Based on FeSO4 has a certain flocculation effect, the author proposes to use the microfiltration membrane process to deeply treat the effluent from electroplating wastewater after chemical reduction precipitation. Using FeSO4 as a reducing agent, convert Cr6+ in wastewater into Cr3+, adjust pH to form Cr(OH)3 precipitates, and iron ions are hydrolyzed to form flocculant with strong flocculation adsorption such as Fe(OH)2 and Fe(OH)3. Cr3+, Cr6+ and some of the suspension in the supernatant are suspended. Cr(OH)3 particles in the flocculation stage, supplemented by a lower flow aeration, promote the contact between the particles, improve the flocculation efficiency, and then use the 0.1 μm microfiltration membrane to intercept the flocs and perform membrane separation treatment to further In addition to the purpose of Cr.
1 Experimental part
The electroplating wastewater used in the experiment was taken from Changhui Guanghui Electroplating Factory, Jiangsu Province, with pH=1.75, total Cr 59 mg/L, Cr6+ 36.8 mg/L.
1.1 Main instruments and reagents
Instrument: novAA300 atomic absorption spectrophotometer, Shimadzu Corporation, Japan; microfiltration membrane module, Tianjin Tianmu Company; A6410 Liulian Mixer, Wuhan Meiyu Company.
Reagents: ferrous sulfate heptahydrate, potassium permanganate, diphenylcarbazide, etc., Shanghai Chemical Reagent No. 3 Plant, analytically pure.
Experimental wastewater: The simulated chromium-containing electroplating wastewater with a concentration of 500 mg/L was prepared with K2Cr2O7, and it can be diluted after use according to the experimental needs.
1.2 Experimental content
Determination of the optimum dosage, pH and initial concentration of CrSO4 in the reduction of Cr in the sedimentation wastewater; determination of the optimal aeration flow rate of the microfiltration membrane; investigation of the treatment of the actual chromium-containing electroplating wastewater by the chemical precipitation-microfiltration membrane combination process Effect. In the experiment, the removal rate of Cr in the solution before and after treatment was determined to determine the removal rate of Cr. The Cr6+ measurement was performed by diphenylcarbazide method, and the total Cr was measured by atomic absorption spectrophotometry.
Firstly, according to the orthogonal experiment, the optimal conditions for chemical precipitation were determined: FeSO4 dosage was 2 500 mg/L, reduction pH=5, precipitation pH=10, initial concentration of chromium in electroplating wastewater was 150 mg/L, and then according to It is required to change the experimental conditions and examine the effects of various factors on the treatment effect.
2 Results and discussion
2.1 Effect of FeSO4·7H2O dosage on reduction of Cr6+
The Cr6+ in the sample solution is completely converted into Cr3+. The theoretical requirement is m(FeSO4·7H2O):m(Cr6+)=16:1, but the actual addition of FeSO4·7H2O should be higher than the theoretical value [4]. When m(FeSO4·7H2O):m(Cr6+)=20:1, the amount of Cr3+ is no longer increased. From this, it can be judged that m(FeSO4·7H2O):m(Cr6+)=20:1.
2.2 Effect of pH on the reduction of Cr6+
Under alkaline conditions, the conversion of Cr2O72- to CrO42- will greatly reduce the oxidizing property, so the environment of the reduction reaction solution should be acidic first [5].
Fix m(FeSO4·7H2O):m(Cr6+)=20:1, change the pH of the solution to determine the effect of pH on the reduction of Cr6+. It is observed that the pH of the wastewater is light green at pH ≈1.0 and the formation of Cr3+ is started. The pH is 2.0. When ~4.0, the wastewater is yellow and a small amount of precipitate is formed, and a small amount of Fe(OH)3 precipitate is formed. At pH ≈4, about 98% of Cr6+ is converted to Cr3+. At this time, Cr3+ is hydrolyzed and precipitated, which promotes the conversion of Cr6+. However, when the pH>5, the CrO42- content in the solution increases, which is converted from Cr2O72-, which is not conducive to the redox of Cr6+. It can be confirmed that the optimum reduction pH range in this experiment should be 3.5~5.0.
2.3 Effect of sedimentation pH on total chromium removal
Fix m(FeSO4·7H2O):m(Cr6+)=20:1, adjust the pH of the wastewater to 3.5~5.0, and after the full conversion of Cr6+ to Cr3+, adjust the pH again, so that Cr3+ completely forms Cr(OH)3 precipitate and confirms the precipitation. The effect of pH during the reaction on the total chromium removal rate. The results show that when pH<4, Cr exists as a +3 free ion; when pH>4, Cr(OH)3 precipitation begins to form. It is conducive to the formation of precipitates, but when the pH>10, the precipitation of Cr(OH)3 begins to dissolve. Because Cr(OH)3 is an amphoteric compound, the pH is too high to convert Cr(OH)3. The experimental results show that the conversion is After Cr3+, the optimum pH ≈9 of the total chromium is removed, ie the precipitation pH ≈9.
2.4 Effect of initial concentration of Cr6+ on total Cr removal
Fixed m(FeSO4·7H2O):m(Cr6+)=20:1, adjusted the pH of wastewater to 3.5~5.0, precipitated pH≈9, changed the initial concentration of Cr6+ in wastewater, and investigated the effect of initial concentration of Cr6+ on total Cr removal. see picture 1.
It can be seen from Fig. 1 that when the initial mass concentration of Cr6+ is between 0 and 400 mg/L, the removal effect of total Cr is not affected by the concentration change. That is, in this attempt, most of the Cr in the wastewater can be reduced by FeSO4. Precipitate and remove.
2.5 0.1 μm microfiltration membrane for the removal of Cr
Take 600 mL of experimental wastewater with a concentration of 100 mg/L of Cr6+, adjust the pH of the wastewater to 3.5-5.0, and then add FeSO4·7H2O to the solution according to m (FeSO4·7H2O):m(Cr6+)=20:1. Stir in a six-point stirrer at 200 r/min for 20 min to allow the redox reaction to proceed sufficiently. Adjust the pH of the sample to pH ≈9, then place the stirring cup on the stirrer for 5~10 min, and finally let stand 30. Min, the supernatant was extracted and treated with a 0.1 μm microfiltration membrane to determine Cr6+ and total Cr in water. After detection, the concentration of Cr6+ and total Cr in the supernatant was 0.12 and 2.1 mg/L, respectively, and 0.1 μm microfiltration. After membrane filtration, Cr6+ could not be detected in the effluent, and the total Cr content was also significantly reduced after chemical precipitation treatment, reaching 0.44 mg/L, indicating that the 0.1 μm microfiltration membrane adsorbed Cr6+ and Cr3+ after being adsorbed by Fe gel. Cr(OH)3 has a good retention effect.
2.6 Effect of aeration on total Cr of effluent
Under the same experimental conditions of 2.5, increase the aeration treatment, the effect of aeration on the total Cr content of the effluent is shown in Figure 2.
Aeration can accelerate the mixing and contact of the reactants, which helps to strengthen the flocculation effect and form a flocculent body with larger particle size [6]. However, considering the aeration rate is greater than 0.2 m3/h, the total Cr content decreases slowly. Increasing the amount of aeration will increase the operating cost. At the same time, it is found that the aeration rate increases to a certain extent, and the strong airflow in the reactor may hinder the migration of water molecules to the membrane surface, resulting in a decrease in membrane flux. The test used an aeration flow rate of 0.2 m3/h.
3 Treatment effect on actual electroplating wastewater
The actual chromium-containing electroplating wastewater was treated by chemical precipitation-microfiltration. The experimental procedure is shown in Figure 3. The parameters obtained in each step were taken as the best values ​​obtained in the above experiments. The results are shown in Table 1.
It can be seen from Table 1 that the combined process has obvious effect on the removal of Cr in the actual chromium-containing electroplating wastewater. After treatment, the Cr6+ and total Cr in the effluent reach the requirements of the Electroplating Pollutant Emission Standard (GB 21900-2008).
4 Conclusion
The optimum operating parameters for the chemical precipitation-microfiltration membrane combination process were determined by experiments: m(FeSO4·7H2O):m(Cr6+)=20:1, and the reduction pH was 3.5~5.0, and the precipitation pH≈9 was formed. The microfiltration membrane with a pore size of 0.1 μm has an aeration rate of 0.2 m3/h and a backwashing time of 10 min. Under the optimum process, the removal rates of Cr6+ and total Cr can reach 99.8% and 98%, respectively, which is superior to the traditional one. The chemical reduction method and the removal effect when the membrane is directly filtered.
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