Comparative adsorption of Pb²⁺ and Cd²⁺ by cow manure and its vermicompost

Organic waste has great potential for use as an amendment to immobilize heavy metals in the environment. Therefore, this study investigates various properties of cow manure (CM) and its derived vermicompost (CV), including the pH, cationic exchangeable capacity (CEC), elemental composition and surface structure, to determine the potential of these waste products to remove Pb²⁺ and Cd²⁺ from solution. The results demonstrate that CV has a much higher pH, CEC and more irregular pores than CM and is enriched with minerals and ash content but has a lower C, H, O and N content. Adsorption isotherms studies shows that the adsorption of Pb²⁺ and Cd²⁺ onto either CM or CV follows a Langmuir model and presents maximum Pb²⁺ and Cd²⁺ adsorption capacities of 102.77 mg g^-1 and 38.11 mg g^-1 onto CM and 170.65 and 43.01 mg g^-1 onto CV, respectively. Kinetic studies show that the adsorption of Pb²⁺ onto CM and CV fits an Elovich model, whereas the adsorption of Cd²⁺ onto CM and CV fits a pseudo-second-order model. Desorption studies indicate that CV is more effective than CM in removing Pb²⁺ and Cd²⁺. FTIR analysis demonstrates that the adsorption of Pb²⁺ and Cd²⁺ onto CM mainly depends on existed aliphatic alcohol, aromatic acid as well as new produced carbonates, whereas that onto CV may be contributed by the existed aliphatic alcohol, aromatic acids as well as some carbonates and phosphates. Thus, vermicomposting disposal of cow manure with destination mineral addition may broaden the way of its recycle and environmental usage.