Comparison of the efficiency of pure and chitosan coated adsorbents in immobilization of soil cadmium

Contamination of soil and water by heavy metals have accelerated since the beginning of the industrial revolution. So cleaning up and removing of the contaminants is one of the major challenges of human societies. There are different methods for remediation of contaminated environments and adsorption and immobilization of heavy metals by the adsorbents is one of them. In addition to being effective and fast, the immobilization technique is simple, inexpensive, and environmentally safe. In recent years, the use of polymeric composites, among the various adsorbents of heavy metals, has attracted the attention of many researchers due to their higher efficiency in comparison of pure adsorbents. Chitosan composites are among those polymeric composites that, due to their properties, can have a high ability to absorb heavy metals in contaminated environments. Chitosan composites have often been used to remove heavy metals from industrial wastewater, however, the efficiency of these composites in immobilization of heavy metals in soils has not been studied. Due to the great variety and environmental safety of these composites, this study aimed to investigate the efficiency of pure and chitosan coated adsorbents in immobilization of soil cadmium.

For this purpose, a pot factorial experiment was conducted in greenhouse conditions using a completely randomized design and three replications. Factors studied were soil cadmium levels (0, 8, 25 and 75 mg/kg soil) and types of adsorbent including pure chitosan, biochar, zeolite and nanomagnetite and composites of chitosan- biochar, chitosan- zeolite and chitosan- magnetite and control (without adsorbents). Each adsorbent was applied to soil at the rate of 0.5% W/W. Uncontaminated soil samples were spiked with different amounts of cadmium sulfate and incubated for two months to achieve relative equilibrium. After two months the samples were treated with different adsorbents and incubated for another two months. Then the amounts of DTPA extractable cadmium and its different chemical forms were determined.

The results showed that the application of adsorbents to soil decreased the concentrations of DTPA extractable cadmium. The results also showed that chitosan composites had higher ability for immobilization of cadmium in the soil than the pure chitosan, biochar, zeolite and nanomagnetite and the highest cadmium immobilization ability was observed for the composite of chitosan- magnetite. Reductions in DTPA- extractable cadmium for pure chitosan, biochar, nanomagnetite, and zeolite were 26.11, 19.38, 18.00 and 7.71% and for composites of chitosan- magnetite, chitosan- biochar and chitosan- zeolite were 34.02, 32.04 and 30.56% respectively when compared to the control treatment. Sequential extraction at the contamination level of 75 mg Cd/kg soil also showed that the use of adsorbents significantly reduced the soluble + exchangeable and carbonate forms of cadmium compared to the control treatment and increased its more stable forms including iron and manganese oxide, organic matter and residue fractions.

According to the results, it can be concluded that coating the pure adsorbents with chitosan by creating more adsorption sites reduces the cadmium mobility in the soils and increased the efficiency of pure adsorbents in the immobilization of cadmium. It was also observed that among the composites that used in this experiment, the highest ability to reduce the cadmium concentration was related to the chitosan-magnetite composite.