Comparative study on Pb²⁺ removal from aqueous solutions using biochars derived from cow manure and its vermicompost

Waste emissions have increased the amount of water and soil contaminated with heavy metals such as Pb. To broaden the methods for the recycling and environmental usage of cow manure (CM) and its vermicompost (CV), CM, CV, and their derived biochars produced by the pyrolysis of CM or CV at 350 and 700 °C were used as adsorbents for Pb²⁺ removal in this batch adsorption experiment to reveal their different Pb²⁺ removal efficiencies and the underlying mechanisms. The batch experiment results revealed that all adsorbents rapidly removed Pb²⁺ within 30 min. A pH between 2.0 and 6.0 positively affected Pb²⁺ removal by CM and its biochar, whereas that by CV and its biochar was only positively affected by pH between 2.0 and 3.0. CV-derived biochar was more effective in the removal of Pb²⁺ than the other absorbents, with the maximum adsorption capacities (Qm) fitted from the Langmuir model reaching approximately 230.0 mg·g^-1 and the desorption rate (DR) being approximately 0.00-0.02%. Material physiochemical characterization, including X-ray diffraction analysis, showed that high pH, high ash content, rich mineral content, and high mineral contents might have been the main reasons for more effective removal of Pb²⁺ from aqueous solutions by CV-derived biochar. Fourier-transform infrared analysis indicated that surface functional groups such as -OH, CO, -COO-, and C-O; original and newly produced carbonate; and phosphate in CV also led to more effective Pb²⁺ removal efficiency from aqueous solution via surface functional group binding. Thus, pyrolyzing CVs may be used to produce biochar as a cost-effective adsorbent for heavy metal remediation in soil and water in the future.