Chemical precipitation-microfiltration treatment of chromium-containing electroplating wastewater
The annual discharge of Cr-containing electroplating wastewater in China is about 1.7 billion m3 [1], in which Cr, especially Cr6+, is a carcinogenic substance and is classified as a national hazardous substance [2]. However, it is difficult to remove chromium from the effluent due to the insolubility of Cr(OH)3 and the difficulty of complete reduction of Cr6+ [3]. The treatment of electroplating wastewater can hardly meet the requirements of the new standard.
Based on the fact that FeSO4 has a certain flocculation effect, the author proposes to use a microfiltration membrane process to treat the effluent of electroplating wastewater after chemical reduction precipitation treatment. The flocculants with strong flocculation and adsorption effects such as Fe(OH)2 and Fe(OH)3 were formed to trap Cr3+, Cr6+ and some Cr(OH)3 particles in suspension in the supernatant, and the flocculation was supplemented with low flow rate aeration to promote inter-particle contact and improve the flocculation efficiency, and then the flocs were retained by 0.1 μm microfiltration membrane for membrane separation to achieve further Cr removal.
1 Experimental section
The electroplating wastewater used in the experiment was obtained from Guanghui Electroplating Plant in Changzhou City, Jiangsu Province, with pH=1.75, total Cr 59 mg/L, and Cr6+ 36.8 mg/L.
1.1 Main instruments and reagents
Instruments: novAA300 atomic absorption spectrophotometer, Shimadzu, Japan; microfiltration membrane assembly, Tianjin Tianmembrane Company; A6410 hexagonal stirrer, Wuhan Meiyu Company.
Reagents: ferrous sulfate heptahydrate, potassium permanganate, diphenylcarbonyl dihydrazine, etc., Shanghai Chemical Reagent Plant No. 3, analytical purity.
Experimental wastewater: simulated chromium-containing electroplating wastewater with a mass concentration of 500 mg/L was prepared with K2Cr2O7, which could be used after appropriate dilution according to the experimental needs.
1.2 Experimental content
The optimum dosage, pH, and initial concentration of FeSO4 for the reduction of Cr in the precipitated wastewater were determined; the optimum aeration flow rate of the microfiltration membrane was determined; the effect of the combined chemical precipitation-microfiltration membrane process on the treatment of real chromium-containing electroplating wastewater was investigated; the Cr removal rate was determined by measuring the changes of Cr6+ and total Cr content in the solution before and after the treatment. Cr6+ was measured by diphenylcarbodihydrazide and total Cr was measured by atomic absorption spectrophotometry.
The optimum conditions for the chemical precipitation were determined based on orthogonal experiments: FeSO4 was added at a mass concentration of 2 500 mg/L, the reduction pH=5, the precipitation pH=10, and the initial mass concentration of chromium in the electroplating wastewater was 150 mg/L. Then, the experimental conditions were changed according to the requirements to investigate the effects of each factor on the treatment effect.
2 Results and discussion
2.1 Effect of FeSO4-7H2O dosing on the reduction of Cr6+
In this experiment, when m(FeSO4-7H2O):m(Cr6+) = 20:1, the amount of Cr3+ did not increase. (FeSO4-7H2O):m(Cr6+) = 20:1.
2.2 Effect of pH on Cr6+ reduction
Cr is converted from Cr2O72- to CrO42- under alkaline conditions and oxidation is greatly reduced, so the reducing solution environment should be acidic in the first place [5].
The pH of the solution was fixed atm(FeSO4-7H2O):m(Cr6+) = 20:1 to determine the effect of pH on the reduction of Cr6+, The pH of Cr6+ was determined to be between 3.5 and 5.0 for this experiment.
2.3 Experiment on the effect of precipitation pH on the removal of total chromium
The pH of the wastewater was adjusted from 3.5 to 5.0 by fixing m(FeSO4-7H2O):m(Cr6+)=20:1. After Cr6+ was fully converted to Cr3+, the pH was adjusted again so that Cr3+ was completely formed into Cr(OH)3 precipitate to determine the effect of pH on total chromium removal during the precipitation reaction. The results showed that Cr was present in the form of +3 valent free ions at pH < 4 and Cr(OH)3 precipitated at pH > 4. This indicated that increasing the pH was favorable for the precipitation, but at pH > 10 the Cr(OH)3 precipitation started to dissolve because Cr(OH)3 is an amphoteric compound and too high a pH would cause the conversion of Cr(OH)3. 9.
2.4 Effect of the initial concentration of Cr6+ on the removal of total Cr
By fixing m(FeSO4-7H2O):m(Cr6+) = 20:1, adjusting the pH of the wastewater from 3.5 to 5.0 and precipitation pH ≈ 9, and changing the initial concentration of Cr6+ in the wastewater, the effect of the initial concentration of Cr6+ on the removal effect of total Cr was investigated and the results are shown in Figure 1.
The results are shown in Fig. 1. As can be seen from Fig. 1, the effect of the initial mass concentration of Cr6+ from 0 to 400 mg/L was not greatly affected by the change of concentration, i.e. most of the Cr in the wastewater could be precipitated and removed by the reduction of FeSO4 within this attempt range.
2.5 Removal of Cr by 0.1 μm microfiltration membrane
The pH of the wastewater was adjusted to 3.5~5.0, and then FeSO4-7H2O was added to the solution according to m (FeSO4-7H2O):m(Cr6+)=20:1, and stirred at 200 r/min for 20 min in a hexagonal stirrer to allow the redox reaction to proceed fully. The pH of the water sample was adjusted to ≈9, then the stirring cup was placed on the stirrer for 5-10 min, and finally left to stand for 30 min. The total Cr content was also significantly reduced to 0.44 mg/L after chemical precipitation treatment, indicating that the 0.1 μm microfiltration membrane has a good retention effect on Cr6+, Cr3+ and Cr(OH)3 after adsorption by Fe gels.
2.6 Effect of aeration on the total Cr in the effluent
Aeration accelerates the mixing and contact of the reactants, which helps to strengthen the flocculation effect and form larger size flocs [6]. On balance, an aeration flow rate of 0.2 m3/h was adopted for this test.
3 Treatment effect on actual electroplating wastewater
The results are shown in Table 1. As can be seen from Table 1, the combined process is very effective in removing Cr from the actual chromium-containing electroplating wastewater, and the Cr6+ and total Cr in the treated effluent meets the requirements of the Electroplating Pollutant Emission Standard (GB 21900-2008).
4 Conclusion
The optimum operating parameters of the combined chemical precipitation-microfiltration membrane process were determined as follows: m(FeSO4-7H2O):m(Cr6+) = 20:1, reduction pH was 3.5-5.0, precipitation pH ≈ 9, microfiltration membrane with a pore size of 0.1 μm, aeration capacity of 0.2 m3/h, and backwashing time of 10 min. The removal rates of Cr6+ and total Cr were 99.8% and 98% respectively, which were better than those of the traditional chemical reduction method and direct membrane filtration.
