CN110420985B

CN110420985B - Method for repairing cadmium-contaminated soil by adopting composite soil repairing agent

Info

Publication number: CN110420985B
Application number: CN201910541010.9A
Authority: CN
Inventors: 袁兴中; 彭叶琼; 蒋龙波; 熊婷; 姚赛; 杨羽茜; 陈鸿鹄
Original assignee: Hunan University
Current assignee: Hunan University
Priority date: 2019-06-21
Filing date: 2019-06-21
Publication date: 2022-02-08
Anticipated expiration: 2039-06-21
Also published as: CN110420985A

Abstract

The invention discloses a method for repairing cadmium-contaminated soil by adopting a composite soil repairing agent, which comprises the following steps: firstly, preparing the composite soil remediation agent, then uniformly mixing the composite soil remediation agent with cadmium-contaminated soil, finally planting rice seedlings, maintaining the water content of the soil to be 50% -80%, wherein the remediation time is the whole growth period of rice, the composite soil remediation agent consists of a hydroxyapatite-calcium silicate composite material and biological carbon, the mass ratio of the hydroxyapatite-calcium silicate composite material to the biological carbon is 1:9-9:1, the addition amount of the hydroxyapatite-calcium silicate composite material to the total soil mass of the soil to be remedied is 5wt%, the preferred mass ratio of the hydroxyapatite-calcium silicate composite material to the biological carbon is 1:9-9:1, the preferred mass ratio of the hydroxyapatite-calcium silicate composite material to the biological carbon is 1:9-4:6, and the more preferred mass ratio is 4:6, the two have synergistic effect, the composite repairing agent can effectively reduce the cadmium content in the soil in a short period of 7 days for repairing, simultaneously convert the cadmium in the soil from an active form to a stable form, effectively reduce the bioavailability, obviously reduce the cadmium metal content in roots and stems of the rice after the rice grows for 152 days, and effectively inhibit the absorption of the rice to the cadmium in the soil.

Description

Method for repairing cadmium-contaminated soil by adopting composite soil repairing agent

Technical Field

The invention belongs to the technical field of soil remediation, and particularly relates to a method for remediating cadmium-contaminated soil by using a hydroxyapatite-calcium silicate composite material in cooperation with a composite soil remediation agent of biochar so as to inhibit rice from absorbing cadmium in the soil.

Background

The heavy metal pollution of soil is one of the most extensive and most harmful environmental problems in the current environmental pollution. Heavy metals in soil are of great concern because of their poor mobility, long persistence, and the inability to be easily degraded. The metal cadmium is considered as a very dangerous environmental pollution element, and the elemental cadmium cannot exist stably in nature and mainly exists in sulfur cadmium ores and zinc ores. Since the beginning of the 20 th century, the production and use of cadmium have been increasing because cadmium and its compounds have been widely used in the manufacturing processes of nickel-cadmium batteries, pigments, alloys, electroplating, plastic products, and the like. The release of cadmium to the environment is counted to about 30000 tons every year all over the world, and 82% -94% of cadmium can enter the soil.

Rice is a food crop cultivated in China in a large quantity, and the cadmium pollution of soil can have serious negative effects on the production of the rice. It is known that plants grow by absorbing nutrients in soil through their root system, and thus plants adsorb cadmium more easily than animals. Cadmium in soil affects photosynthesis and respiration of plants, affects metabolism of nutrient sources of the plants, destroys shapes of plant cells and affects health of the plants. Scientists find that the cadmium adsorption capacity of the root system of rice is far stronger than that of corn, soybean and other crops due to the specificity of genes, and the cadmium is accumulated in roots, stems, leaves and grains of rice in large quantity, so that the yield, the quality and the whole farmland ecosystem of rice are influenced, and the health of human beings is endangered through a food chain.

The remediation technology of the cadmium-polluted soil is mainly divided into an in-situ remediation technology and an ex-situ remediation technology. The in-situ repair mode mainly comprises a physical technology, a chemical technology and a biological repair technology. The physical technology comprises a soil dressing and soil turning method, heat treatment separation, electrokinetic repair, isolation embedding and the like. Chemical techniques include chemical fixation, chemical leaching, chemical redox, and the like. Bioremediation techniques include phytoremediation, microbial remediation, animal remediation, and the like. The bioremediation method generally has the problems of low remediation efficiency, high treatment cost and the like. The traditional physical repair methods such as landfill, physical leaching, soil turning repair and the like have large engineering quantity and high cost, and often cause the damage of the soil structure and the loss of certain nutrient elements. Chemical fixation restores soil by adding a chemical restoring agent into soil, and is emphasized due to the characteristics of simple operation, low investment and the like, however, the conventional soil restoring agent such as lime, biochar, compost and the like has different problems, for example, biochar can reduce the biological effectiveness of heavy metal pollutants in soil through adsorption, and the restoring effect is realized. However, organic matters in the soil are easily mineralized, and the change of the pH value of the soil can also cause desorption, so that heavy metal pollutants can be released again, the restoring effect of the biochar is unstable, rebounding occurs, and in addition, the problems that the reduction efficiency of the existing other restoring agents is not obvious enough, the physicochemical property of the soil is adversely affected and the like exist.

Disclosure of Invention

In order to solve the problems of unstable repairing effect, poor repairing effect and strong cadmium adsorption capacity of rice root systems of the existing single activated carbon, the invention provides a method for repairing cadmium-polluted soil by adopting a composite soil repairing agent.

The technical scheme of the invention is as follows:

a method for repairing cadmium-contaminated soil by adopting a composite soil repairing agent comprises the following steps:

firstly, preparing the composite soil repairing agent, then uniformly mixing the composite soil repairing agent and cadmium-polluted soil, finally planting rice seedlings, maintaining the water content of the soil to be 50% -80%, repairing for the whole growth period of the rice,

the composite soil remediation agent consists of a hydroxyapatite-calcium silicate composite material and biochar, the mass ratio of the hydroxyapatite-calcium silicate composite material to the biochar is 1:9-9:1, and the addition amount of the composite soil remediation agent accounts for 5wt% of the total soil mass of the soil to be remediated.

The biological carbon is poplar-based biological carbon.

The mass ratio of the hydroxyapatite-calcium silicate composite material to the poplar-based biochar is 1:9-4: 6.

The hydroxyapatite-calcium silicate hydrate in the composite soil remediation agent is synthesized by recovering phosphorus from calcium silicate in phosphate under the condition that the pH value is 5 +/-0.2.

The preparation method of the compound soil remediation agent comprises the following steps:

ultrasonically dispersing calcium chloride into water to obtain a calcium chloride dispersion liquid;

ultrasonically dispersing sodium silicate into water to obtain a dispersion liquid of the sodium silicate;

dropwise adding the calcium chloride solution into the sodium silicate solution under ultrasonic wave for reaction, washing with deionized water after the reaction is finished, and filtering to obtain a precipitate product, namely calcium silicate hydrate;

adding the calcium silicate hydrate into a phosphate solution, continuously reacting for 24 hours, adding acid liquor or alkali liquor to enable the pH of the mixed liquor to be 5.0 +/-0.2, filtering to obtain the hydroxyapatite-calcium silicate composite material,

and mixing the hydroxyapatite-calcium silicate composite material and biological carbon according to the mass ratio of 1:9-9:1 to obtain the soil remediation agent.

The liquid-solid ratio of the calcium chloride solution and the liquid-solid ratio of the sodium silicate solution are both 9-11, the mass ratio of the calcium chloride to the sodium silicate is 1.0-1.4, and the reaction time of the calcium chloride to the sodium silicate is 20-30 h.

The concentration of phosphorus in the phosphate solution is 50-70mg/L, the mass ratio of the calcium silicate hydrate to the phosphorus in the phosphate is 8: 1-12: 1, the reaction is carried out at room temperature for 20-30h, and the rotating speed is 120-240 rpm.

H in the acid liquor⁺The concentration of (A) is 0.01-0.05M.

OH in the alkali liquor^-The concentration of (A) is 0.01-0.05M.

The repair time is 7-152 days.

The invention has the technical effects that:

the invention relates to a method for repairing cadmium contaminated soil by using a composite soil repairing agent, which comprises the steps of firstly preparing calcium silicate hydrate by using ultrasonic-assisted sol-gel reaction, generating hydroxyapatite by using the reaction of the calcium silicate hydrate and a phosphate solution, recovering phosphorus in the solution by using the calcium silicate hydrate, forming hydroxyapatite-calcium silicate, separating the hydroxyapatite-calcium silicate composite material from the solution, mixing and compounding the hydroxyapatite-calcium silicate composite material with biochar according to the mass ratio of 1:9-9:1 to prepare the composite soil repairing agent, mixing the composite soil repairing agent with contaminated soil, finally planting rice seedlings, and maintaining the water content of the soil to be 50% -80%, so that the cadmium contaminated soil is repaired in the whole growth cycle of rice by using the composite repairing agent, and the rice is inhibited from absorbing cadmium in the soil.

The calcium silicate hydrate is a loose laminated structure, has a uniform shape and a large number of gaps, can recover phosphate from a solution more easily to form hydroxyapatite through a single crystallization process, can be built on the basis of a calcium silicate framework easily to form a fine and uniform fiber structure, and finally forms a hydroxyapatite-calcium silicate composite material, wherein the composite material comprises the hydroxyapatite as a main component, the calcium silicate is a small amount, and the calcium silicate not only serves as the framework, but also can effectively adsorb heavy metal ions. Compared with single hydroxyapatite, the composite material has a larger and more pore structures and very strong heavy metal adsorption capacity. The hydroxyapatite-calcium silicate composite material is a stable substance, can adsorb heavy metal cadmium in a plant absorbable state in soil in a mode of electrostatic attraction, production precipitation or mineral, forms fixed ions which are difficult to desorb in the environment, can be used by being compounded with biochar, can prevent the cadmium adsorbed on the activated carbon from being released into the soil again and entering the rice body, solves the problem that the soil polluted by the cadmium and repaired by the biochar is easy to mineralize, and achieves a long-term repairing effect.

Meanwhile, the surface of the biochar is rich in carboxyl, hydroxyl, acid anhydride and a series of functional groups, and has more negative charges, so that heavy metal ions (Cu, Cd, Pb and Zn) can be effectively fixed, the biochar has unique surface properties, morphological structures and abundant and discrete pore systems, and can be used together with hydroxyapatite-calcium silicate to better improve the physical and chemical properties of soil, improve the pH value of the soil, increase the negative charges on the soil surface, enhance adsorption and reduce the bioavailability of metal ions.

Preferably, the mass ratio of the hydroxyapatite-calcium silicate composite material to the biochar in the composite repairing agent is 1:9-9:1, and the further preferred ratio is 4:6, the two have synergistic effect, and the composite repairing agent can effectively reduce the cadmium content in the soil in a short period of 7 days for repairing; in the period of 152 days of rice growth, cadmium is continuously repaired and fixed, the cadmium content in soil is reduced, the cadmium in the soil is converted from an active form into a stable form, the bioavailability is effectively reduced, the cadmium metal content in roots and stems of rice is obviously reduced after the rice grows for 152 days, and the absorption of the rice to the cadmium in the soil is effectively inhibited.

In conclusion, the method for repairing the cadmium-polluted soil by using the hydroxyapatite-calcium silicate hydrate and the biochar is expected to overcome the defect that the hydroxyapatite-calcium silicate hydrate and the biochar are independently applied, achieves the long-term heavy metal-polluted soil repairing effect, and is low in cost, simple, efficient and environment-friendly.

Compared with the prior art, the invention also has the advantages that:

1. besides strong heavy metal adsorption regulation capacity, the hydroxyapatite-calcium silicate hydrate composite material can also play a role in improving the physicochemical property of soil after being applied to the soil, such as increasing the water-soluble carbon content of the soil, increasing available phosphorus in the soil and the like.

2. The main component in the composite repairing agent is biochar, the source of the biochar is wide, the synthesis and transportation cost is low, and the cost is greatly reduced; and the biochar and the hydroxyapatite-calcium silicate are non-toxic, green and environment-friendly materials, and do not bring toxic action to the soil environment after being applied to the soil.

Drawings

FIG. 1 is a scanning electron micrograph of calcium silicate hydrate prepared according to example 1 of the present invention;

FIG. 2 is a scanning electron microscope image of a hydroxyapatite-calcium silicate composite material prepared in example 1 of the present invention;

FIG. 3 is a graph showing the content of heavy metal cadmium in soil after the cadmium-contaminated soil is repaired by hydroxyapatite-calcium silicate hydrate and biochar with different addition ratios;

FIG. 4 is a comparison graph of the content of cadmium in different forms extracted from soil after the cadmium-contaminated soil is repaired by hydroxyapatite-calcium silicate hydrate and biochar with different addition ratios;

FIG. 5 is a graph showing the comparison of the heavy metal content in each organ of rice in soil after repairing cadmium contaminated soil with hydroxyapatite-calcium silicate hydrate and biochar at different addition ratios.

Detailed Description

The invention will be further described with reference to the accompanying figures 1 to 5 of the specification and preferred embodiments, without thereby limiting the scope of the invention.

Example 1

A method for repairing cadmium contaminated soil by using hydroxyapatite-calcium silicate hydrate and biological carbon in a synergistic manner comprises the following steps:

(1) preparation of composite soil repairing agent

(a) Preparing a hydroxyapatite-calcium silicate composite material:

dissolving 6g of calcium chloride in 60ml of water to obtain a calcium chloride solution;

dissolving 5g of sodium silicate in 50ml of water to obtain a sodium silicate solution;

the resulting calcium chloride solution was slowly added dropwise to the sodium silicate solution under sonication conditions to produce a white sol and gradually hardened and sampled, taken out and incubated in a plastic container for 24 hours at room temperature. In order to remove free calcium hydroxide, washing and filtering with deionized water after the reaction is finished, and finally drying at 80 ℃ for 24 hours to obtain calcium silicate hydrate, wherein a scanning electron microscope picture is shown in figure 1;

0.26gKH₂PO₄Dissolving in 1L deionized water to prepare phosphate solution;

adding 0.6g of calcium silicate hydrate into the 1L of phosphate solution, continuously reacting for 24 hours in a constant temperature shaking table at the speed of 25 ℃ and 180rpm, adding 0.01mol/L HCl or 0.01mol/L NaOH to enable the pH of a mixed solution to be 5, filtering after the reaction is finished, washing with water, and finally drying for 12 hours at the temperature of 60 ℃ to obtain the hydroxyapatite-calcium silicate composite material, wherein a scanning electron microscope picture is shown in figure 2;

(b) mixing the hydroxyapatite-calcium silicate composite material with poplar-based biochar to prepare a composite soil remediation agent;

(2) preparing cadmium-polluted soil:

the test soil is collected from a hoeing experimental base of Hunan agricultural university, cadmium-polluted soil is prepared by a method of artificially adding cadmium solution, the cadmium-polluted soil is cultured at a constant temperature of 25 ℃ for two months, so that the cadmium form tends to be stable, and the total cadmium content in the soil is measured to be 1.2mg/kg by a graphite furnace digestion method;

(3) repairing cadmium-polluted soil for cultivating rice:

preparing five groups of cadmium-polluted soil stabilized in the step (2), wherein each 4kg of cadmium-polluted soil is placed in a basin platinum, and the addition amount of the soil remediation agent accounts for 5% of the mass of the soil. Wherein, the first group of mixed materials added into the cadmium polluted soil are hydroxyapatite-calcium silicate hydrate and biochar 1:9(H1B9) in proportion, and are uniformly mixed; the second group of mixed materials added into the cadmium contaminated soil are hydroxyapatite-calcium silicate hydrate and biochar 2:8(H2B8) which are uniformly mixed; the third group is that the mixed material added into the cadmium polluted soil is hydroxyapatite-calcium silicate hydrate and biological carbon of 4:6(H4B6) which are mixed evenly; and the fourth group of mixed materials added into the cadmium polluted soil in the proportions of hydroxyapatite-calcium silicate hydrate, biological carbon of 10: 0(H10B0), mixing well; the fifth group did not add any repairing agent as blank control group (CK);

transplanting seedlings with the seedling age of 30 days into pots, wherein 3 holes are formed in each pot, 1 plant is formed in each hole, watering is carried out every day during the growth period, the soil is kept moist, the water content of the soil is kept to be about 60% of the maximum field water capacity (namely 2-3cm of water layer is kept), and 2 parallel samples are formed in each treatment group;

applying base fertilizer for 3 times, applying 1g of urea (containing 46% of nitrogen) to each pot 20 days after transplanting and 5 days before heading, and moving the pots randomly during the growth period to ensure sunshine;

(4) cadmium contaminated soil remediation condition test

And (3) testing the content of cadmium in soil: the soil remediation effect is expressed by the fixation rate of cadmium, after rice grows for 7 days, 28 days, 42 days, 84 days and 152 days, the TCLP extraction method is used for extracting cadmium in five groups of cadmium-polluted soil respectively, and an inductively coupled plasma emission spectrometer (ICP-OES) is used for measuring the cadmium content, and the result is shown in figure 3, wherein the fixation rate is 1-treatment group heavy metal content/blank control group heavy metal content; meanwhile, after the rice grows for 152 days, the form and the proportion of the cadmium element in the soil are known by a BCR four-step extraction method, and different forms and proportions of the cadmium element are measured by an inductively coupled plasma emission spectrometer (ICP-OES), wherein a comparison chart of the different forms of the cadmium is shown in FIG. 4;

testing the cadmium content in each organ of the rice: after the rice had grown for 152 days, the cadmium content in the roots, stems, leaves and fruits of rice planted in five groups of cadmium-contaminated soil was measured by TCLP extraction method, respectively, and the cadmium content was measured by inductively coupled plasma emission spectroscopy (ICP-OES), as shown in fig. 5 and table 2.

TABLE 1 comparison table of the fixation effect of different proportions of remediation agents on cadmium in soil

Treatment method (fixed rate of cadmium)	H1B9	H2B8	H4B6	H10B0
						7d	31.1％	27.3％	32.1％	42.1％
21d	25.7％	33.6％	39.8％	50.4％
					42d	30.7％	31.9％	38.5％	46.2％
70d	26.1％	27.2％	48.9％	50.0％
					152d	16.8％	21.3％	60.7％	57.3％

TABLE 2 cadmium content (mg/kg) of various organs of rice added with remediation agents in different proportions in cadmium contaminated soil

Rice organ	CK	H1B9	H2B8	H4B6	H10B0
						Root of herbaceous plant	5.42	3.54	3.34	2.89	3.07
Stem of a tree	0.42	0.36	0.31	0.22	0.28
						Leaf of Chinese character	0.39	0.22	0.14	0.12	0.14
(Fruit)	0.43	0.32	0.22	0.17	0.20

As can be seen from fig. 1-2, the microstructure of the prepared calcium silicate is a loose laminated structure, the hydroxyapatite-calcium silicate composite material is an excellent uniform fiber structure, the surface has more pore structures, and the fiber structure may correspond to calcium silicate;

as can be seen from fig. 3 and table 1, the fixing effect is better in the early stage (before 42 days) in the two groups of H1B9 and H2B8, but the fixing effect in the later stage is slightly rebounded, the material in the group of H4B6 fixes cadmium stably, the content of cadmium is continuously reduced, the fixing rate of cadmium reaches 60.7% after 152 days, and the optimal group in the composite repairing agent is H4B 6;

as can be seen from fig. 4, compared with CK, the composite repairing agent reduces the weak acid state of F1, increases the residue state of F4, and particularly, has the best effect on H4B6, reduces the weak acid state of F1 by 6.3, and increases the residue state of F4 by 7.9, which indicates that the composite repairing agent can reduce the bioavailability of cadmium to some extent;

as can be seen from FIG. 5 and Table 2, after 152 days, Cd accumulated mostly in the roots of rice, and the adsorption capacity of roots, stems, leaves and fruits to cadmium ions was root > stem > leaf > fruit. Compared with the blank control plant, the cadmium content in the roots, stems and leaves of the rice is reduced by adding the repairing agent, particularly the roots, stems, leaves and fruits of H4B6 are respectively reduced by 53.3%, 52.3%, 30.8% and 39.5%.

In conclusion, the composite repairing agent composed of the hydroxyapatite-calcium silicate hydrate and the biological carbon has an effective fixing effect on heavy metals in rice organs, especially when the ratio of the hydroxyapatite-calcium silicate hydrate to the biological carbon is 4:6(H4B6), the synergistic effect is far greater than a single effect, and compared with the single hydroxyapatite-calcium silicate hydrate, the cost is effectively reduced.

Claims

1. A method for repairing cadmium-contaminated soil by adopting a composite soil repairing agent is characterized by comprising the following steps:

the composite soil remediation agent consists of a hydroxyapatite-calcium silicate composite material and biochar, the mass ratio of the hydroxyapatite-calcium silicate composite material to the biochar is 4:6, and the addition amount of the composite soil remediation agent accounts for 5wt% of the total soil mass of the remediated soil;

the hydroxyapatite-calcium silicate hydrate in the composite soil remediation agent is synthesized by recovering phosphorus from calcium silicate in phosphate under the condition that the pH is =5 +/-0.2;

dropwise adding the dispersion liquid of the calcium chloride into the dispersion liquid of the sodium silicate under ultrasonic waves to react, washing with deionized water after the reaction is finished, and filtering to obtain a precipitate product, namely calcium silicate hydrate;

adding the calcium silicate hydrate into a phosphate solution, continuously reacting for 24 hours, adding acid liquor or alkali liquor to enable the pH of the mixed liquor to be =5.0 +/-0.2, filtering to obtain the hydroxyapatite-calcium silicate composite material,

mixing the hydroxyapatite-calcium silicate composite material and biological carbon according to a mass ratio of 4:6 to obtain the soil remediation agent; the biological carbon is poplar-based biological carbon.

2. The method according to claim 1, wherein the liquid-solid ratio of the dispersion liquid of the calcium chloride and the liquid-solid ratio of the dispersion liquid of the sodium silicate are both 9 to 11, the mass ratio of the calcium chloride to the sodium silicate is 1.0 to 1.4, and the reaction time of the calcium chloride to the sodium silicate is 20 to 30 hours.

3. The method according to claim 1, wherein the concentration of phosphorus in the phosphate solution is 50-70mg/L, the mass ratio of the calcium silicate hydrate to the phosphorus in the phosphate is 8: 1-12: 1, the reaction is carried out at room temperature for 20-30h, and the rotation speed is 120-240 rpm.

4. A process according to claim 1, characterized in that H is contained in the acid liquor⁺The concentration of (A) is 0.01-0.05M.

5. The method according to claim 1, characterized in that OH in the lye^-The concentration of (A) is 0.01-0.05M.

6. The method of claim 1, wherein the repair time is 7 to 152 days.