Sustainable Wastewater Treatment for Reuse and Materials Recovery Through Engineered Chitosan Materials

In this study, chitosan-based adsorbents, namely chitosan powder, chitosan flake, chitosan beads, and chitosan sponges, were synthesized and assessed using a standard adsorbate. A chitosan form was selected and modified to meet specific application requirements based on the results obtained from material characterization and adsorption studies. The modified chitosan beads, copper-loaded chitosan, and nano-chitosan were utilized as bio-sorbents in wastewater treatment. Both metal ions (cadmium and copper) and anions (phosphate) removal were investigated for their uptake capacity, mechanism and kinetics. Also, the effect of other adsorption process parameters such as adsorbent dosage, solution concentration, and temperature have been assessed to achieve the maximum removal/recovery of the most valuable materials. For Cu (II) application, the maximum adsorption capacity was 54.29 mg/g and was obtained at pH 5 for chitosan beads. In the desorption/adsorption process, the maximum adsorption capacity of chitosan beads dropped slightly from 54.29 to 51.39 mg/g. An adsorbent with a structural change, due to a layer of embedded metal ions, was used as a new adsorbent for other pollutants. Therefore, the saturated chitosan with Cu(II) ions was directly applied for phosphate removal from the aqueous solution without being regenerated to test its reusability. The maximum adsorption capacity for phosphate on copper-loaded chitosan was 21.24 mg P/g achieved at a pH of 6-7. The experimental data were best represented using a second pseudo-order kinetic model and the Langmuir isotherm model. Cadmium was another metal ion investigated in this study using chitosan-based adsorbents. The adsorption of cadmium onto chitosan beads and nano-chitosan was assessed through equilibrium and kinetic experiments. At equilibrium, the adsorption capacity was about 1.90 mmol/g for nano-chitosan and 1.65 mmol/g for chitosan. The best-fit isotherm was the Sips, and the best fit kinetic model was the Elovich, followed by the pseudo-second order. These results indicate a multimechanistic adsorption process of lone pair electron chelation and surface complexation. A novel model based on two mechanistic stages was developed and analyzed using the Freundlich isotherm model to provide a more scientific basis for the multimechanistic adsorption process.The adsorption uptake capacity was vital for selecting the suitable adsorbent for multiple water treatment applications. High removal efficiencies even under low influent concentrations are significant to meet environmental regulatory discharge limits. For this purpose, a two-stage batch treatment was proposed and optimized. Further optimization of a two-stage treatment was done to minimize contact time using the Sips and Elovich models for adsorption capacity and kinetics, respectively. The optimized system was able to reduce the adsorbent quantity by 62% compared to the single stage system for the same removal performance (95%). Overall, the work demonstrates that chitosan is a highly effective adsorbent for metals and certain organic contaminants; the structural modification can provide moderate performance improvements, while pre-loading with copper is an excellent way to improve anion removal such as phosphate.