kinetic study of the adsorption process conformed to a pseudo-second-order model. Density functional theory (DFT) computations clarified the adsorption mechanism of Cd(II) and Pb(II) by carboxyl (-COOH) and hydroxyl (-OH) functional groups. The findings demonstrated that functional groups contribute lone-pair electrons for the adsorption of heavy metal ions. The carboxyl (-COOH) functional group exhibited a greater affinity for binding Cd(II) and Pb(II) ions than the hydroxyl (-OH) group, which explains the improved adsorption efficiency seen in biochar treated with hydrochloric acid. These findings offer theoretical validation for the use of hydrochloric acid-modified biochar as an efficient adsorbent for the remediation of sludge contaminated with Cd(II) and Pb(II).