Study On The Performance And Mechanism Of Heavy Metal Complexes Removal Based On Iron Ions Replacement Strategy

The problem of heavy metal pollution has already aroused widespread concern.The removal difficulty of heavy metal ions largely depends on their existence state in environmental media,and the removal of heavy metal complexes is more difficult,so the treatment of heavy metal complexes has become one of the urgent problems to be solved in the field of environmental protection.At present,the treatment process for heavy metal complexes is mainly based on the combination of oxidation decomplexation and traditional alkaline precipitation,which process is relatively cumbersome and subject to many restrictive factors.Aiming at the cumbersome process,low removal efficiency,and poor adaptability of complex water quality existing in the existing removal methods,this study selected the emerging iron ion replacement technology with excellent performance,simple process,and low cost.With the goal of achieving high-efficiency removal of heavy metal complexes,several new heavy metal complexes removal technologies based on iron ion replacement strategies have been proposed.The main factors affecting the removal process were investigated,and the possible mechanism of the removal was explored.The main research contents are as follows:（1）Aiming at the high concentration of lead complexes,a Fe（Ⅲ）/flue gas desulfurization gypsum（FGDG）system was constructed,and the feasibility of removing Pb-EDTA and the key factors affecting the removal process were analyzed.The results show that the Fe（Ⅲ）/FGDG system removes Pb-EDTA quickly,and the removal rate exceeds 90%after 1 min.When the initial p H value is 2,the molar ratio of Fe（Ⅲ）/Pb（Ⅱ）is 1:1,and the initial concentration of Pb-EDTA is 700 mg Pb/L,the best removal rate of 97.35%can be achieved.It shows that the system is suitable for high concentration of Pb-EDTA,and the consumption of iron salt is not high.The investigation of the removal mechanism of Pb-EDTA showed that Fe³⁺replaced Pb²⁺,Pb-EDTA was converted to Fe-EDTA,and lead was released into free state,and then FGDG co-precipitated with free Pb²⁺to form lead sulfate.In general,the Fe（Ⅲ）/FGDG system has the ability to remove lead complexes quickly and efficiently,and has great application potential.（2）Aiming at the low concentration of Pb-EDTA,a heterogeneous replacement of ferrous phosphate and ferric phosphate was proposed,and the difference in the removal process of the two was compared.Both ferrous phosphate and ferric phosphate can reduce Pb-EDTA from 50mg Pb/L to less than 1 mg Pb/L,and the leaching concentration of total iron is less than 50mg/L.In addition,the maximum adsorption capacity of ferrous phosphate under hypoxia conditions is 436.68 mg Pb/g,which is significantly higher than 80.44 mg Pb/g under aerobic conditions.Even if the initial concentration of Pb-EDTA is as high as 300 mg Pb/L,ferrous phosphate can reduce the Pb concentration to less than 1 mg Pb/L.The study on the removal mechanism of Pb-EDTA showed that the synergistic displacement of Fe²⁺and Fe³⁺contributed to the efficient removal of Pb-EDTA by ferrous phosphate.Furthermore,ferrous phosphate is less affected by coexisting salt ions and organic ligands than ferric phosphate.In the treatment of the detonator wastewater containing lead complexes,it was found that the residual Pb concentration after ferrous phosphate treatment was 0.94 mg Pb/L（lower than Chinese emission standards）,and the residual Pb concentration after ferric phosphate treatment was 16.35 mg Pb/L.It can be considered that ferrous phosphate is more suitable for the treatment of lead complexes in actual wastewater.（3）Aiming at the high salinity in actual wastewater,a new strategy for removing Cu-EDTA from high-salt wastewater by using sulfide-modified nano-zero-valent iron（S-NZVI）was proposed.S-NZVI still maintains a considerable adsorption capacity for Cu-EDTA（～83 mg Cu/g）under high salt concentration（25 g/L Na Cl）.Similarly,under the interference of Ca Cl_2,Mg Cl₂,Na₂SO₄,and Na NO₃ coexisting anions and cations（25 g/L）,S-NZVI still maintains excellent adsorption performance（～83 mg Cu/g）.The removal rate of Cu-EDTA by S-NZVI is extremely fast,it can almost completely remove 50 mg Cu/L Cu-EDTA within 1 min,and the k_obs is about 1.5 g/（mg min）.S-NZVI has a wide p H working range,in the p H range of 2-9,Cu-EDTA can be completely removed within 5 minutes.S-NZVI can be recycled.Add 50 mg Cu/L Cu-EDTA before each run,in the 6 consecutive runs,Cu-EDTA was almost completely removed within 1 min each time.In addition,S-NZVI maintains excellent Cu-EDTA removal performance under conditions of high organic ligand concentration（the molar ratio of EDTA:Cu is 10:1）,and S-NZVI pairs with the other 5 carboxyl ligands（glycine,citrate,DTPA,NTA and tartaric acid）complexed copper also shows strong removal ability.The mechanism of S-NZVI to remove Cu-EDTA is that the Fe²⁺and Fe³⁺released from the oxidation of S-NZVI synergistically complete the replacement of Cu-EDTA,and the free Cu²⁺were subsequently reduced/removed by S-NZVI and co-precipitated by iron oxides and S^2-.（4）Due to strict discharge standards for Cd²⁺in industrial wastewater,the use of sulfur-modified zero-valent iron（S-NZVI）to remove low-concentration Cd-EDTA in high-salinity wastewater was proposed,and explore the feasibility of meeting the discharge standard and possible removal mechanism.When the dosage of S-NZVI was only 0.05 g/L,the 2000μg Cd/L Cd-EDTA was almost completely removed.The removal process is almost unaffected by salinity.Even if the salinity is 50 g/L,the adsorption capacity of S-NZVI still reaches 39.5 mg Cd/g,and the residual Cd concentration in the supernatant is less than 50μg Cd/L,which is in line with Emission Standards of GB 8978-1996（less than 0.1 mg/L）.In addition,S-NZVI can almost completely remove Cd-EDTA in the p H range of 2-7.It shows good removal performance for the other four Cd carboxyl complexes（DTPA,citrate,glycine and tartaric acid）.In addition,S-NZVI also shows good performance in the presence of high concentrations of coexisting ions（Ca Cl₂,Mg Cl₂,Na₂SO₄）and organic substances（thiourea,acetone）.However,the performance of S-NZVI will be inhibited to a certain extent by high concentrations of complexing substances（such as EDTA,imidazole）and oxidizing substances（such as NO₃^-）.The mechanism of S-NZVI to remove Cd-EDTA is that the Fe²⁺and Fe³⁺released by S-NZVI oxidation cooperate to complete the replacement of Cd-EDTA.Fe-EDTA generates ligand-to-metal charge transition（LMCT）under natural light,which process promoted the replacement process of Cd²⁺to a certain extent.And the finally released Cd²⁺was captured by S-NZVI and removed in the form of Cd S and iron oxide-Cd.In summary,the study on the removal performance and mechanism of heavy metal complexes based on the iron ions replacement strategy not only provide new ideas for the treatment of heavy metal complexes in wastewater,but also provide technical support for the promotion of iron ion replacement methods in the field of heavy metal complexes removal.