Surface characteristics of vermicompost produced with different mineral addition and its effect on Cd immobilization in soil

This study aims to seek in situ remediation materials with a high immobilization efficiency for heavy metals. The mineral-based vermicompost was prepared using cow dung mixed with iron powder and phosphate rock powder minerals. A vermicomposting experiment was also carried out. The mineral-based vermicompost was then examined using a series of structural characterization tools. The remediation effectiveness was assessed to screen the suitable amendments for soil. The adsorption-desorption characteristics were analyzed for the available properties and chemical forms of cadmium (Cd) in soil. The results demonstrated that the addition of iron powder and phosphate rock powder reduced the pH, C/N, and organic matter content in the mineral-based vermicompost, while significantly increasing the total nutrient content. Brunauer-Emmett-Teller (BET) specific surface area measurement, Scanning electron microscopy (SEM) and X-ray energy dispersive analysis (EDS) showed that the mineral-based vermicompost increased the specific surface area and pore volume, whereas, there was a decrease in the pore diameter, compared with the vermicompost (VC). The mineral addition also changed the percentage of mineral elements in the vermicompost. Fourier transform infrared spectroscopy (FTIR) showed that the mineral-based vermicompost had more alcohol or carboxylic acid aromatic compounds and polysaccharides, but fewer carbohydrates and lipids, compared with the vermicompost. X-ray diffraction (XRD) showed that more mineral fractions were found in the mineral-based vermicompost, compared with the untreated. Most mineral components were found in the VCFe+P treatment, such as phosphate and nano-zero-valent iron. Soil culture and adsorption/desorption experiments showed that the application of mineral-based vermicompost enhanced the adsorption efficiency of Cd in the soil, where there was a decrease in the desorption rate of Cd. The best Cd adsorption was achieved in the soil of mineral-based vermicompost with cow manure, 20% iron powder and 20% phosphate rock powder (VCFe+P). The adsorption capacity increased by 137.66%, and the desorption rate decreased by 7.34%-12.76%, compared with the VC application. The VCFe+P treatment was reduced by 50.02% of the available Cd contaminants. Furthermore, The VCFe+P also promoted the transformation of Cd from exchangeable frictions to Fe/Mn oxide fractions and the residual fractions. In summary, the mineral-based vermicompost was produced from a mixture of cow dung. The better adsorption surface and immobilization on soil Cd were achieved in the 20% iron powder and 20% phosphonate powder. There was a great application prospect in the soil remediation of Cd pollution.