Computer simulation modelling of the simultaneous adsorption of Cd, Cu and Cr from aqueous solutions by agricultural clay soil: A PHREEQC geochemical modelling code coupled to parameter estimation (PEST) study

Although minerals such as gold and platinum are important for improving life, their processing comes with social and environmental problems. During the processing of these minerals, toxic elements such as cadmium, copper and chromium are released along with wastewater into the environment including soils and surface water leading to plausible migration to groundwater. The removal of these elements from aqueous solutions is of utmost importance however, where no remediation is implemented, the ability of the soil to retain the elements and prevent migration is crucial. Moreover, there is a need for reliable and accurate computational modelling programs that requires minimum set of experiments for calibration in order to determine the success of the sorption of elements by natural and man-made adsorbents since laboratory and field experiments can be expensive, time consuming and often require repetition. In this study, the ability of an agricultural clay soil to adsorb and retain cadmium (Cd), copper (Cu) and chromium (Cr) was investigated by simulating different conditions using PHREEQC (which stands for, pH, redox, equilibrium written in the C language) geochemical modelling code coupled to parameter estimation (PEST). The role of pH (2-9) on the adsorption of Cd, Cu and Cr onto generalised weak, strong and super strong sites of the agricultural soil was determined and used to calibrate the model. The soil was found to be effective in preventing the mobility of Cd, Cu and Cr to groundwater however, the presence of competing ions and plant exudates increased their mobility. PHREEQC geochemical modelling code coupled with PEST was found to be a quick and a reliable tool to determine the performance of natural and man-made adsorbents in different conditions. Thus, PHREEQC coupled to PEST is a suitable decision making tool for environmental sustainability.