| Heavy metals are a class of environmental pollutants with biotoxicity,high enrichment and non-degradability.Leather and electroplating are two major industrial sectors suffered from heavy metal pollution,which is explicitly listed in Action Plan for Prevention and Control of Water Pollution.Seventy million tons of tannery wastewater is discharged in China every year,of which the chrome tanning process wastewater accounts for about 8%.The retanning effluent should be treated separately due to high concentration of trivalent chromium,but the concentration of the residual chromium in effluent is still high above 10 mg/L after the widely used chemical precipitation technology.The annual discharge of electroplating wastewater in China is as high as 40 billion tons,in which the most difficult disposal fraction is the electroless nickel plating wastewater and the concentration of nickel is prone to exceed the standard.However,the cost-effective treatment technology with sort of empirical was pursued only based on the removal efficiency,ignoring the speciation study on heavy metals and their incident variations during primary treatment processes.The speciation of heavy metals in wastewater has a great impact on the selection of reliable treatment scheme,especially on the effectiveness of advanced treatment technology,thus the relevant research is urgently desired.To solve the above issues concerning the exceeding concentration of heavy metals in the treating process,the paper conducted researches in terms of Cr speciation after and during alkaline precipitation in tannery effluent,and Ni speciation variation during a hybrid sequential treatment scheme in electroless plating wastewater at the molecular level,employing comprehensive analytical methods involving optical spectrometry,chromatography and mass spectrometry.The obtained results are as follows:To investigate the speciation of chromium in retanning effluent after precipitation treatment,the practical wastewater from a large leather manufacturer was collected for alkaline precipitation tests.Employing various analytical methods such as sequential microfiltration-ultrafiltration,size exclusion chromatography(SEC),Fourier transform infrared spectroscopy(FTIR),X-ray photoelectron spectroscopy(XPS)and liquid chromatography-mass spectrometry(LC-MS),the molecular speciation of residual Cr in effluent was uncovered,which was mostly located in two size ranges,i.e.the 13-100 nm colloidal fraction and the <4 nm dissolved fraction.Cr(Ⅲ)was coordinated by carboxyl groups or hydroxyl groups in both fractions,while the complexation by nitrogen-containing groups was excluded.Based on the comprehensive analyses as well as the feed chemicals in leather retanning process,the structures of Cr(Ⅲ)complexes in both fractions were proposed.Cr(Ⅲ)cross-linked the carboxyl groups from polyacrylic acid chains to form the network gel structure in the 13-100 nm fraction,while the complex structure of Cr(Ⅲ)in the <4 nm fraction was formed through hydroxyl-carboxyl chelation by masking agents such as tartrate and citrate.Although polyoxyethylene ether was abundantly present,it was responsible for the complexation of Cr(Ⅲ)in neither fraction.During the alkaline precipitation process,the study focused on two typical Cr(Ⅲ)-bearing complexes(Cr(Ⅲ)-collagen and Cr(Ⅲ)-citrate)ubiquitous in tannery effluent yet with distinct treatment efficiencies,as Cr(Ⅲ)was much more difficult to remove in the Cr(Ⅲ)-citrate solution.With the aid of multiple analytical techniques including potentiometric titration,absorption spectrometry,SEC,XPS and zeta potential,etc.the speciation of both the Cr-bearing complexes along with solution p H increase was revealed.It was found that a lower removal efficiency towards Cr(Ⅲ)of Cr(Ⅲ)-citrate was significantly associated with higher oligomers.The coordination effect between Cr(Ⅲ)and citrate ligand enhanced as solution p H increased,with certain amount of hydroxide entered into the complex interior.The constantly deprotonated carboxyl groups as well as integrated hydroxyl groups are excellent bridging groups,promoting the polymerization of monomers to form oligomers,and the molecular size of the Cr(Ⅲ)-citrate complex continued to increase simultaneously.The postulated Cr(Ⅲ)-citrate species towards higher oligomers were discovered.Beyond that,along with the increased solution p H,both charge neutralization and sweeping effects were presented among the gradually emerging flocs in the Cr(Ⅲ)-collagen system together with the electric double layer compression effect derived from salinity,thus resulting in a larger floc size and higher Cr(Ⅲ)removal efficiency in saline solutions.The residual Cr in tannery wastewater was inner orbital coordination compound,whereas the residual Ni in the electroless plating effluent was external orbital complex,which was totally different in speciation.Typical treatment scheme for electroless nickel(EN)plating wastewater consist of physicochemical,biological,Fenton and biological aerated filter(BAF)processes.To study the speciation variation of nickel along the sequential treatment process,the outfall of the four segmented units was sampled for analysis.The ion chromatography(IC)and SEC was tandem with inductively coupled plasma mass spectrometry(ICP-MS)separately,combined with LC-MS method,the molecular speciation variation of the residual Ni-bearing complexes was resolved.The chromatography hyphenated IC-ICP-MS and SEC-ICPMS results demonstrated that the dominant Ni species in the EN effluents was similar to EDTA-Ni but with a smaller size,totally different from free Ni(II).The LC-MS experiment further distinguished a series of reasonable molecular formulas of Nibearing complexes.Although Ni concentration declined continuously along the process,the number of detected Ni-bearing complexes gradually increased but with lower molecular weight.Most of the detected mononuclear complexes had higher double bond equivalence(BDE)than EDTA-Ni,whereas it was believed that the similar stereo ring shape was widespread in the EN effluent.In summary,the speciation of chromium and nickel in the wastewater of domestic key industry involving heavy metals was associated with complicated organometallic complexes.Therefore,this study provides a theoretical basis for further treatment of low concentration metal-organic complexes. |
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