Decomplexation-Mediated Elimination Of Interference From Typical Organic Acids To Chelation Adsorption Of Heavy Metals

Low molecular weight organic acids(LWOAs)are widely used in non-cyanide electroplating,PCB manufacturing and other correlated industries.Because their structures contain amino,carboxylic acid and phosphonic acid groups,it's easy for LWOAs to form stable and water-soluble complexes with typical heavy metal ions(HMIs),which significantly enhances the migration and transformation of HMIs.Therefore,the LWOAs-complexed HMIs are difficult to be effectively removed by the existing technologies such as flocculation and precipitation,oxidative decomposition,ion exchange,membrane separation and so on.Thus,it has become a hot topic to control the LWOAs-complexed HMIs.The chelating adsorption technology has many advantages,such as high capacity and high selectivity to HMIs.However,in the practical application,the adsorption performance is vulnerable to the interference of LWOAs,which reduces the adsorption capacity and selectivity to HMIs.Therefore,the effect of LWOAs on the selective adsorption of HMIs by chelating adsorbents was studied.Then,a novel targeted strategy is proposed to enhance the selective separation of HMIs.It has important research significance and application prospects to eliminate the interference of LWOAs,improve the separation performance of HMIs in the coexistence of LWOAs,and realize the low-cost recycle of HMIs.In this study,PAMD resin was selected to adsorb typical HMIs(Cu(?)as a model)in the coexistence of LWOAs.The effects of strong ligand(EDTA)and weak ligand(HEDP)on the selective adsorption of Cu(?)were comparatively studied.The main influencing mechanism of typical LWOAs on the selective adsorption of HMIs was illustrated as follows:(1)The deprotonated amino site combined with HMIs by forming a ternary complex;(2)The deprotonated amino site coordinates with HMIs by displacing LWOAs ligands;(3)The protonated amino sites adsorb the LWOAs-complexed Cu(?)through electrostatic interaction.Compared with strong organic acid ligands,the deprotonated amino site is capable of replacing weak organic acid ligands to enhance the adsorption selectivity of Cu(?)in the coexistence of LWOAs.Based on this conclusion,a novel Fe(?)/UV/O2 technology was developed to break the coordination of LWOAs with HMIs and enhance selective separation of HMIs.Under optimized conditions,selective oxidation and decarboxylation of the strong organic acid ligand EDTA was realized,which greatly promoted the ligand replacement to adsorb Cu(?).Hence,the adsorption selectivity of Cu(?)on common chelating resins was improved,and the adsorption performance of Cu(?)achieved in EDTA-coexisting complex system,was close to that of single-component Cu(?)system.The main conclusions were listed as follows:1.The amino sites in four different chemical environments on the PAMD surface were quantified by the proton consumption model(PCM)and XPS analysis after in-situ lyophilization.The content of amino sites was 2.78 mmol/g,4.44 mmol/g,3.28 mmol/g and 7.40mmol/g,respectively.The acidity coefficient pKa was 3,6.46,8.64 and 10.83,respectively.2.Through the theoretical calculations and potentiometric titration analysis of complex configuration,the configurations and stability constants of several main complexes of Cu(?)-HEDP were determined:bidentate complexes[Cu(?)-HEDP]2-,[Cu(?)-HEDP]0 by one phosphonic acid group,bidentate complexes[Cu(?)-HEDP]-by two phosphonic acid groups with the stability constants as 12.64,3.80 and 7.07.3.3.The effect of strong ligand EDTA and weak ligand HEDP on the selective adsorption of Cu(?)onto PAMD at different pHs and salinities was comparatively studied.It has been found that the deprotonated amino sites was the dominant Cu(?)adsorption site.The deprotonated amino groups on the solid phase compete with the organic acids in the liquid phase for binding Cu(?).HEDP was more inclined to be replaced by the protonated amino site while EDTA was more inclined to form a ternary complex with the deprotonated amino site as the configuration of EDTA-Cu(?)-amino group.Both the Electrostatic adsorption of LWOAs-complexed HMIs and the co-adsorption by forming ternary complexes reduced the selective adsorption of Cu(?),while the ligand replacement enhanced the selective adsorption of Cu(?).4.A novel Fe(?)/UV/O2 process was developed for selective oxidative and decarboxylation,which weakened the complexation of Cu(?)with LWOAs and improved the selectivity of common chelating resins for Cu(?).It was found that EDTA decomposition might be a non-radical reaction initiated by light-induced ligand-to-metal electron transfer(LMCT),rather than the interaction with hydroxyl radical(·OH)and peroxy radical(O2·-).Oxygen was mainly used as the electron acceptor of Fe cycle transformation.5.The parameters of Fe(?)/UV/O2 process were optimized:initial pH 3.0,Fe(?)/EDTA molar ratio 0.5,light source 300W mercury lamp,continuous aeration,reaction time 10 min.Under optimized condition,the 100%selective oxidative decarboxylation efficiency of EDTA was achieved.Conventional inorganic salts have no significant effect on the decomposition of EDTA.Treating wastewater containing 2 mM EDTA complexed heavy metals,at flow rate of 6 t/h,56 g of Fe(?)were added into a ton of water,and 0.05 Kw·h power were consumed by a ton of water(not considering energy consumption of aeration).Therefore,it is considered that this technology has considerable technical and economic advantages.