Adsorption of Cd(II) and Pb(II) onto Humic Acid–Treated Coconut (Cocos nucifera) Husk

This study was conducted to characterize the adsorption of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ 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 ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ 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 ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ 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 ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ ions by HACH was independent of pH and ionic strength. The maximum adsorption capacities of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ of HACH were 47.28 and $66.26\mathrm{mg}{\mathrm{g}}^{-1}$ and were greater than 24.24 and $21.21\mathrm{mg}{\mathrm{g}}^{-1}$ for RCH. The adsorption of both ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ onto HACH was a chemical sorption, and ${\mathrm{Pb}}^{2+}$ adsorption needed a longer reaction time than ${\mathrm{Cd}}^{2+}$ adsorption. The adsorption reaction rates of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ of HACH were 0.056 and $3.17\mathrm{mg}{\mathrm{g}}^{-1}{\min }^{-1}$. Some anions and cations affected the adsorption of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ onto HACH. Based on the results from adsorption tests under different conditions, RCH and HACH are effective for individual removal of ${\mathrm{Cd}}^{2+}$ and ${\mathrm{Pb}}^{2+}$ 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.