Removal of cadmium from a citrate-bearing solution by floatable microsized garlic peel

Paddy field soil contaminated by cadmium may produce cadmium-containing corn due to the paddy's strong ability to accumulate cadmium. The in situ washing of soil with an organic acid is one of the suitable technical choices for the remediation of cadmium-contaminated soils. The limiting factor of this method lies in the recycling and reuse of the huge amount of washing effluent in an efficient and economical way. In present study, the simulated solutions were used to examine the adsorption efficiency of cadmium on a biosorbent which was synthesized by using garlic peel as the raw material. The biosorption behavior of cadmium on garlic peel was systematically studied in the presence of a citrate ligand. Presented here for the first time, garlic peel with buoyant properties was carefully collected and used for the preparation of the adsorbent, and verified to have a prominent advantage in efficiently separating from the solution after adsorption because of its floatability. Results show that the presence of citrate has a significant inhibition effect on the adsorption behavior of cadmium on the floating garlic peel, at the optimal pH of 4.0, which can be ascribed to the competitive affinity to the cadmium from the citrate ligand. SEM shows that floating garlic peel has a ruffled epidermis in the flat surface and porous microstructure in the transversal surface, making it durable enough and favorable for adsorption; and -COOH was determined by FTIR to be the main functional group contributing to the adsorption capability of garlic peel. Cadmium can be eluted off the garlic peel after adsorption, and the garlic peel can be then reused for the next cycle of adsorption with little decrease in adsorption capacity, even after ten adsorption/desorption cycles. The real leach liquor of cadmium-contaminated soil sample by 0.01 mol L^-1 citric acid solution was used for testing, and it was found that after three adsorption uses, almost all the cadmium in the leach liquor had been recovered by the floating garlic peel. The above research results provided a possible route to recycle the soil washing solution by biosorption, giving a great perspective in the remediation of paddy field soil contaminated by cadmium.