Efficient cadmium removal from aqueous solutions using a sample coal waste activated by rhamnolipid biosurfactant

A sample coal waste activated by rhamnolipid biosurfactants was used as an efficient adsorbent for the adsorption of cadmium from aqueous solution. The effects of three factors, namely, initial solution pH (3-11), absorbent to cadmium ratio (12.5-162.5) and contact time (3-31 h), on cadmium removal were studied and optimized using a central composite type response surface methodology. The two factors that play a key role in the adsorption process are pH and absorbent dosage. Optimum adsorption conditions achieved at pH 9, absorbent to cadmium ratio of 125 and equilibrium contact time of 10 h, resulted in more than 99% cadmium removal. Kinetic studies revealed that a maximum removal can be achieved before 10 min of adsorption process following a pseudo-second order model. The selectivity study in bimetal aqueous systems using copper, lead and zinc metals showed the adsorption order of Cu²⁺ > Cd²⁺ > Zn²⁺ > Pb²⁺. The cadmium adsorption on activated coal waste was also found to follows the Temkin isotherm model with a correlation coefficient of 92.43%.