Prediction of Cu(II) retention, sorption mechanism and biochar effect in tropical soil

Abstract

The sorption behavior of potentially toxic metals in soils influences their spread to other environmental compartments and their bioavailability to life forms. The present study evaluated the sorption of Cu(II) ions in two representative soils from Yala and Tindinyo regions along River Yala in Lake Victoria basin, Kenya. The sorption experiments were carried out in single component solution systems using the batch technique. The equilibrium data was modeled using Henry’s isotherm model and four two-parameter isotherm models. Freundlich and bi-phasic Henry’s isotherm models best described the data, while both negative constants for Langmuir and Elovich necessitated rejection of the models. The y-intercept values for Henry’s monophasic fitting were negative, but the corresponding soil solution values of 0.43 μg/mL and 0.45 μg/mL for the Yala and Tindinyo soils, respectively, were taken as the background Cu(II) solution levels. We propose that in cases where the y-intercepts are negative, the bi-phasic Henry’s model can be used to predict a soil’s critical retention limit for a given pollutant, while the corresponding soil solution amount gives the permissible level in the soil solution. Using bi-phasic Henry’s model, the critical Cu(II) retention limits were determined as 300.11 μg/g and 301.48 μg/g for Yala and Tindinyo soils, respectively. The corresponding soil solution amounts were 3.47 μg/mL and 3.36 μg/mL for Yala and Tindinyo soils, respectively. The background and equilibrium soil solutions were below and above the WHO limit of 2 μg/mL for drinking water, respectively, while the sorbed amount was above the WHO limit of 36 μg/g soil. These findings indicate that a contaminant in soil may not pose any toxicity risk unless its concentration exceeds the critical retention limit, and that a pollutant’s toxicity is dependent on the soil solution amount rather than the total amount in soil. Dissolved organic carbon was more dominant than clay fractions in influencing the degree of sorption in both soils. Surface complexation between Cu(II) ions and carboxylic functional groups is hypothesized as a plausible sorption mechanism. Immobilization of Cu(II) in soil was significantly increased by amendment with fish-scales-derived biochar which, therefore, has promising potential application in mitigating environmental contamination by metals.