JPS6234433B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is suitable for reducing hexavalent chromium in hexavalent chromium-contaminated water, and includes a treatment agent for hexavalent chromium-contaminated water that is made by solidifying ferrous sulfate with gypsum and has a long-lasting reducing power. Concerning a method for treating chromium-contaminated water. Land where chromium slag has been landfilled or disposed of has become a major pollution problem due to the large amount of harmful hexavalent chromium eluted, and it is necessary to take appropriate measures to prevent the leaching of hexavalent chromium. . Many treatment methods have been proposed to prevent the elution of hexavalent chromium, but in general, the hexavalent chromium-contaminated water is made acidic or neutral and treated with ferrous sulfate, sodium sulfite, or bisulfite. A method is used in which chromium is reduced to trivalent chromium by the action of a reducing agent such as soda, and then converted to weakly alkaline and insoluble chromium hydroxide. As a result of various studies, the present inventors have found that by directly adding ferrous sulfate to hexavalent chromium eluted from chromium slag, it is possible to not only reduce it to trivalent chromium ions, but also to oxidize it to water-insoluble hydroxide. It was discovered that it can be separated as a chromium precipitate. The chemical reaction can be considered as follows. Ca(OH) ₂ +FeSO ₄ →CaSO ₄ +Fe(OH) ₂ ...(1) 2Na ₂ CrO ₄ +6FeSO ₄ +8H ₂ SO ₄ → Cr ₂ (SO ₄ ) ₃ +3Fe ₂ (SO ₄ ) ₃ +2Na ₂ SO ₄ +8H ₂ O ...(2) Cr ₂ (SO ₄ ) ₃ +3Ca(OH) ₂ →2Cr(OH) ₃ +3CaSO ₄ ...(3) First, the pH decreases by the substitution reaction of formula (1), and the pH becomes 9.
From the vicinity, the reduction reaction of equation (2) and the precipitation reaction of equation (3) proceed almost simultaneously, and the reaction is completed when chromium hydroxide is completely precipitated. The above reaction does not proceed without free sulfuric acid, but as shown in equations (4) and (5) below, ferrous sulfate and ferric sulfate liberate sulfuric acid through hydrolysis. 2FeSO ₄ +5H ₂ O+1/2O ₂ →2Fe(OH) ₃ +2H ₂ SO ₄ ...(4) Fe ₂ (SO ₄ ) ₃ +6H ₂ O →2Fe(OH) ₃ +3H ₂ SO ₄ ...(5) Especially sulfuric acid Hydrolysis of ferric iron is accelerated in the presence of free sulfuric acid, so the pH is lower than with ferrous sulfate. It is thought that as the amount of hexavalent chromium increases, the amount of ferric sulfate generated by its reduction increases, and the pH further decreases. Furthermore, the chromium hydroxide produced when the hexavalent chromium in the chromium slag is reduced by ferrous sulfate is a gel-like precipitate, which co-precipitates with a large amount of iron hydroxide, forming an impermeable layer. This layer is formed from the surface of the chromium slag disposal site in order to prevent the elution of hexavalent chromium from the chromium slag disposal site, as the adsorption capacity for hexavalent chromium is extremely large when ferrous sulfate is in excess of the equivalent amount of hexavalent chromium. The method used is to provide a mixed layer of ferrous sulfate, earth, sand, or crushed stone to a certain thickness, and then provide a protective layer of soil or asphalt on top of this. This method can prevent hexavalent chromium from leaching to the ground surface due to rainwater. However, if there is a flow of underground water or continuous spring water inside the ground, hexavalent chromium will be eluted into the water, resulting in hexavalent chromium-contaminated water, which will inevitably flow into gutters and the like. The amount of such hexavalent contaminated water increases or decreases depending on the day, and the amount of hexavalent chromium contamination also varies, but because it flows out constantly every day, it is necessary to collect it quickly without spreading it elsewhere. , it is necessary to perform a permanent reduction treatment, and in order to carry out such a reduction treatment, there is a need for the development of a reducing agent that can demonstrate sufficient reduction ability for a long time and an economical and easy-to-manage reduction treatment system. has been done. The present invention meets these demands, and
A treatment agent for hexavalent chromium-contaminated water that can sustainably and efficiently reduce and insolubilize hexavalent chromium in hexavalent chromium-contaminated water, and a treatment agent for hexavalent chromium-contaminated water that uses this treatment agent to efficiently detoxify hexavalent chromium-contaminated water. The purpose is to provide a water treatment method. That is, the present invention uses ferrous sulfate, gypsum and water, and if necessary, resin emulsion, bituminous emulsion,
A treatment agent for hexavalent chromium-contaminated water is prepared by adding rubber latex, etc., mixed and solidified, and a pit or a place (hereinafter referred to as a reduction zone) in which a basket filled with the treatment agent is installed in the flow path of the hexavalent chromium-contaminated water. ) to 1
One or more reduction zones are provided, and hexavalent chromium-contaminated water is allowed to flow through these reduction zones. The ferrous sulfate used in the present invention is tetrahydrate, pentahydrate,
Although either hydrate or heptahydrate can be used, commercially available industrial heptahydrate (FeSO ₄ .7H ₂ O) is generally used. The gypsum used in the present invention is anhydrite or hemihydrate gypsum (both α type and β type), and generally hemihydrate gypsum is used. Furthermore, the water may be either freshwater or seawater, such as general tap water, groundwater, river water, lake water, industrial water, etc. Examples of the resin emulsion, bituminous emulsion, rubber latex, etc. that may be used as necessary in the present invention are as follows. Resin emulsions include petroleum resins, rosin and its derivatives, coumaron indene resins, styrene-butadiene resins, styrene-acrylate resins, vinyl acetate resins, vinyl acetate-acrylate copolymer resins, polyacrylate resins, alkyd resins, Emulsions of polyvinyl chloride resins, polyvinylidene chloride resins, polyethylene resins, epoxy resins and their curing agents, and any of anionic, nonionic, and cationic emulsions can be used. The bitumen emulsion is an emulsion of asphalt, tar, pitch, etc., and any of anionic, nonionic, and cationic emulsions can be used. Rubber latex includes natural rubber, synthetic rubber, such as isoprene rubber, styrene/butadiene rubber, styrene/isoprene rubber, nitrile rubber, chloroprene rubber,
Anionic, nonionic, and cationic latexes such as butyl rubber and butadiene rubber can be used. These resin emulsions, bituminous emulsions, rubber latexes, etc. (hereinafter simply referred to as resin emulsions, etc.) can also be used in combination. These resin emulsions and the like can increase the waterproofness of the solidified processing agent and slow down the elution rate of ferrous sulfate. The treatment agent for hexavalent chromium-contaminated water of the present invention can generally be produced from the above-mentioned materials in the following manner. That is, ferrous sulfate is weighed and mixed with gypsum as a solidifying agent, mixed with a specified amount of water or water to which resin emulsion, etc. has been added, placed in a suitable container or mold, solidified, and the weight is determined after demolding. It is made by crushing it into chunks of about 1 to 10 kg in size. Although the ratio of ferrous sulfate and gypsum can be varied, it is preferable that the weight ratio of ferrous sulfate to gypsum is in the range of 50:50 to 90:10. The amount of water used is usually about 40 parts by weight per 50 parts by weight of gypsum. Furthermore, when using resin emulsion etc., it is usually used within 10% by weight of the sum of ferrous sulfate and gypsum. Next, the action of the processing agent according to the present invention will be explained. (1) When the solidified treatment agent is immersed in water contaminated with hexavalent chromium, ferrous sulfate dissolves from the treatment agent, reduces hexavalent chromium, insolubilizes it in the form of trivalent chromium hydroxide, and precipitates together with iron hydroxide. do. As the ferrous sulfate dissolves, the treatment agent lump becomes porous and the contact surface with the hexavalent chromium-contaminated water becomes wider, and the ferrous sulfate inside the lump also gradually dissolves, all of which work as a reducing agent. (2) Since the elution of ferrous sulfate occurs gradually and uniformly over a long period of time, the reducing action can be sustained for a long period of time. Further, since ferrous sulfate is not consumed by gypsum solidification, all the reducing agents in the treatment agent effectively act as reducing agents. Since the treatment agent of the present invention is solidified with gypsum, the elution of ferrous sulfate into water is about 30% on the first day, but after that it elutes linearly at a slow and constant rate, 100% elution occurs in 8 days.
That is, the amount of elution until 100% elution is approximately averaged, so that the reducing power of ferrous sulfate is effectively maintained during this time. Ferrous sulfate, which does not solidify into gypsum, dissolves into water instantaneously, so it will wash away under running water, and the reducing power of ferrous sulfate cannot be maintained for a long time. If cement, water glass, etc. are used as a solidifying agent, they will react with ferrous sulfate, which will consume the ferrous sulfate and reduce the reducing power. For example, when trying to solidify ferrous sulfate with cement, the ferrous sulfate reacts with CaO in the cement and becomes gypsum. (3) It can be easily molded into various shapes, has strength, and is easy to handle. It can be made into various shapes such as cylindrical, hexagonal prism, square, charcoal, oval, etc., and is strong, so when it is actually used, it is placed in a basket and immersed in hexavalent chromium-contaminated water. It is easy to handle and dispose of by simply taking it out and disposing of it as it is a solid substance. Since the treatment agent of the present invention has the above-mentioned actions and characteristics, by using the treatment agent, it is possible to permanently reduce water contaminated with hexavalent chromium discharged from disposal sites such as chromium slag. In other words, one pit is installed in the water path that collects and flows groundwater, spring water, rainwater, etc. from the chrome slag disposal site.
If one or more pits are provided, and one or more of the pits is filled with the treatment agent of the present invention in a basket, hexavalent chromium-contaminated water is reduced and rendered harmless when it passes through the pits before being discharged. be done. The amount of treatment agent to be put into the pit is determined based on the flow rate and degree of contamination of the hexavalent chromium-contaminated water. In the case of a treatment agent in which the elution of ferrous sulfate in the treatment agent is completed in 6 days, the treatment agent may be replaced with a new treatment agent on the 7th day. It will be more effective if the pit into which the processing agent is placed can be stirred as appropriate. This is because a concentration gradient occurs in the elution of ferrous sulfate due to its specific gravity, and the concentration is higher at the bottom of the pit and becomes thinner toward the top. A good way to stir this is, for example, by sinking a helical pipe into the bottom of the pit, making a number of small holes in the pipe in different directions, and occasionally blowing air through the pipe from outside to stir.
Thereby, the hexavalent chromium-contaminated water can be uniformly reduced. In the water discharged after reducing hexavalent chromium-contaminated water in a pit using ferrous sulfate,
Hexavalent chromium can be removed, but the pH is slightly acidic and ferrous sulfate is also present. Therefore, let this water flow as it is,
If the downstream water is to be combined with water contaminated with hexavalent chromium flowing out from other than the pit path,
This hexavalent chromium can be reduced. However, if acidic water cannot be directly discharged, calcium carbonate, limestone, etc. can be placed downstream of the pit containing the treatment agent to neutralize it. By using this method, there is no loss of ferrous sulfate and it can be effectively used for reduction treatment, and it is possible to completely render hexavalent chromium-contaminated water harmless by replacing the treatment agent about once a week, and it is extremely easy to manage. Easy and economical. The treatment agent and method for hexavalent chromium-contaminated water of the present invention can be applied not only to hexavalent chromium-contaminated water from chromium slag disposal sites, but also to hexavalent chromium-contaminated water from various wastewaters. Next, an example will be described. In addition, the test method and materials used in the following examples are as follows. Test method: Elution test of ferrous sulfate from ferrous sulfate gypsum solid (treatment agent) Ferrous sulfate is solidified by adding gypsum, water, etc., and the material is 7 days old and has a diameter of about 10 cm. was immersed in running water, and samples were taken out at regular intervals. A part of this sample was crushed, immersed in a specified amount of water, thoroughly stirred, and then filtered. 5 ml of sulfuric acid (1+1) was added to the filtrate, and titrated with N/10 potassium permanganate. The consumption amount was converted into ferrous sulfate (FeSO ₄ .7H ₂ O). The elution amount of ferrous sulfate (FeSO ₄ .7H ₂ O) was expressed as the elution amount (g) per 100 g of the sum of ferrous sulfate and gypsum. Materials used; Ferrous sulfate FeSO ₄・7H ₂ O Gypsum Hemihydrate gypsum Water Tap water Synthetic rubber latex Styrene/butadiene
Rubber latex (nonvolatile content 50% by weight) Bituminous emulsion Cationic asphalt emulsion MK
-1 (evaporation residual fraction 58% by weight, penetration of evaporation residue 145) Example 1 Ferrous sulfate, gypsum, water,
Mixed with synthetic rubber latex etc. Diameter 10cm Height 10cm
The mixture was poured into a cylindrical mold to solidify, left for 24 hours, and then removed from the mold to obtain the treatment agent of the present invention.

[Table] 〓Determined as a percentage. 〓
For these treatment agents, samples of 7-day-old materials were used to examine changes over time in the amount of ferrous sulfate leached into water. The results are shown in FIGS. 1 to 4. Figure 1 shows the relationship between the elution amount of ferrous sulfate and the number of days depending on the ratio of ferrous sulfate and gypsum, and Figures 2 to 4 show the relationship between the ferrous sulfate content and the number of days, respectively.
% and 80% (corresponding to 40%, 30%, and 20% of gypsum, respectively) without the addition of synthetic rubber latex and with addition (10% by weight of gypsum) of ferrous sulfate elution and the relationship between the number of days. It is a graph. 1st
From the figure, it can be seen that 30 to 50% by weight of ferrous sulfate from the processing agent is eluted in 24 hours, but thereafter it elutes slowly and linearly at a constant rate, and it takes 6 days for almost 100% elution. A similar test using only ferrous sulfate quickly dissolves 100%. From Figures 2 to 4, it can be seen that when synthetic rubber latex was added in an amount of 10% of the amount of gypsum, the elution curve showed a similar tendency, but it took 8 days to complete the elution. Example 2 80 parts by weight of ferrous sulfate and 20 parts by weight of hemihydrate gypsum were mixed, 14 parts by weight of water and 2 parts by weight of bitumen emulsion were added and mixed, poured into a plastic bucket, and solidified. After demolding, it was crushed to obtain the processing agent of the present invention. Diameter 10 cm and height 10 cm as in Example 1 with the same composition.
A 7-day-old cylindrical specimen was prepared and tested for the elution of ferrous sulfate into water, and results similar to those obtained when synthetic rubber latex was added were obtained. Example 3 Formula No. 3 in Table 1 (80% by weight of ferrous sulfate)
Then, a solid treatment agent in the form of a lump with a diameter of about 10 to 30 cm was made, and a pit with the structure shown in Figure 5 was installed in the water flow path that collected water flowing from the actual chromium slag disposal site, and the hexavalent chromium-contaminated water was collected. I processed it. In other words, pit 2 (specification 600m/m x 600m/m x
1600 m/m), 80 kg of solid processing agent was placed in basket 4 and immersed. The hexavalent chromium-contaminated water that continuously gushes out from the chromium slag disposal site and the rainwater-contaminated water are collected and discharged through this pit 2. The flow rate of the hexavalent chromium-contaminated water was 7 tons/day, and the amount of hexavalent chromium was 60 to 90 mg/day. In the pit 2, a helical pipe 5 with a large number of small holes is provided below the basket 4 containing the processing agent, and a compressor 6 equipped with a timer 7 passes through this pipe 5 for 5 minutes at a rate of 15 to 20 minutes every 30 minutes. of air and stirred. As a result, the amount of hexavalent chromium in the contaminated water flowing into pit 1 was 86 mg/day, but in pits 2 and 3,
In the water leaving the water, the concentrations were 0.08 mg/ and 0.05 mg/, respectively, and it was possible to remove hexavalent chromium almost completely. By replacing the treatment agent in pit 2 with a new one every week, it was possible to maintain good drainage with hexavalent chromium removed. Figure 6 is a system diagram showing an application example where hexavalent chromium-contaminated water is treated according to the above reduction treatment method and discharged into rivers (Nishiki River and Uenbetsu River), and the numbers in the frame indicate the chromium concentration ( Cr ⁶⁺ ppm). In the figure, pit 9 contains the treatment agent of the present invention, and in pit 8 the Cr ⁶⁺ concentration is 86 ppm, but when exiting pits 9 and 10, Cr 6+ is 0.08 ppm, respectively.
It becomes 0.05ppm. At pit point A,
Water with a Cr ^{6+ content} of 64.2ppm is coming out, but this decreases to 0.06ppm downstream by merging with the wastewater from pit system B.

[Brief explanation of the drawing]

The drawings show examples of the present invention, and Figure 1 is a graph showing the relationship between the elution amount of ferrous sulfate and the number of days depending on the ratio of ferrous sulfate and gypsum, and Figures 2 to 4 are graphs showing the relationship between ferrous sulfate and gypsum ratios. Iron content is 60%, 70% and respectively
A graph showing the relationship between the elution amount of ferrous sulfate and the number of days depending on the presence or absence of addition of synthetic rubber latex at 80%, Fig. 5 is an explanatory diagram showing an example of the structure of a pit for carrying out the present invention, and Fig. 6 The figure is a system diagram showing an example of application when hexavalent chromium-contaminated water is treated and discharged into a river according to the present invention. 1, 2, 3, 8, 9...Pitsuto.

Claims (1)

[Scope of Claims] 1. A treatment agent for hexavalent chromium-contaminated water, which is obtained by mixing ferrous sulfate, gypsum, water, and optionally resin emulsion, bituminous emulsion, rubber latex, etc., and solidifying and molding the mixture. 2. Treatment of hexavalent chromium-contaminated water by adding ferrous sulfate, gypsum, water, and if necessary resin emulsion, bituminous emulsion, rubber latex, etc. to the flowing water path of hexavalent chromium-contaminated water, mixing and solidifying the mixture. Hexavalent chromium pollution is characterized by providing one or more pits or areas (hereinafter referred to as reduction zones) in which baskets filled with the agent are installed, and flowing hexavalent chromium-contaminated water through these reduction zones. How to treat water.