Efficiency of Natural and Modified Bentonite and Rice Husk on Immobilization of Cadmium and Its Effect on Some Biological Properties of Soil

Rapid development of industrialization, heavy metal and radionuclide contaminants from industrial activities have posed a major threat to the environment owing to their toxicity, non-biodegradability and persistent accumulation. So various ecosystems are continuously contaminated with high levels of high-risk chemicals with different structures and levels of toxicity. Cadmium is one of the high-risk elements that enters the environment and can accumulate in the body of fish and other aquatic organisms, plants and livestock and be transferred to the human body. Therefore, the remediation of contaminated soils with cadmium in order to protect human health is very important. One method for remediation of pollutants is immobilization of them in the soil by adsorbents. Among the absorbents, bentonite has been identified for its unique properties, including high surface area and cation exchange capacity and adsorptive affinity for organic and inorganic ions, low cost and ease of access. If the physical and chemical properties of natural bentonites are improved by a special modification process, the adequate supplies for environmental purposes can be obtained. Among the biosorbents, rice husk has also been reported to be suitable for adsorption of cadmium and other heavy metals. This research was designed with the aim of decreasing the amount of cadmium in the soluble and exchangeable phase of a polluted soil under laboratory conditions in the presence of bentonite and rice husk. Considering that biological properties of the soil are an indicator of soil health and quality, so, after application of adsorbents, biological properties and some soil ecophysiological indices were also investigated. The experiment was done with 13 treatments and 3 replications in a completely randomized design. Treatments were bentonite (B) and modified bentonite with iron (B-Fe), manganese (B-Mn), iron and manganese together (B-Fe-Mn), rice husk (RH), modified rice husk with phosphoric acid (RH-P) in two levels (2 and 5%) and control treatment (without adding adsorbent). Modification of bentonite was done with iron chloride (FeCl3.6H2O), manganese chloride (MnCl2.6H2O) and a mixture of FeCl3.6H2O and MnCl2.6H2O. Some of the characteristics of bentonite and rice husk adsorbents including pH, electrical conductivity, cation exchange capacity and organic carbon were measured. The contaminated soil with CdCl2 was treated with adsorbents and incubated for 2 months under constant lab conditions. After the incubation time, soil biological properties such as basal respiration, substrate-induced respiration (SIR), microbial biomass carbon (MBC), activity of some enzymes and also some ecophysiological indexes were measured. The results showed that the basal respiration, SIR, MCB, activity of phosphatase, dehydrogenase and urease were less in the control treatment. The basal respiration and phosphatase activity in RH-P 5% treatment were 2.6 and 2.25 times more than those in the control, respectively. SIR and urease activity were highest in RH-P 5% treatment. The application of adsorbents to contaminated soil reduced soluble and exchangeable cadmium fraction. The lowest amount of soluble and exchangeable fraction of cadmium was in RH-P 5% treatment that showed 2.5 times reduction in comparison to control. In other words, immobilization of cadmium from these fractions improved soil conditions and caused increasing of biological soil properties and activity of microorganisms. The metabolic quotient was higher in the control treatment, probably due to lower microbial content, and decreased by adding adsorbents. Microbial quotient in control treatment was lower than other treatments which prove again the lower biomass carbon of control treatment. Carbon availability that is the ratio of basal respiration to SID, also was more in control in comparison to other treatments, perhaps due to the suppress or inhibition of dormant or zymogenous microbes by cadmium in the control treatment which can be stimulated to growth in the SIR experiment. The results of this study revealed that cadmium with concentration of 30 mg kg-1has a toxic and inhibitory effect on the microbial activity of the soil. The addition of bentonite and rice husk adsorbents in particular modified form reduced mobility of cadmium and thus improved the biological properties of the soil and also had a positive effect on ecophysiological indexes.
The use of these adsorbents can be a cost effective, succeeded, and operative management strategy for immobilization of cadmium in contaminated soils that reducing the risk of plant reclamation, washing and entry into groundwater and food cycle.