Effect of Citric Acid and Phosphorus Coexistence on Cadmium Adsorption by Soil

Abstract

In brown-red soil, the effect of phosphorus and citric acid co-existence on the adsorption of cadmium was studied using indoor experiments and isothermal equilibrium adsorption analysis. After treatment with different doses of phosphorus and citric acid, the fractions of cadmium were altered by varying dry and wet conditions. The results showed that:① Soil treated with 10 mg·L-1 of CdCl2 solution showed no notable effect on cadmium adsorption when a low concentration of phosphorus was added (40 mg·L-1); however, higher a concentration of added phosphorus (80 mg·L-1) significantly increased cadmium adsorption (an increase of 78 g·kg-1 and 7.89% compared to the control treatment); ② Using a 40 mg·L-1 phosphorus solution, the addition of citric acid proportionally reduced cadmium adsorption. This inhibition effect was more notable for the soil treated with low-dose phosphorus (40 mg·L-1) than the high-dose treatment with 1 mmol·L-1 and 5 mmol·L-1 citric acid (cadmium adsorption decreased by 30.89% and 40.97%, respectively). The effect of citric acid was not significant, however, at higher concentrations of phosphorus. When the concentration of citric acid reached 5 mmol·L-1, cadmium adsorption was only 1% lower than without citric acid treatment; ③ Periodic dry-wet alternation significantly promoted the transformation of cadmium from a weak acid extractable and reducible state to an oxidizable and residual state in the soil. That is, the availability of cadmium in soil subjected to the combined action of phosphorus and citric acid decreased with an increase in wet and dry alternations.