Resource Recovery Of Cr(VI) From Electroplating Wastewaterand Preparation Of Novel Chelating Fibers

With the rapid development ofindustrialactivities, heavy metal wastewater is a serious threat to human health and environment. Amongst them, Cr(VI) is a highly toxic metal that has been linked to cancer for its carcinogenicity and oxidability. Chromium pollution has become the environmental issues of common concern to all countries in the world.At present, the main treatments of Cr(VI)-containing wastewater areby chemical reduction to Cr(III) followed by precipitation with alkali.However, these methods produce a large amount of chromium-rich sludge that must be disposed of, which can cause serious pollution. Therefore, development of a simple, lowcost andresource recovery treatment for chromium-containing wastewater has good application potential.Ion exchange fiber is a kind of novel adsorption and separation materials similar with the ion exchange resin. The chemical functional groups of IEF include acid, alkali and chelating structure. Wherein, chelating fiber has better selectivity for metal ionsfor its stable coordinate bonds. With its obvious dynamical advantages of short mass-transfer distance, fast adsorption rate and high purification efficiency, chelating fiber has been widely used in the highly purification of harmful substances, separation of trace elements and protection from harmful gases in recent years.This paper was focused on the resource recovery of Cr(VI) from electroplating wastewater and preparation of novel chelating fibers. The work can be summarized as follows:The static adsorption performance of PAN-TETA fiber for Cr(VI) was researched in various conditions, such asthe counter ion form of PAN-TETA fiber, p H, initial Cr(VI) concentration, temperature, etc. Thecontent and distribution of characteristic elements(C, N, O, Cr) along the fiber cross-section were determined by SEM-EDSduring the adsorption process. The results shown that the static adsorption capacity and regeneration performance of Cl-form fiber are superior to that of the OHform fiber. The maximum adsorption capacity is obtained under p H 2~3. It was found that the adsorption kinetic model was consistent with the pseudo-second order kinetic model(R2> 0.998) and the adsorptive equilibrium can be achieved within 30 min. The maximum Cr(VI) adsorption capacity of the fresh ion exchange fiber was 425.1mg/g. The SEM-EDS results shown that the amine group existed in the surface and cross-sectionof PAN-TETA fiber were participated in the adsorption of Cr(VI), and thechromium contentdistribution in the cross-section was relatively homogenous. When it was used as adsorbent to treat the electroplating wastewater containing chromium, the refreshed functional fiber still keeps excellent adsorption capacity and removal ratio for Cr(VI)after 100 cycles of adsorption and desorption, with the removal rate was higher than 91%.Based on the results of static adsorption experiment, the column adsorption of Cr(VI) by PAN-TETA fiber was tested. Themaximum qb andqe was up to 291.2mg/g and 426.3 mg/g, respectively.Thomas model can be satisfactory for the design of PAN-TETA fiber columns in a range of feasible concentrations(R2>0.9). The coexisting SO42-have some effect on the adsorption of Cr(VI),and the SO42- ions adsorbed on the active sites of PAN-TETA matrix could be gradually replaced by chromate species during the column adsorption process. The results of the elution experiments shown that the Cr(VI) loaded on the fiber could be desorbed rapidly and efficiently by Na OH solution. The neutral portion contain a lot of Cr(VI) and little OH-,which can be reused in the plating industry. the alkaline portion contain a lot of OH-, which was reused in the next regeneration cycle.The laboratory and pilot-scale adsorption-desorptionof Cr(VI) from mixed electroplating wastewater and chromium-plating rinse water were comprehensively investigated. The PAN-TETA material possesses fineregenerationand physicochemical stability after 80 adsorption-elution cycles for the removal of Cr(VI) from mixed electroplating wastewater. However, the operating exchange capacity werereduced to 230.1 mg/g ~180.9 mg/g.After 5 adsorption-regeneration cycles for the removal of Cr(VI) fromchromium-plating rinsewater, the PAN-TETA fiber exhibited excellent adsorption characteristics for chromate species, and the operation capacity was higher than 320mg/g. The results shown that the fiber could be well used in the treatment of chromium-containing electroplating wastewater, especially in the treatment of chromium-plating rinse water.The resource recovery process of Cr(VI) was developed through the pilot-scale multi-column facility from the chromium-plating rinse wastewater.The operation capacities had fluctuated from 228.4 mg/g to 324.6 mg/g during 5 adsorption-elution cycles although the rate of capacity reduction slightly faster compared with the laboratory results.The SO42- and Cl- ions adsorbed on the active sites of FFA-1 fiber matrix would be gradually replaced by chromate species through the extension of adsorption time, which would be conducive to improving the operation capacity and purity of eluent, simultaneously.During 5 times elution operation, the Cr(VI) concentration of formed neutral elution were up to 30 g/L and could be recycled in the electroplating industry.A new kind of functional fiber(PP-St-NH2) was prepared through nitration and reduction on polypropylene grafted styrene(PP-St) fiber.VH2SO4/VHNO3 exerts the most impact on the nitration of PP-St fiber and the conversions of mono-nitro substituentwas nearly 100%. The exchange capacity of PP-St-NH2 fiber could be up to 5.78mmol/g under the optimized reduce conditions.The functional fiber containing amine group maintains good micro-morphology and has no obvious thermal decomposition below 300℃.Three kinds of chelating fibers PP-St-IDA, PP-St-PAA and PP-St-SCB were synthesized by the reactions of PP-St-NH2 with sodium chloroacetate, phosphorous acid, salicylaldehyde, respectively.Under the corresponding optimum reaction conditions, the conversions of chelating structures iminodiaceticacid, amino phosphonate and schiff base are higher than 90%, respectively.the maximumadsorption capacity of the synthetic PP-St-IDA, PP-St-PAA and PP-St-SCB fibers for nickel ion was 102.6mg/g, 51.6mg/gand 44.1mg/g, respectively. The order of the selectiveadsorptionproperty for nickel ionwas PP-St-IDA >PP-St-PAA≈PP-St-SCB. By the compared experiments, the selective adsorptionproperty of PP-St-IDA fiber for nickel ion was better than the PAN-base weak acid fiber. Therefore, the PP-St-IDA chelating fiber has a good prospect of application in the nickel-containing wastewater treatment.