WATER DESALINATION AND HEAVY METALS REMOVAL USING CHITOSAN/BENTONITE/NATROSOL-CARBOXYMETHYL CELLULOSE COMPOSITES MEMBRANE

Abstract

Accumulation of heavy metals in the environment is of great concern because the accumulation has caused severe impacts in the ecosystems. Chromium(VI) and cadmium(II) are among the most hazardous metals. This research examined adsorption of toxic chromium and cadmium ions from aqueous solutions using chitosan, chitosan/bentonite composite, and chitosan/Natrosol-carboxylmethyl cellulose composites and membrane. Chitin was extracted from Blue crab shells, which was cleaned, dried, ground and sieved into the desired particle size of 0.850 mm. The powdered crab shell was converted to chitin by the process of deproteination and demineralisation and chitosan was extracted from chitin by the process of deacetylation, which was later purified by precipitation method. The purified chitosan was finally modified into different adsorbent materials: chitosan/bentonite, and chitosan/bentonite/Natrosol-carboxymethyl cellulose composites. The materials were characterised using Fourier Transform Infrared spectroscopy, Scanning electron microscopy-Energy Dispersive X-ray spectroscopy, Thermogravimetric Analyser and X-ray diffraction spectroscopy. The effects of the initial pH of the solutions, contact time, concentration, biomass doses, temperature, and the ionic interference of sodium, magnesium, and copper ions on the adsorption of these metals were investigated. Experimental results showed that the adsorbent materials were effective for removal of chromium and cadmium ions from aqueous solutions. Adsorption of the metal ions were dependent on equilibrium pH. Maximum removal of chromium and cadmium ions was attained at pH 5 and 6 with 85.32%, and 98.53%, for unmodified chitosan, pH 6 and 7 with 99.5% and 99.1%, for chitosan/bentonite while chitosan/bentonite/natrosol-carboxymethyl cellulose composites is at pH 7 with 99.33% and 93.5%, respectively. The equilibrium adsorption of chromium and cadmium ions on chitosan biomass was attained within 60 min for both modified and unmodified chitosan. The adsorption kinetic data showed that biosorption of chromium and cadmium ions on the materials followed pseudo second order model. Metal ions removal depends, on adsorbent dose; as the dose increased from 0.25 to 2.00 g, the percentage metal ions removed also increased from 96.56% to 99.90% for the adsorbents. The maximum adsorption capacities from Langmuir isotherm for chromium and cadmium are 411.7 and 769.3 for chitosan, 435.165 and 228.0 for chitosan/bentonite as well as 347.71 and 467.980 mg/g for chitosan/bentonite/Natrosol-CMC adsorbents respectively. Effective desorption of metal ions from metal-loaded adsorbents was achieved with 0.3 mol/dm3 HCl. Incorporating both the bentonite and the cellulosic materials for modification improves the blending matrix and increase the number of active sites. Adsorption thermodynamic data indicated physico-chemical adsorption process. Maximum adsorption capacity using chitosan/bentonite/Natrosol-CMC composites and membrane for metals removal increases with increasing height and thickness during the continuous flow system studies. The experimental data from the continuous flow system were subjected to kinetic studies and the result was well fitted to Thomas kinetic. In overall, chitosan and its derivatives are alternative and effective adsorbent materials for removal of cadmium and chromium ions from aqueous solutions. Finally an appreciable desalination was observed with continuous flow system studies using chitosan/bentonite/Natrosol-carboxymethyl cellulose composite membrane for salt and other substances removal.