Method for removing heavy metals in rice straws

Abstract

The invention provides a method for removing heavy metals in rice straws, which comprises the following steps of: crushing the rice straws, and soaking the crushed rice straws in alkali liquor to remove the heavy metals. The method is simple, convenient, efficient and economical, the heavy metal in the rice straw can be removed in a targeted mode, and the removal efficiency is remarkably improved.

Description

Technical Field

[0001] The invention relates to efficient utilization of heavy metal-contaminated straw, and in particular to a method for removing heavy metals from rice straw. Background Art

[0003] At present, there are physical, chemical, biological and agronomic remediation methods for contaminated soil, but these methods have their own limitations. For example, physical remediation technology is labor-intensive, costly and only applicable to small areas of contaminated soil; chemical remediation technology has many technical operation requirements and there is a risk of changing soil structure and secondary pollution; agronomic remediation measures have a long cycle and poor results. Phytoremediation technology has the advantages of simplicity, high efficiency and low cost, and is an environmentally friendly heavy metal pollution remediation technology. Traditional high-accumulation plant remediation of farmland heavy metal pollution has the problem of changing soil structure, which needs to be restored after remediation. Using high-accumulation rice varieties to carry out arsenic-contaminated rice field remediation does not change the properties and structure of the rice field, which can provide new ideas for the phytoremediation of arsenic-contaminated rice fields. However, the improper disposal of a large amount of rice straw produced after soil remediation not only easily causes secondary pollution of heavy metals, but also endangers the quality and safety of agricultural products.

[0004] In recent years, landfill, pyrolysis and other methods have been developed worldwide to treat heavy metal-accumulating plants. However, due to the lack of effective strategies for enriching heavy metals, most treatment methods are usually accompanied by high energy consumption and secondary pollution risks. However, the safe use and disposal of high-heavy metal-accumulating straw from contaminated farmland remains a major challenge facing my country's agriculture and rural areas. Direct use of such straw as fertilizer, feed and base material will have adverse effects on soil, livestock and agricultural products. Summary of the invention

[0005] In view of the above-mentioned shortcomings of the prior art, the object of the present invention is to provide a method for removing heavy metals from rice straw, so as to solve the problem of difficulty in removing heavy metals from straw with high heavy metal accumulation in the prior art.

[0006] The first aspect of the present invention provides a method for removing heavy metals from rice straw, comprising the following steps:

[0007] The rice straw is crushed and soaked in alkaline solution to remove heavy metals.

[0008] The rice straw may be arsenic-enriched rice straw, which is grown in arsenic-contaminated soil and is enriched and formed in the rice straw. The arsenic content in such rice straw is generally above 10 mg / kg.

[0009] Preferably, the heavy metal is arsenic.

[0010] Preferably, the arsenic content of the rice straw is 10-15 mg / kg, 15-20 mg / kg, 20-25 mg / kg, 25-30 mg / kg, 30-35 mg / kg or more than 35 mg / kg. For example, the arsenic content of the rice straw can also be 6 mg / kg, 8 mg / kg, 10 mg / kg, 12 mg / kg, 14 mg / kg, 16 mg / kg, 18 mg / kg, 20 mg / kg, 22 mg / kg, 24 mg / kg, 26 mg / kg, 28 mg / kg or 30 mg / kg.

[0011] Preferably, the rice straw is crushed to a particle size of no more than 20 mm. For example, the rice straw can be crushed to a particle size of 0.25-0.425 mm, 0.425-0.5 mm, 0.5-0.6 mm, 0.6-0.71 mm, 0.71-0.85 mm, 0.85-1.0 mm, 1.0-1.18 mm, 1.18-1.40 mm, 1.40-2.0 mm, 2.0-3.35 mm, 3.35-4.75 mm, 6.0-8.0 mm, 8.0-10.0 mm, 10.0-12.0 mm, 12.0-14.0 mm, 14.0-16.0 mm, 16.0-18.0 mm, 18.0-20.0 mm.

[0012] The applicant found that the smaller the particle size after crushing, the more helpful it is for the subsequent removal of heavy metals; however, if the particle size is too small, it will increase the cost and have no significant effect on improving the removal efficiency of heavy metal arsenic. The applicant found through trial that the alkali solution soaking treatment technology in this application can effectively remove arsenic from rice straw within the range of 0.425 to 10 mm.

[0013] More preferably, the rice straw is crushed to a particle size of no more than 10 mm. For example, the rice straw can be crushed to a particle size of 0.25-0.425 mm, 0.425-0.5 mm, 0.5-0.6 mm, 0.6-0.71 mm, 0.71-0.85 mm, 0.85-1.0 mm, 1.0-1.18 mm, 1.18-1.40 mm, 1.40-2.0 mm, 2.0-3.35 mm, 3.35-4.75 mm, 5.0-6.0 mm, 6.0-8.0 mm, 8.0-10.0 mm.

[0014] Preferably, the concentration of the alkali solution is 1-10 g / 100 mL based on the volume of water. For example, the concentration of the alkali solution can also be 1-2 g / 100 mL, 2-4 g / 100 mL, 4-6 g / 100 mL, 6-8 g / 100 mL, or 8-10 g / 100 mL.

[0015] The higher the alkali solution concentration, the better the arsenic removal effect. However, too high an alkali solution concentration has no significant effect on improving the arsenic removal effect. At the same time, it will increase the water used for washing the treated straw and the subsequent wastewater treatment costs.

[0016] More preferably, based on the volume of water, the concentration of the alkali solution is 1 g / 100 mL, 2 g / 100 mL, 4 g / 100 mL, 6 g / 100 mL, 8 g / 100 mL, or 10 g / 100 mL.

[0017] Preferably, the temperature of the alkali solution immersion treatment is 25-100° C. For example, the alkali treatment temperature can also be 25-30° C., 30-40° C., 40-50° C., 50-60° C., 60-70° C., 70-80° C., 80-90° C., 90-100° C.

[0018] If the temperature of the alkaline solution treatment is too low, the arsenic in the straw cannot be fully dissolved in the alkaline solution, resulting in poor arsenic removal effect; if the temperature of the alkaline solution treatment is too high, it will increase the heat input in the treatment process, thereby increasing the treatment cost. At the same time, high-temperature cooking with alkaline solution will remove a large amount of lignin and hemicellulose in the straw, resulting in a large sample loss.

[0019] More preferably, the temperature of the alkali solution immersion treatment is 25°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, or 90°C.

[0020] Preferably, the alkali solution soaking treatment time is at least 0.5h. For example, the alkali solution soaking treatment time can also be 0.5h-5h, 5h-20h, 20h-35h, 35h-50h, 50h-65h, 65h-80h, 80h-96h.

[0021] If the alkali solution soaking treatment time is too long, the treatment efficiency will be affected; if the alkali solution soaking treatment time is too short, the arsenic in the straw will not be fully dissolved, affecting the removal effect.

[0022] More preferably, the alkali solution immersion treatment time is 2 hours, and can also be 1 hour, 1.5 hours, 3 hours, 6 hours, 12 hours, or 24 hours.

[0023] Preferably, 5 to 20 mL of the alkali solution is used for soaking 1 g of the rice straw, such as 5 to 8 mL, 8 to 10 mL, 10 to 12 mL, 12 to 14 mL, 14 to 16 mL, 16 to 18 mL, or 18 to 20 mL of the alkali solution.

[0024] More preferably, with respect to 1 g of the rice straw, 10 mL of the alkali solution is used for soaking treatment, and 8 mL, 12 mL, 14 mL, 16 mL, 18 mL, and 20 mL of the alkali solution can also be used for soaking treatment.

[0025] Preferably, the alkali in the alkali solution is any one or more of NaOH, KOH, Ca(OH)2, Na2CO3, NH4OH or Mg(OH)2, and the solvent in the alkali solution is water.

[0026] Although some acid solutions can also remove arsenic, the overall removal effect is worse than that of alkaline solutions. In addition, considering the potential utilization value of arsenic removal from rice straw, alkaline solution treatment can provide the alkaline environment required for subsequent treatment.

[0027] More preferably, the alkali in the alkali solution is NaOH or KOH.

[0028] In a more preferred embodiment, the alkali in the alkali solution is NaOH and / or KOH, and based on the volume of water, an arsenic removal efficiency of more than 75% can be achieved under the conditions that the alkali solution concentration is 1-4 g / 100 mL, the rice straw is crushed to a particle size of no more than 10 mm, the soaking temperature is 25-90° C., and the soaking time is not less than 0.5 h.

[0029] In a more preferred embodiment, the alkali in the alkali solution is Ca(OH)2, and based on the volume of water, an arsenic removal efficiency of more than 75% can be achieved under the conditions that the alkali solution concentration is 2-6 g / 100 mL, the rice straw is crushed to a particle size of no more than 10 mm, the soaking temperature is 50-90°C, and the soaking time is not less than 2 h.

[0030] The second aspect of the present invention also claims the use of the above-mentioned alkali solution for removing heavy metals from heavy metal-enriched rice straw.

[0031] Preferably, the alkali in the alkali solution is any one or more of NaOH, KOH, Ca(OH)2, Na2CO3, NH4OH or Mg(OH)2, and the solvent in the alkali solution is water.

[0032] More preferably, the alkali in the alkali solution is NaOH or KOH.

[0033] Preferably, the heavy metal is arsenic.

[0034] Preferably, the concentration of the alkali solution is 1-10 g / 100 mL based on the volume of water. For example, the concentration of the alkali solution can also be 1-2 g / 100 mL, 2-4 g / 100 mL, 4-6 g / 100 mL, 6-8 g / 100 mL, or 8-10 g / 100 mL.

[0035] More preferably, based on the volume of water, the concentration of the alkali solution is 1 g / 100 mL, 2 g / 100 mL, 4 g / 100 mL, 6 g / 100 mL, 8 g / 100 mL, or 10 g / 100 mL.

[0036] Normal rice straw refers to the straw remaining after the harvest of healthy rice without serious pests and diseases. It can be returned to the field or other forms of resource utilization. The heavy metal content of normal rice straw should meet the relevant national and industry standards, but there is currently no specific limit standard value for arsenic in rice straw. The existing standards are mainly for the arsenic content in rice grains and rice, not straw. The relevant national regulations stipulate that the limit of inorganic arsenic in rice grains is 0.35 mg / kg, and the limit of inorganic arsenic in rice is 0.2 mg / kg. These standard values ​​can provide a reference for restoring arsenic-enriched rice straw to the normal range.

[0037] As described above, the method of the present invention has the following beneficial effects:

[0038] The present application provides a simple, efficient and economical method, which can remove heavy metals in rice straw in a targeted manner and significantly improve the removal efficiency. The straw treated by this method can be safely reused or disposed of naturally. DETAILED DESCRIPTION

[0039] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0040] Before further describing the specific embodiments of the present invention, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments described below; it should also be understood that the terms used in the examples of the present invention are for describing the specific embodiments rather than for limiting the scope of protection of the present invention; in the present specification and claims, unless otherwise expressly stated herein, the singular forms "a", "an" and "the" include plural forms.

[0041] When the embodiments give numerical ranges, it should be understood that, unless otherwise specified in the present invention, both endpoints of each numerical range and any numerical value between the two endpoints can be selected. Unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those generally understood by those skilled in the art. In addition to the specific methods, equipment, and materials used in the embodiments, according to the grasp of the prior art by those skilled in the art and the record of the present invention, any methods, equipment, and materials of the prior art similar or equivalent to the methods, equipment, and materials described in the embodiments of the present invention can also be used to realize the present invention.

[0042] Through various attempts and practices, the applicant of the present application unexpectedly discovered that soaking rice straw containing excessive heavy metals, especially arsenic, in alkaline solution can remove the heavy metals quickly and effectively, thereby facilitating the subsequent further utilization and disposal of the rice straw.

[0043] In a specific embodiment, the rice straw can be arsenic-enriched rice straw, which is grown in arsenic-contaminated soil and enriched and formed in the rice straw. The general arsenic content in this kind of rice straw is above 10 mg / kg, which can be obtained by ICP-MS testing.

[0044] In order to further illustrate the technical solution and its technical effects and technical problems of the present application, the following specific examples and comparative examples of the present application provide a method for removing heavy metals from rice straw, wherein the heavy metal is arsenic, and the specific steps are as follows:

[0045] 1) crushing the arsenic-enriched rice straw;

[0046] 2) soaking the crushed rice straw with the reagents shown in Table 1, with 10 mL of alkali solution used for 1 g of rice straw;

[0047] 3) The treated rice straw is rinsed with tap water, dried and then the arsenic content thereof is detected by ICP-MS.

[0048] The arsenic content in the arsenic-enriched rice straw is 7220.84 μg / kg.

[0049] Examples 1 to 7, Comparative Examples 1 to 2 and Blank Control Example

[0050] The specific reagent types and reagent concentrations used in Examples 1 to 7, Comparative Example 1, Comparative Example 2 and the blank control example are shown in Table 1; wherein in the Examples, Comparative Examples and the blank control example in Table 1, the particle size of the straw after crushing is 0.250 to 0.425 mm, the soaking treatment temperature is 50° C., and the time is 2 h.

[0051] After testing, the removal effect is shown in Table 1.

[0052] Table 1

[0053]

[0054] The solvent of the removal reagent is water.

[0055] From the results in Table 1, it can be seen that at the same concentration, the arsenic removal effect of monovalent alkali (NaOH or KOH) is better than that of divalent alkali (Ca(OH)2 or Mg(OH)2), and the use of NaOH aqueous solution as an arsenic removal agent has the best effect on arsenic removal. Most preferably, under the same conditions, the NaOH aqueous solution has the best removal effect at a concentration of 4g / 100mL.

[0056] Example 8

[0057] Rice straw with different arsenic contents was selected as raw material, and the other aspects were the same as those in Example 3. The arsenic removal effect is shown in Table 2.

[0058] Table 2

[0059]

[0060] The results in Table 1 show that regardless of the initial arsenic content of the straw, the arsenic removal effect reached more than 95%, showing consistency.

[0061] Examples 9 to 12, Comparative Example 3

[0062] The difference between Examples 9 to 12 and Comparative Example 3 and Example 3 is that the particle sizes after crushing are different, and the other aspects are the same. The removal effect is shown in Table 3.

[0063] Table 3

[0064]

[0065] The results in Table 3 show that when the NaOH aqueous solution with a concentration of 4g / 100mL is still used as the arsenic removal agent, the total arsenic removal rate is only 42.06% when the straw is crushed to 100mm; when the straw is crushed to 0.425mm-10mm, the arsenic removal efficiency is increased to more than 90%. Most preferably, the straw crushing particle size of 0.250-0.425mm has the best removal effect. In addition, when the straw particle size is further reduced to less than 0.250mm, its total arsenic removal rate is almost the same as the removal rate under the crushing particle size of 0.250-0.425mm. It can also be seen that too small a crushing particle size will not only increase the processing cost, but also have no significant effect on improving the removal efficiency of heavy metal arsenic.

[0066] Embodiments 13 to 17

[0067] The difference between Examples 13 to 17 and Example 2 is only the change in one or more of the soaking temperature, time, and concentration of the removal agent. The specific changes are shown in Table 4, and the removal effects are also shown in Table 4.

[0068] Table 4

[0069]

[0070] The experimental data in Table 4 show that under normal temperature conditions, whether 2g / 100mL or 4g / 100mL NaOH aqueous solution is used as the arsenic removal agent, as long as the soaking time is not less than half an hour, an arsenic removal efficiency of more than 75% can be achieved. In addition, the experiment also found that increasing the temperature can further improve the arsenic removal efficiency, but too high a temperature will not only increase the processing cost, but also cause a large loss of lignin and hemicellulose in the straw, which may affect the subsequent reuse of rice straw. Therefore, when selecting the optimal treatment temperature, it is necessary to comprehensively consider the arsenic removal efficiency and the reuse value of the straw.

[0071] Examples 18 to 21

[0072] The difference between Examples 18 to 21 and Examples 6 to 7 is only in the concentration of the remover and / or the immersion treatment temperature, and the others are the same. The removal effects are detailed in Table 5.

[0073] Table 5

[0074]

[0075] The data in Table 5 show that within the temperature range of 50 to 90°C, more than 75% of the arsenic in rice straw can be effectively removed by soaking in a 2 to 6 g / 100 mL Ca(OH)2 aqueous solution for 2 hours.

[0076] In summary, this application not only provides assistance for selecting suitable arsenic removers and optimizing removal conditions, but also provides important reference information for the effective utilization of rice straw.

[0077] The above examples are for the purpose of illustrating the embodiments disclosed by the present invention and are not to be construed as limitations of the present invention. In addition, the various modifications listed herein and the variations of methods and compositions in the invention are obvious to those skilled in the art without departing from the scope and spirit of the present invention. Although the present invention has been specifically described in conjunction with various specific preferred embodiments of the present invention, it should be understood that the present invention should not be limited to these specific embodiments. In fact, various modifications obvious to those skilled in the art as described above to obtain the invention should be included within the scope of the present invention.

Claims

1. A method for removing heavy metals from rice straw, characterized in that: The rice straw is crushed and soaked in alkaline solution to remove heavy metals.

2. The method according to claim 1, characterized in that The heavy metal is arsenic; and / or, based on the mass of the rice straw, the content of the heavy metal is not less than 0.2 mg / kg; and / or, crushing the rice straw to a particle size not exceeding 20 mm; preferably not exceeding 10 mm; And / or, the method further comprises a rinsing and / or drying step.

3. The method according to claim 1, characterized in that Based on the volume of water, the concentration of the alkali solution is 1 to 10 g / 100 mL.

4. The method according to claim 1, characterized in that The temperature of the alkaline solution soaking treatment is 25-100°C.

5. The method according to claim 1, characterized in that The alkali solution soaking treatment time is at least 0.5h.

6. The method according to claim 1, characterized in that With respect to 1 g of the rice straw, 5 to 20 mL of the alkali solution is used for soaking treatment.

7. The method according to claim 1, characterized in that The alkali in the alkali solution is any one or more of NaOH, KOH, Ca(OH)2, Na2CO3, NH4OH or Mg(OH)2, and the solvent in the alkali solution is water.

8. Alkaline solution is used to remove heavy metals from rice straw enriched with heavy metals.

9. The use according to claim 8, characterized in that The alkali in the alkali solution is any one or more of NaOH, KOH, Ca(OH)2, Na2CO3, NH4OH or Mg(OH)2, and the solvent in the alkali solution is water.

10. The use according to claim 8, characterized in that The heavy metal is arsenic; and / or, based on the volume of water, the concentration of the alkali solution is 1-10 g / 100 mL.

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