Adsorption and desorption characteristics of metal(oid)s in the yellow soils of a typical karst area, southwest China

Yellow soil is a type of soil rich in hydrated iron oxide that is extensively distributed in the typical karst areas of Guizhou Province, southwest China. Concomitantly, the higher contents of metal(oid)s are observed in the soil of karst areas in Guizhou Province, and there are few reports to study the inner relationship between local soil and metal(oid)s in the here. Hence, the present study mainly aimed to understand the adsorption and desorption characteristics of metal(oid)s in yellow soils of a typical karst area using kinetics and adsorption isotherms. The effects of temperature, pH and electrolytes on adsorption were also examined. The risk of metal(oid)s in soil was also evaluated by geoaccumulation and tomlinson pollution load indexes. The results showed that the pH value of yellow soil collected from Guizhou Province was 6.41, making it a weakly acidic soil, and the average concentrations of organic matter and CaCO₃ were approximately 1.12% and 0.88%, respectively. The adsorption of metal(oid)s did not significantly vary with an increase in the CaCO₃ concentration in the yellow soil. The metal(oid)s adsorption of the yellow soil improved with an increasing clay content. The Hg content in the yellow soil far exceeded the standard value (GB 15618–2018), with a corresponding cumulative evaluation of grade 1 according to the geoaccumulation index. Tomlinson pollution load indexes exhibited that the comprehensive score of the selected metal(oid)s in the yellow soil was 0.60, corresponding to no pollution. The adsorption behavior of As onto the yellow soil can be interpreted through pseudo-second-order kinetics and the Langmuir model, where the maximum adsorption was observed at 1318 mg/kg. The adsorption of most metal(oid)s increased with increasing temperature. The adsorption of Pb, Cr, Cu, Zn and As did not obviously fluctuate with an increasing NaNO₃ concentration. Notably, local related departments should take measures to decrease the Hg content in this soil.