Applications of Polyvinylamine in Removal of Heavy Metals from Wastewater by Polymer-Enhanced Ultrafiltration and Adsorption
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In this study, water-soluble polyvinylamine (PVAm) was used as chelating agent for heavy metal removal from wastewater by polymer-enhanced ultrafiltration (PEUF). The effects of parameters involved in the ultrafiltration (UF) process, the interaction properties of PVAm and heavy metals, as well as the batch operation of PEUF process were investigated. In addition, the synthesis of thiol functionalized PVAm and its applicability for Hg(II) adsorptive removal were studied. The removal of eight toxic heavy metals (e.g., Co(II), Cu(II), Ni(II), Pb(II), Fe(III), Cd(II), Zn(II), and Mn(II)) from water by a PVAm-enhanced ultrafiltration was investigated. By forming stable PVAm-metal complexes in the aqueous solution, the heavy metals can be separated from water using UF membrane. The removal rate for Pb(II), Cu(II), and Fe(III) can achieve as high as 99%, 97%, and 98% by PVAm-enhanced ultrafiltration, respectively. The sulfate divalent anion was found to be able to cause the precipitation of the soluble PVAm-metal complexes. The mechanism of the precipitation formation and its effect on the performance of PEUF were investigated. Further, this technique was used for Hg(II) removal from wastewater. A mercury removal as high as 99% was obtained. Over the feed mercury concentration range tested (0 - 50 ppm), the PVAm dosage used did not affect the mercury rejection considerably, while water flux was reduced significantly at a higher dosage of PVAm. A flux vs pressure relationship typical of UF of macromolecular solutes was observed, and the limiting flux appeared to follow the gel layer formation model. The fouled membrane surface was cleaned periodically with dilute hydrochloric acid to recover the membrane permeability. Mercury removal with the PEUF was also tested with a simulated chlor-alkali wastewater that contained mercury and other chemicals (i.e., sodium chloride and sulphate), and the accompanying compounds in the feed solution was shown to influence the performance of PEUF for mercury removal. A mathematical model for batch operation of PEUF process for mercury removal was developed. Its applicability was testified using three different water-soluble polymers (i.e., PVAm, polyethyleneimine (PEI), and poly(acrylic acid) (PAA)) by comparing with the experimental data. The performance of the three polymers for mercury removal by PEUF process decreased as the order PVAm > PEI > PAA at the same polymer concentration and operating conditions. The membrane fouling was found to have profound influences on the modelling of batch operation of PEUF process. For a given recovery task, the mercury concentration in the feed, the mercury recovery rate, the batch operating time and the membrane area needed to achieve the desired recovery can be predicted if the concentration dependence of the perm-selectivity of the process (i.e., when the UF membrane and the water-soluble polymer are selected) is known. To enhance the removal efficiency and the selectivity towards Hg(II), the PVAm was chemically functionalized by thiol groups. The synthesized PVAm derivative (denoted as PVAm-SH) was found to be insoluble in water and showed good adsorption capability for Hg(II) in aqueous solution. The adsorption isotherms and kinetics were investigated. Thermodynamic estimation showed that the Hg(II) sorption onto PVAm-SH is endothermic.