Effect of biochars on the bioavailability of cadmium and di-(2-ethylhexyl) phthalate to Brassica chinensis L. in contaminated soils

Soil co-contamination of potentially toxic elements (PTEs) and phthalate esters has become prominent due to its potential adverse effect on human food supply. There is limited information on using wood- and animal-derived biochars for the remediation of co-contaminated soils. Therefore, a pot experiment was conducted using Brassica chinensis L. as a bio-indicator plant to investigate the effect of P. orientalis biochar and pig biochar application on the bioavailability of cadmium (Cd) and di-(2-ethylhexyl) phthalate (DEHP) and on plant physiological parameters (malondialdehyde, proline and soluble sugars). Biochar materials were applied to two soils containing low (LOC) and high (HOC) organic carbon content at rates of 0, 0.5, 1, 2, and 4%. To better understand the influence of biochar, physicochemical properties and X-ray diffraction (XRD), energy dispersive X-ray spectrometry (EDS), Fourier transform-infrared spectrometry (FTIR), scanning electron microscopy (SEM) were characterized. Biochar application increased soil pH, organic carbon content, and available phosphorus content. Increasing biochar application rates decreased DTPA-extractable Cd and extractable DEHP concentrations in both soils. Biochar application reduced the plant uptake of both Cd and DEHP from co-contaminated soils; the maximum reduction of Cd (92.7%) and DEHP (52.0%) was observed in 2% pig biochar-treated LOC soil. The responses of plant physiological parameters to increased biochar applications indicated that less Cd and DEHP were taken up by plants. Pig biochar was more effective (P < 0.05) at reducing the bioavailability of Cd and DEHP in both soils than P. orientalis biochar; therefore, pig biochar had greater potential for improving the quality of the crop. However, the highest application rate (4%) of pig biochar restricted plant seed germination. Key factors influencing the bioavailability of Cd and DEHP in soils were soil organic carbon content, biochar properties (such as surface alkalinity, available phosphorus content and ash content) and biochar application rates.