The alkali digestion pretreatment method in the United States Environmental Protection Agency (USEPA) Method 3060A could underestimate the content of Cr(VI) in Cr-contaminated soils, especially for soils mixed with chromite ore processing residue (COPR), which leads to a misjudgment of the Cr(VI) level in soils after remediation, causing secondary pollution to the environment. In this study, a new pretreatment method to analyze Cr(VI) concentration in contaminated soils was established. The impacts of soil quality, particle size, alkali digestion time and the rounds of alkali digestion on Cr(VI) detection in contaminated soils was explored and the alkali digestion method was optimized. Compared with USEPA Method 3060A, the alkaline digestion time was prolonged to 6 h and multiple alkali digestion was employed until the amount of Cr(VI) in the last extraction was less than 10% of the total amount of Cr(VI). Because Cr(VI) in COPR is usually embedded in the mineral phase structure, the hydration products were dissolved and Cr(VI) was released gradually during the alkaline digestion process. The amount of Cr(VI) detected showed high correlation coefficients with the percentage of F1 (mild acid-soluble fraction), F2 (reducible fraction) and F4 (residual fraction). The Cr(VI) contents detected by the new alkaline digestion method and USEPA Method 3060A showed significant differences for soil samples mixed with COPR due to their high percentage of residual fraction. This new pretreatment method could quantify more than 90% of Cr(VI) in Cr-contaminated soils, especially those mixed with COPR, which proved to be a promising method for Cr(VI) analysis in soils, before and after remediation.

Soil is a major sink for microplastics (MPs) which can interact with organic contaminants affecting their transport behavior. The impact of MPs in soil matrix is complicated depending on soil-MPs, soil-contaminants and MPs-contaminants interactions, however the study on their interactions is limited. In this study, polyethylene (PE) was selected to investigate its effects on 2,4,6-tribromophenol (TBP) adsorption in soils. TBP adsorption on soils with/without MPs followed pseudo-second order kinetics, and the adsorption isotherm data fitted well with Langmuir and Freundlich models. Compared to pure soil, the presence of PE and aged PE (APE) in soil (1 %, w/w) reduced the adsorption capacity of TBP by 29.4 % and 32.4 %. Meanwhile, the TBP desorption efficiency from soil with PE and APE exceeded that observed in pure soil 31.5 %, which were 36.3 % and 35.7 %, respectively. Consequently, the mobility of TBP in soil could be enhanced increasing its risk to groundwater. With the increment of environmental pH, MPs dosage and ionic strength, the TBP adsorption amount decreased. MPs could compete with TBP for adsorption on soil particles exhibiting an inhibitory effect, since the adsorption energies of MPs-soil and TBP-soil from DFT calculation are strong and on the same level, while the interactions between TBP and MPs are mainly due to van der Waals force. Moreover, the inhibitory effect of APE was weaker than PE, because APE carried more negative charge, resulting from the oxygen-containing functional groups generated during aging, which reduced its affinity towards soil, consequently more vacant sites on soil were left for TBP adsorption.