Application of Chitosan Cross-linked with κ-Carrageenan for Removal of Cadmium Ions from Water and Soil Systems

Due to the negative effects on human being health, the decrease of cadmium bioavailability in waters and soils is necessary. The main origins of cadmium ions in environment consist of batteries, phosphate fertilizers, mining, pigments, stabilizers, and alloys. Many methods such as ion exchange, chemical precipitation, flotation, ultrafiltration, nanofiltration membranes, reverse osmosis, and electrocoagulation have been used for the removal of cadmium. Notably, adsorption is proven the most practical technique for heavy metal ions removal of pollutants from wastewater and contaminated soils. Among the various adsorbents, chitosan has introduced to be an efficient one, due to its unique characteristics such as antimicrobial activity, biocompatibility, non-toxicity, and being low-cost bio-adsorbent. Chitosan is a derivative of N-deacetylated of chitin, a naturally occurring polysaccharide taken from crustaceans i.e. shrimps and crabs, and fungal biomass. The presence of amine and hydroxyl groups in the backbone of chitosan gives the polymer its high binding capacity in adsorption processes. Chitosan can decrease the metal ion concentration to near zero. This work evaluates the modified chitosan’s potential as a bio-adsorbent in the water system and also its potential as a soil amendment in the soil system in terms of the adsorption and desorption of Cd2+. It is also worth noting that there is no report on the removal of cadmium ions by ionically crosslinked chitosan/κ-carrageenan materials, especially in soil systems. The chitosan-based magnetic bio-adsorbent was prepared through in situ co-precipitation of iron ions in the presence of chitosan with high molecular weight. The surface (0-30cm) soil samples were collected from a field in University of Maragheh in the North East of Iran. Some physio- chemical properties of the soil used in this study were determined. Adsorption of cadmium on the bio-adsorbent was investigated using batch experiments. After adsorption, the adsorbent loaded with cadmium ions was washed with distilled water before treating it with 90 ml of 0.1M ethylenediaminetetraacetic acid (EDTA) for the determination of the metal desorption. The experimental data of Cd2+ adsorption and desorption isotherm were fitted by Freundlich and Longmuir models. The crystalline nature and phase analysis for pure chitosan and magnetic chitosan bio-adsorbent was confirmed by XRD analysis. The diffractogram of chitosan consisted of two typical crystalline peaks at 2θ= 10.8A° and 20.42A°, corresponding to the partial crystalline structure of chitosan and the hydrated crystals of the remained α-chitin chains in pure chitosan, respectively. The characteristic peaks of chitosan in the XRD pattern of the magnetic bio-adsorbent disappeared, indicating of the amorphous structure of chitosan. It suggests that the addition of magnetite nanoparticles obviously affects the crystallinity of chitosan. On analyzing the values of r2 and RMSE obtained using Freundlich and Langmuir models, it was observed that Freundlich model provided the best fit for the experimental adsorption and desorption data at the ranged of the Cd2+ concentration studied in the soil and water systems. To evaluate the efficiency of the modified chitosan as an efficient bio-adsorbent in water and soil system, the difference between adsorption and desorption amounts, Δq, was calculated. The less amounts of Δq, the more efficient adsorbent in a water system. This means that the adsorbent can be reused several times. In contrast, in a soil system, a positive relationship was found between the amounts of Δq and the efficiency of the adsorbent. This means that the adsorbent can immobilize the adsorbatesand therefore, may be used as a metal immobilizing amendment in soil. As the initial concentrations raised, the amounts of Δq increased in the water system; therefore, it seems that the bio-adsorbent may not efficient at high initial concentrations. In the soil system, the more amounts of Δq decreases, the more efficiency of the adsorbent as a cadmium immobilization increases. Therefore, the bio-adsorbent used can be relatively efficient as a soil modifier. The results revealed the magnetic bio-adsorbent based on chitosan can be sorb Cd2+ from water and soil systems. The maximum adsorption capacity (b) of cadmium onto the adsorbent appeared to increase from the water system to the soil system, from 750.2 to 992.7 µmol/g, respectively. On analyzing the values of r2 and RMSE obtained using Freundlich and Langmuir models, it found that Freundlich model provided the best fit for the experimental adsorption and desorption data at the ranged of the Cd2+ concentration studied in both water and soil systems. By comparing the amounts of Δq, the difference between adsorption and desorption amounts, the bio-adsorbent is not economically feasible at high initial concentrations in the water system. But, the more decrease amounts of Δq in the soil system, the more increase efficiency of the adsorbent as a cadmium immobilization. So that, the bio-adsorbent used can be relatively economic as a soil modifier.