Removal Of Heavy Metals From Aqueous Solutions By Complexation-ultrafiltration With Chitosan

Heavy metal pollutants exhibit the following characteristics: wide source, long time with toxicity, easily cumulating and hardly resuming, and so have negative effects on the health of human being and aquatic organisms. The main traditional technologies for the treatment of heavy metal containing wastewater are chemical treatment method, ion exchange process, adsorption and so on. These methods just simply transfer heavy metals from one place to another, and thus cannot thoroughly solve the pollution problems which are caused by heavy metals, much less recycle the heavy metals. Wastewater, produced by many industries, contains heavy metals of low concentration. The wastewater amount is so large that traditional technologies cannot recycle heavy metals economically. In such present situation that water resources become increasingly tense and the discharge standards become increasingly stringent, the process of complexation-ultrafiltration with polymers, aiming to reuse water and recycle heavy metals, can treat heavy metal containing wastewaters effectively and economically, and hence has a positive significance to slow the lack of resource and reduce environmental pollution.Removal of heavy metal ions from aqueous solutions by the process of complexation-ultrafiltration with polymer was studied. Polyethersulphone (PES) membrane was chosen and chitosan was used as complexing agent to remove heavy metal ions by complexation-ultrafiltration. Based on the principle of removal of heavy hetals from aqueous solutions by complexation-ultrafiltration with chitosan, the coupling process was investigated systematically, including rejection coefficient of heavy metals by complexation-ultrafiltration with chitosan, decomplexation of chitosan-metal complex, membrane fouling and membrane cleaning. The process operational parameters and membrane resistance were studied. Effects of miscellaneous factors on the rejection coefficient of heavy metal ions were investigated. Various cleaning methods were applied to clean the contaminated membranes and their effects were also studied. The regeneration of the chitosan by acidification using diafiltration technique was further performed. The effect of regenerated chitosan on the heavy metal removal was evaluated.Firstly, PES membrane was chosen and chitosan was used as complexing agent to remove lead and nickel ions by complexation-ultrafiltration. Effects of solution pH, heavy metal ion/chitosan ratio, ionic strength and Ca²⁺ on the rejection coefficient of lead and nickel were investigated. Rejection coefficient of the heavy metals and membrane flux with concentration time were also studied. The value of pH was found to be the key parameter in the process of complexation-ultrafiltration. The rejection coefficient of heavy metal ions can reach very hight level at appropriate pH with suitable metal ion/chitosan ratio. The rejection coefficient of lead goes high to over 99% at pH 6.0 with the Pb²⁺/chitosan ratio 0.25; the rejection coefficient of nickel goes close to 60% at pH 8.0 with the Ni²⁺/chitosan ratio 0.25. The increase of ionic strength and Ca²⁺ is not beneficial to the heavy metal removal by the process of complexation-ultrafiltration.Secondly, the images of membrane surface were observed, the membrane resistance was analyzed, and effects of several cleaning methods on membrane cleaning were also investigated. It was found that membrane fouling was mainly caused by the deposition of the chitosan-metal complex in the ultrafiltration process. Several kinds of cleaning agents were used to remove the contaminants from the membrane. The HNO3 solution with pH 2.0 showed the best cleaning effect. The flux of the cleaned membranes shows no significant difference with that of fresh membranes, and meets the requirement to be reused. Thus the technology is expected to save the cost of consumptive material.Finally, recycling of chitosan in the concentrate was carried out. The chitosan-metal complex was acidified and then the chitosan was regenerated by diafiltration. The regenerated chitosan was used to remove Pb²⁺ and Ni²⁺ from respective aqueous solutions by complexation-ultrafiltration, and the rejection coefficients of Pb²⁺ and Ni²⁺ reached 96.2% and 51.07% accordingly, showing no significant difference with that obtained by the fresh chitosan.Results showed that the process of complexation-ultrafiltration with chitosan can effectively remove heavy metal ions from aqueous solutions, reuse water and chitosan can be effectively regenerated.