Adsorption property of Pb and Cd by substrate residues after vermicomposting of cow dung mixed with minerals

Abstract: High concentrations of heavy metals lead (Pb) and cadmium (Cd) in soil have triggered a serious threat to plant growth, animal health, and ecological environment for human survival. Non-biodegradable Pb and Cd contamination in soil can be easily transferred to agricultural crops through the soil-plant-food chain, easy to endanger human body health. Alternatively, in situ stabilization is an effective technology to reduce the toxicity of such heavy metals. However, it is particularly important to choose the cost-effective materials with significant stability effects in soil. This study aims to seek in situ stabilized remediation materials with a high adsorption efficiency of heavy metal. A vermicomposting experiment of cow dung mixed with high calcium and magnesium minerals or sepiolite was conducted to analyze the variation of substrate residues properties and the adsorption characteristics of Pb2+ and Cd2+. Firstly, the growth indexes of earthworms were selected for the feasibility of vermicomposting cow dung mixed with minerals. Then, the substrate residues were characterized, including pH value, cation exchange capacity, morphology under a Scanning Electron Microscopy (SEM), mineral components under X-Ray Diffraction (XRD), and surface functional groups under a Fourier Transform Infrared Spectroscopy (FTIR). Finally, batch adsorption experiments were carried out to evaluate the adsorption efficiency of Pb2+ and Cd2+ from aqueous solution. The results demonstrated that the earthworms survived well with high calcium and magnesium minerals or sepiolite addition. The earthworm treatment reduced pH value, C/N, and organic content, while, increased the cation exchange capacity, and the specific surface area of substrate residues. High calcium and magnesium minerals or sepiolite addition both increased the pH value, cation exchange capacity, and specific surface area of substrate residues. FTIR analysis showed that the earthworm treatment had more alcohol or carboxylic acids and aromatic substance, while, less lipid material and polysaccharide substance in substrate residues, compared with the untreatment. The addition of high calcium and magnesium mineral also produced more Me-O groups in substrate residues. XRD analysis showed that more silicate and soluble salts were found in the earthworm treatment, compared with the untreatment. The addition of high calcium and magnesium mineral produced silicate, CaO and MgO components, whereas, the sepiolite addition produced silicate and MgO components in substrate residues. The batch adsorption experiment showed that the earthworm treatment had better adsorption capacity of Pb2+and Cd2+ in aqueous solution. The adsorption efficiency of Pb2+and Cd2+ was the best for the substrate residues rom the vermicomposting process of cow dung mixed with high calcium and magnesium minerals. The adsorption rates of Pb2 + and Cd2 + were 77.8% and 59.7%, respectively, whereas, the desorption rate of adsorption states Pb2+ and Cd2+ were 0.02% and 6.66%, respectively. It infers that the adsorption mechanism of VG for Pb2+and Cd2+ involved in the physical adsorption, ion exchange, electrostatic adsorption, precipitation, and complexation. A high adsorption efficiency of Pb2+and Cd2+ from aqueous solution was achieved under the higher pH value, cation exchange capacity and specific surface area, abundant functional groups and mineral components in VG. Therefore, it is feasible to produce substrate residues with high Pb2+and Cd2+ adsorption capacity via adding high calcium and magnesium mineral into cow dung for vermicomposting. The findings can provide an insightful theoretical basis to effectively optimize key parameters for the vermicomposting of cow dung mixed with minerals, and thereby to promote the recycling efficiency of organic solid wastes and the safe utilization of heavy-metal contaminated soil.