Adsorption of Cd(II) and Pb(II) onto humic acid-treated coconut (cocos nucifera) husk

This study was conducted to characterize the adsorption of Cd²⁺ and Pb²⁺ by raw coconut husk (RCH) and humic acid-treated coconut husk (HACH). Grounded and sieved coconut husk (CH) from Sri Lanka was treated with humic acid to use as the adsorbent in laboratory experiments. Capacities of HACH and RCH for adsorption of Cd²⁺ and Pb²⁺ at different pH, ionic strengths, initial metal concentrations, and contact times between adsorbent and adsorbate were measured. The effects of binary metal solution, anions, and cations on adsorptions of Cd²⁺ and Pb²⁺ by treated and nontreated CH in batch experiments were also examined. The surface characteristics of HACH and RCH were investigated by scanning electron micrography (SEM), X-ray energy dispersion analysis (EDAX), methylene blue (MB), and the Brunauer, Emmett, and Teller (BET) methods to explain the results. Results showed that adsorption of Cd²⁺ and Pb²⁺ ions by HACH was independent of pH and ionic strength. The maximum adsorption capacities of Cd²⁺ and Pb²⁺ of HACH were 47.28 and 66.26 mg g^-1 and were greater than 24.24 and 21.21 mg g^-1 for RCH. The adsorption of both Cd²⁺ and Pb²⁺ onto HACH was a chemical sorption, and Pb²⁺ adsorption needed a longer reaction time than Cd²⁺ adsorption. The adsorption reaction rates of Cd²⁺ and Pb²⁺ of HACH were 0.056 and 3.17 mg g^-1 min^-1. Some anions and cations affected the adsorption of Cd²⁺ and Pb²⁺ onto HACH. Based on the results from adsorption tests under different conditions, RCH and HACH are effective for individual removal of Cd²⁺ and Pb²⁺ from aqueous solutions. However, humic acid treatment effectively removes metal ions from single metal solutions, although the removal of metal ions in the presence of coexisting ions and other anions and cations could be retarded.