CN109626591B - Method for synergistically reducing hexavalent chromium by utilizing microorganisms and hematite under illumination condition - Google Patents

Abstract

The invention discloses a method for synergistically reducing hexavalent chromium by utilizing microorganisms and hematite under the illumination condition, which is characterized in that the hexavalent chromium is synergistically reduced by adding the hematite and iron reducing bacteria capable of carrying out extracellular electron transfer into wastewater containing the hexavalent chromium under the illumination condition. The method utilizes hematite minerals and extracellular breathing iron reducing bacteria which widely exist in the natural environment to realize the high-efficiency treatment of chromium pollution by synergistically utilizing light energy and chemical energy, solves the problems of low treatment efficiency, complex synthesis of photocatalytic materials, poor environmental compatibility, intolerance of microorganisms, high treatment cost and the like of the existing method, and simultaneously avoids the problem of secondary pollution in the treatment process.

Description

A method for synergistic reduction of hexavalent chromium by microorganisms and hematite under light conditions

technical field

The invention relates to a treatment technology for hexavalent chromium in wastewater, in particular to a method for utilizing environmental microorganisms and hematite minerals to synergize photocatalytic reduction of hexavalent chromium, and belongs to the field of environmental engineering water treatment.

Background technique

Chromium and its compounds are widely used in industry, but in the process of production and use, it is easy to produce toxic chromium-containing waste residue and waste water. With economic development, the amount of chromium salts in industrial production is increasing, and chromium pollution is becoming more and more serious. The toxicity of chromium is related to its valence. Hexavalent chromium is 100 times more toxic than trivalent chromium, and is easily absorbed and accumulated in the body. Hexavalent chromium is a lasting hazard to the environment and can invade the human body through digestion, respiratory tract, skin and mucous membranes. It has been reported that when the air contains different concentrations of chromic anhydride, there are different degrees of hoarseness and nasal mucosal atrophy, and in severe cases, it can also cause perforation of the nasal septum and bronchiectasis; intrusion through the digestive tract can cause vomiting and abdominal pain; Dermatitis and eczema when invasive; carcinogenic hazard by prolonged or short-term exposure or inhalation. Trivalent chromium in water is mainly adsorbed on solid substances and exists in sediments, while hexavalent chromium is mostly soluble in water and is stable. How to convert hexavalent chromium in wastewater into trivalent chromium that is easy to precipitate and less harmful is the focus of current wastewater chromium pollution control.

Xu et al. used alginic acid-coated _TiO2 as a photocatalytic material and added ferric ions and found that it had a good effect on the reduction of hexavalent chromium. However, the absorption band of TiO ₂ is very narrow, and the material synthesis is expensive. Due to the consumption of alginic acid, it does not have the ability to continuously reduce and maintain the original form (Xu SC, Pan SS, Xu Y, et al. Efficientremoval of Cr (VI). ) from wastewater under sunlight by Fe(II)-doped TiO ₂ spherical shell. [J]. Journal of Hazardous Materials, 2015, 283(283):7-13.).

Middleton et al. used Shewanella oneidensis MR-1 to reduce hexavalent chromium, and found that at low concentrations of chromium, chromic acid could be continuously added to reduce it multiple times. However, it has a certain metabolic inhibition on microorganisms at high concentrations, and the rate of bioreduction of hexavalent chromium is slow (Middleton S S, Latmani R B, Mackey M R, et al. Cometabolism of Cr(VI) by Shewanella oneidensis MR-1 produces cell-associated reduced chromium and inhibits growth [J]. Biotechnology & Bioengineering, 2003, 83(6): 627-37.).

Under acidic conditions, Ye et al. can excite the organic complex of Fe(III) with ultraviolet light to achieve the degradation of the organic complex and the reduction of Fe(III); the generated Fe(II) converts Cr(VI) to Cr(III) ). However, this method requires a lot of energy consumption, not only needing ultraviolet conditions, but also adding a lot of organic complexes as electron donors (Ye Y, Jiang Z, Xu Z, et al. Efficient removal of Cr(III)-organic complexes from water using UV/Fe(III)system:Negligible Cr(VI)accumulation and mechanism[J].Water Research,2017,126:172.).

Microbial-assisted photocatalytic reduction reaction refers to the fact that after the semiconductor material is excited by light, some microorganisms that can carry out extracellular electron transfer or the extracellular secretions released by the microorganism are used as the hole trapping agent of the semiconductor material, and the reduction of photogenerated electrons is used to carry out the photocatalytic reduction reaction. The process of reducing other substances, so as to realize the conversion of biomass energy and light energy into chemical energy.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for the efficient reduction of hexavalent chromium by coupling the extracellular respiration of microorganisms to generate electricity and the photocatalysis of natural minerals, so as to solve the problems of low processing efficiency, complex synthesis of photocatalytic materials and environmental compatibility in the current method. To solve the problems of poor performance, microbial intolerance, and expensive treatment costs, the use of hematite minerals and extracellular respiration iron-reducing bacteria that are widely present in the natural environment can achieve synergistic utilization of light energy and chemical energy, efficiently treat chromium pollution, and avoid chrome pollution. The problem of secondary pollution in the treatment process.

Semiconductor minerals will generate oxidative photo-generated holes and reductive photo-generated electrons after being excited by a certain wavelength of light, which often exist in pairs and undergo a large number of recombinations. Many microorganisms that can carry out transmembrane electron transfer, such as Shewanella and Geobacter, can fill the holes generated by photoexcitation of semiconductor minerals with extracellular electrons, and some hole trapping agents can also react with holes, so that Photogenerated electrons reduce hexavalent chromium to trivalent chromium. Trivalent chromium forms hydroxide precipitation when the pH is above 8, so as to achieve the separation of chromium element from the liquid phase.

The method for reducing hexavalent chromium provided by the present invention is to add hematite and iron-reducing bacteria capable of extracellular electron transfer into wastewater containing hexavalent chromium, and the two synergistically reduce hexavalent chromium under light conditions.

The hematite used in the method for reducing hexavalent chromium of the present invention can be natural hematite or artificially synthesized hematite. Preferably, the particle size of the hematite is nanoscale or microscale, that is, hematite with an average particle size of 1 nm to 10 μm.

The required hematite can be synthesized by the following method: (1) Weigh Fe(NO ₃ ) ₃ ·9H ₂ O solid and dissolve it in water to prepare Fe(NO ₃ ) ₃ · with a concentration of 0.8M to 1.2M. 9H ₂ O solution; (2) heating a certain volume of water to boiling, adding the Fe(NO ₃ ) ₃ ·9H ₂ O solution dropwise to the continuously boiling water under stirring, and then standing to cool to room temperature; (3) Put the suspension obtained in step (2) into a dialysis bag, put it into ultrapure water for dialysis, change the water regularly until the conductivity of the dialysis water is similar to the conductivity of the pure water, and store it in a sealed bottle and protected from light.

The above-mentioned iron-reducing bacteria that can carry out extracellular electron transfer include Shewanella, Pseudomonas aeruginosa, Alcaligenes faecalis, Geobacter, etc., preferably Shewanella oneida MR-1 (Shewanella oniedensisi MR-1). ) (Deposit Number ATCC 700550).

The strains can be preserved by a variety of preservation methods, such as freeze-drying preservation method, cryogenic preservation method, liquid nitrogen preservation method, mineral oil sealing method, solid koji preservation method, sand tube preservation method, agar puncture preservation method, etc. Before use, activate the preserved bacteria in the culture solution, and then subculture the bacteria solution after activation or transfer culture. The bacteria were resuspended in an isotonic solution. To avoid the effect of the extracellular secretions of iron-reducing bacteria on the reduction of hexavalent chromium, the washing effect can be achieved by repeated centrifugation, discarding the supernatant, and resuspension.

In the method for reducing hexavalent chromium of the present invention, the amount of iron-reducing bacteria added is preferably 0.1-1.0, preferably 0.2-1.0, to control the OD ₆₀₀ of the final bacteria amount. The concentration of hematite is controlled from 15mg/L to 300mg/L.

In the method for reducing hexavalent chromium of the present invention, it is preferable to adjust the pH of the wastewater containing hexavalent chromium to 6.5-7.5 before adding hematite and iron-reducing bacteria. If the pH of the wastewater is lower than 6, the trivalent chromium produced by the reduction of iron-reducing bacteria is toxic to the microorganism itself. Therefore, under acidic conditions, a complexing agent of trivalent chromium can be added while adding hematite and iron-reducing bacteria, such as EDTA (ethylenediaminetetraacetic acid), TEOA (triethanolamine), etc., reduce the toxicity to microorganisms by complexing trivalent chromium. The amount of complexing agent added depends on the amount of trivalent chromium produced.

If the hexavalent chromium-containing wastewater lacks nutrients for the growth of iron-reducing bacteria, lactate can be added at a concentration of 10mM to 45mM.

In the method for reducing hexavalent chromium of the present invention, the illumination conditions may be placed under a xenon lamp simulating sunlight or other artificial light sources with visible light band, and the illumination intensity is 10mW/cm ² -100mW/cm ² , or in natural Under sunlight, maintaining the temperature in the range of 25-35 °C, the hexavalent chromium is continuously reduced.

Through the method for synergistic reduction of hexavalent chromium by hematite and iron-reducing bacteria provided by the present invention, the tolerance of microorganisms to hexavalent chromium is improved, the reducing ability of microorganisms to hexavalent chromium is accelerated, and artificial synthesis is saved. The cost of semiconductors for catalytic reduction.

Description of drawings

Fig. 1 is the comparative identification result of the XRD pattern (top) of the hematite sample synthesized in Example 1 and the standard hematite pattern (bottom).

Fig. 2 is the curve of the synergistic reduction of hexavalent chromium by iron-reducing bacteria and minerals under light or dark conditions in a neutral environment in Example 1.

Fig. 3 is the curve of photocatalytic reduction of hexavalent chromium by iron-reducing bacteria under the conditions of light or dark, with or without hematite in wastewater at pH=5.5 in Example 2.

Fig. 4 is the curve of reduction of hexavalent chromium by iron-reducing bacteria under dark conditions in the wastewater of pH=5.5 with or without the addition of EDTA complexing agent.

Fig. 5 is the curve of reducing hexavalent chromium under the condition of adding EDTA to the wastewater with pH=5.5, light or dark, with or without iron-reducing bacteria in Example 3.

Detailed ways

The present invention will be described in detail below with reference to the accompanying drawings through specific embodiments, but the present invention is not limited thereto.

The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained from commercial sources unless otherwise specified.

Example 1:

32.32 g of Fe(NO ₃ ) ₃ ·9H ₂ O solid was weighed and dissolved in 80 mL of water to prepare a 1M Fe(NO ₃ ) ₃ ·9H ₂ O solution. Take 1L of ultrapure water into a conical flask, add a magnet, and heat it to boiling on a magnetic heating stirrer. The solution was stirred and the Fe( _{NO3 ) 3.9H2O} solution was added dropwise to the continuously boiling water at a rate of 0.5 mL/min using a peristaltic pump. After the solution was added, the heating was stopped, and it was allowed to stand to cool to room temperature naturally.

The dialysis bag (3.5-5KD, SpectraPor) was soaked in ultrapure water for 1 hour before use, cut into multiple sections (about 30cm), put the cooled suspension into the dialysis bag, tie both ends tightly, and put it in ultrapure water During dialysis, the water was changed every 12 h until the conductivity of the dialysis water was similar to that of pure water.

Fig. 1 is the XRD pattern of the hematite synthesized by the above-mentioned hydrothermal method. Compared with the standard XRD pattern of hematite, it can be considered that the material synthesized by the hydrothermal method is indeed hematite.

The normally transferred bacterial solution of Shewanella oniedensisi (Shewanella oniedensisi MR-1, deposit number ATCC 700550) was mixed with 50% glycerol in a volume ratio of 1:1 in a container, and stored at -80 degrees Celsius. When in use, the preserved bacterial solution was added to the LB medium at a volume ratio of 1:20, and the transfer was carried out in the same proportion after activation. Shewanella is a facultative bacteria, and in order to make it grow faster, all cultures are carried out under aerobic conditions. The transferred bacteria were washed twice by centrifugation and resuspended for later use.

The above-mentioned synthetic hematite, and the resuspended bacterial solution and lactate are added in the neutral wastewater containing chromium (5mg/L) (pH is equal to 7), so that the OD ₆₀₀ value of the mixed solution after adding is 0.2, and the lactate The concentration was 30 mM, and the hematite concentration was 150 mg/L. Then, the mixed solution was placed under a xenon lamp simulating sunlight with a light intensity of 45mW/cm ² , or under natural sunlight, and the temperature was maintained in the range of 25-35°C. The treatment efficiency of 50% can be achieved by illumination for 6 h, as shown in Figure 2, it can be seen that the reduction of Cr and chromium in the system by illumination under neutral conditions has a significant promoting effect in terms of reaction speed and degree of reaction.

Example 2:

The hematite synthesis method and bacterial culture method in Example 1 were adopted. The synthetic hematite, bacterial liquid and lactate are added to the acid wastewater containing chromium (10mg/L) (pH is equal to 5.5), so that the OD ₆₀₀ value of the mixed solution after adding is about 0.2, and the lactate concentration is 30mM, The hematite concentration was 150 mg/L. Then the mixture is irradiated under light conditions (the irradiation conditions are the same as in Example 1), and the reduction efficiency of hexavalent chromium can reach more than 85% in 25h, as shown in the kinetic curve of "light with bacteria and hematite" in Figure 3 Show. It can be seen from Figure 3 that the reduction of hexavalent chromium by iron-reducing bacteria in the dark is basically the same with or without hematite, but the reduction effect is significantly improved in the presence of hematite and light. The processing efficiency of more than 80% can be achieved.

Example 3:

The hematite synthesis method and bacterial culture method in Example 1 were adopted. Since the trivalent chromium reduced from the acidic wastewater is in an ionic state and has certain toxicity to microorganisms, EDTA is used as the trivalent chromium complexing agent. The synthetic hematite, bacterial liquid, lactate and EDTA are added to the acid wastewater containing chromium (10mg/L) (pH is equal to 5.5), so that the OD ₆₀₀ value of the mixed solution after adding is about 0.2, and the lactate concentration is 30mM, the hematite concentration is 150mg/L, and the EDTA concentration is 0.66mM. Then, the mixture was irradiated under light conditions (the irradiation conditions were the same as those in Example 1), and the reduction efficiency of hexavalent chromium of 80% could be achieved in 5 hours, as shown in the kinetic curve of "light with bacteria and hematite" in Figure 5. Comparing Figure 4 and Figure 5, it can be seen that the photoreduction effect of iron-reducing bacteria and hematite is significantly improved with the addition of EDTA compared to without EDTA. The added EDTA complexing agent is complexed with trivalent chromium, thereby reducing the toxicity to microorganisms. Experiments show that EDTA can promote the reduction of hexavalent chromium by iron-reducing bacteria under acidic conditions. In addition, the addition of EDTA promoted the efficient reduction of hexavalent chromium by iron-reducing bacteria under acidic conditions, and the treatment efficiency reached more than 80% in 3 h.

Finally, it should be noted that the above embodiments are only for further understanding of the present invention and not for limiting it. However, those skilled in the art will appreciate that various substitutions and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the present invention should not be limited to the contents disclosed in the embodiments, and the scope of protection of the present invention shall be subject to the scope defined by the claims.

Claims (11)

1. A method for reducing hexavalent chromium comprises adding hematite and iron reducing bacteria capable of performing extracellular electron transfer into wastewater containing hexavalent chromium, and synergistically reducing hexavalent chromium under illumination condition.

2. The method of claim 1, wherein the hematite is natural hematite or synthetic hematite having a particle size of the order of nanometers or microns.

3. The method of claim 2, wherein the hematite has an average particle size of from 1nm to 10 μ ι η.

4. The method of claim 1, wherein the hematite is prepared by:

1) weighing Fe (NO)₃)₃·9H₂Dissolving O solid in water to prepare Fe (NO) with the concentration of 0.8-1.2M₃)₃·9H₂O solution;

2) heating a volume of water to boiling, and adding Fe (NO) under stirring₃)₃·9H₂Dropwise adding the O solution into continuously boiling water, and then standing and cooling to room temperature;

3) filling the turbid liquid obtained in the step 2) into a dialysis bag, putting the dialyzed turbid liquid into ultrapure water for dialysis, periodically changing water until the conductivity of the dialyzed water is similar to that of the pure water, and then bottling, sealing and storing in dark place.

5. The method of claim 1, wherein the iron-reducing bacteria capable of extracellular electron transfer are shewanella, pseudomonas aeruginosa, alcaligenes faecalis, and/or geobacillus.

6. The method of claim 5, wherein the Shewanella is Shewanella inekensis MR-1(Shewanella niedensis MR-1).

7. The method of claim 1, wherein the iron-reducing bacteria are added in an amount to control the final bacteria amount OD₆₀₀0.1-1.0, and the hematite concentration is 15-300 mg/L.

8. The method of claim 1 wherein the pH of the hexavalent chromium-containing wastewater is adjusted to a pH of 6.5 to 7.5 prior to the addition of the hematite and the iron reducing bacteria.

9. The method of claim 1 wherein when the hexavalent chromium-containing wastewater is acidic, the complexing agent for trivalent chromium is added simultaneously with the hematite and the iron-reducing bacteria.

10. The method of claim 1 wherein a quantity of lactate salt is added for the growth of iron-reducing bacteria when the hexavalent chromium-containing wastewater is depleted of nutrients for the growth of iron-reducing bacteria.

11. The method according to claim 1, wherein the illumination condition is that the illumination intensity is 10-100 mW/cm when the artificial light source simulating sunlight is placed under the illumination condition²Or in natural sunlight, maintaining the temperature in the range of 25-35 ℃.

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