CN109454101B

CN109454101B - Method for removing soil cadmium by using ferric trichloride and phosphate

Info

Publication number: CN109454101B
Application number: CN201811171163.0A
Authority: CN
Inventors: 李�柱; 吴龙华; 赵婕; 郝金才
Original assignee: Jiangsu Firefly Environmenta Science Technology Co ltd; Institute of Soil Science of CAS
Current assignee: Jiangsu Firefly Environmenta Science Technology Co ltd; Institute of Soil Science of CAS
Priority date: 2018-09-30
Filing date: 2018-09-30
Publication date: 2020-10-13
Anticipated expiration: 2038-09-30
Also published as: CN109454101A

Abstract

The method for removing the cadmium in the soil by utilizing ferric trichloride and phosphate comprises the steps of adding a solution of 20-200 mM ferric trichloride/kg of soil into cadmium-polluted soil, adjusting the water content of the soil to be 100% of the soil weight, reacting until the pH of a system is stable, adding water, stirring and mixing, standing for settling soil particles, leading out an upper layer solution, adjusting the pH of the solution to 7, and adding soluble phosphate which is 0.2-2 wt.% of the soil weight for settling; and (4) carrying out solid-liquid separation on the precipitate, recovering the solid, and repeatedly returning the liquid to the soil for utilization or discharge. The invention aims at the cadmium-polluted soil, takes mild and nontoxic ferric trichloride and phosphate as remediation materials, can realize the efficient removal of cadmium in the soil, has small damage to the soil property and low phosphate usage amount, and can recover the phosphate remediation materials and metal resources, reduce the remediation water and safely discharge or recycle the remediated water.

Description

Method for removing soil cadmium by using ferric trichloride and phosphate

Technical Field

The invention belongs to a soil pollution treatment technology in the field of environmental protection, and relates to a method for removing soil cadmium by using ferric trichloride and phosphate.

Background

With the development of industry and agriculture, soil pollution harms ecological safety and human health, and becomes one of the main environmental problems affecting the sustainable development of society. In China, the total overproof rate of soil pollution is 16.1%, wherein the proportion of slightly, moderately and severely polluted points is 11.2%, 2.3%, 1.5% and 1.1%, respectively. Wherein the number of the inorganic heavy metal pollutant over-standard points accounts for 82.8 percent of all over-standard points, and the over-standard rate of the first point of the cadmium pollution of the soil is 7.0 percent. Cadmium is easily enriched in organisms in the soil environment and enters human bodies through food chains to cause chronic poisoning. Cadmium pollution events frequently occur in China, such as a carambola event in 2006, a Liuyang cadmium pollution event in 2009, a Guangxi dragon river cadmium overproof event in 2012 and the like.

At present, the remediation method aiming at the cadmium pollution of soil comprises a stable remediation technology, a plant absorption remediation technology, a leaching removal remediation technology and the like. The stable restoration technology has the best effect on the cadmium low-pollution soil, the plant absorption restoration is used for restoring the cadmium low-pollution soil, the cadmium stabilization is difficult to realize for the moderate to severe cadmium-polluted soil, and the plant restoration requires the restoration time of decades to decades and cannot be applied. The leaching remediation technology can rapidly remove the polluted cadmium from the soil, make up for the defects of stable remediation and plant absorption remediation, but has the defects of great damage to the soil structure, and a great amount of leaching solution generated after leaching, wherein the leaching solution contains a great amount of soil base ions such as Ca and Mg and complexing agents such as common citric acid and EDTA, and is difficult to treat by simple and rapid technology. Thereby making it difficult to put the leaching technique into practical use in contaminated soil. The technology utilizes phosphate and ferric trichloride, can efficiently and quickly remove cadmium in the polluted soil, breaks through the idea of stabilizing the cadmium in the polluted soil by the traditional phosphate, utilizes a mild reagent ferric trichloride to activate the cadmium in the soil into a solution, utilizes the phosphate to quickly remove the cadmium in the solution, and returns the treated solution to the soil, thereby realizing efficient remediation of the cadmium-polluted soil and quick and pollution-free discharge or reutilization of the activated liquid.

Disclosure of Invention

The technical problem to be solved is as follows: the invention aims to provide a method for removing soil cadmium by combining ferric trichloride and phosphate, which can be implemented in situ, is relatively environment-friendly, and can be used for quickly and efficiently removing soil cadmium, so that the ecological and environmental risks of soil cadmium are reduced.

The technical scheme is as follows: the method for removing the cadmium in the soil by utilizing ferric trichloride and phosphate comprises the steps of adding a solution of 20-200 mM ferric trichloride/kg of soil into cadmium-polluted soil, adjusting the water content of the soil to be 100% of the soil weight, reacting until the pH of a system is stable, adding water, stirring and mixing, standing for settling soil particles, leading out an upper layer solution, adjusting the pH of the solution to 7, and adding soluble phosphate which is 0.2-2 wt.% of the soil weight for settling; and (4) carrying out solid-liquid separation on the precipitate, recovering the solid, and repeatedly returning the liquid to the soil for utilization or discharge.

The soluble phosphate is water soluble or weak acid soluble salt.

The water soluble or weak acid soluble salt is potassium phosphate, dipotassium hydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate, trisodium phosphate, disodium hydrogen phosphate or calcium phosphate.

After the ferric trichloride is added, the pH value of a soil solution system is stabilized at 3-4.

And (3) adjusting the pH of the solution before precipitation to 7-7.5, and adding the ferric trichloride solution into soil, wherein iron ions can be hydrolyzed to form ferric hydroxide precipitate. While in the soil solid phase, due to Cl^-And H⁺When released, cations such as Ca and Zn exist in the soil (see figure 1), and the Ca and the Zn are main influencing elements in the invention. Therefore, the pH value is determined according to the concentration of Zn ions in the ferric trichloride activation solution, and when the concentration of the Zn ions is high, the pH value can be adjusted to 7.5, so that the competition of Zn and Cd on phosphate radical is reduced. The pH value of the phosphate precipitation reaction system is 7.5-9.

According to the method for removing the cadmium in the soil by using the ferric trichloride and the phosphate, when the liquid is recycled, the pH value of the solution is adjusted to 3-4, and then the recycling operation is carried out, or the solution is discharged after being treated.

Has the advantages that: the invention takes mild and nontoxic ferric trichloride and phosphate as the repair materials, the repair materials and the generated solid matters can be recycled, water resources can be repeatedly utilized in the repair process, the solid matters can be safely discharged after the repair, and the repair technology has the characteristics of high repair efficiency, environmental friendliness and in-situ implementation. In the range of pH 3-4 of the activated solution, the soil activation rate is high, and the soil structure (iron-aluminum-manganese oxide) is not damaged; the pH value of the system is adjusted to 7-7.5 before the phosphate precipitation reaction, the pH value of the precipitation reaction is controlled to be about 7.5-9 in the system, the phosphate dosage is small, and the precipitation efficiency is high. When the concentration of ferric trichloride is 50mM, 1 per mill of phosphate is adopted, according to the technical scheme, the cadmium in the soil is 31.3 mg.kg^-1Reduced to 3.52 mg/kg^-1The removal rate reaches 88.7 percent, and the remediation water can reach the II-grade surface water and the farmland irrigation water quality standard.

Drawings

FIG. 1 is a schematic diagram showing the pH value and the concentration of an activating element of a solution after oscillation for 24 hours under different ferric trichloride concentration treatments, wherein the soil-to-liquid ratio is 1: 2;

FIG. 2 is a graph showing the pH of a solution and the concentration of activated cadmium at different activation times of 30 mM (upper) and 50mM (lower) ferric trichloride at a soil to liquid ratio of 1: 2;

FIG. 3 is a graph showing the activated cadmium concentration in 50mM soil solution in comparison with ferric chloride after shaking for 6 hours;

FIG. 4 is a schematic diagram of the calculation and study of the precipitation reaction of an iron trichloride activated solution under different pH conditions;

FIG. 5 is a schematic diagram of the precipitation reaction of an activated solution of ferric trichloride in a calculation and study system under different pH conditions in the presence of 1.5 wt.% P;

FIG. 6 is a schematic diagram of the precipitation reaction at different concentrations P after adjusting different pH values in an iron trichloride activating solution of a research system;

FIG. 7 is a schematic flow chart of the present invention.

Detailed Description

The following detailed description does not limit the technical solutions of the present invention in any way, and all technical solutions obtained by means of equivalent substitution or equivalent transformation fall within the scope of the present invention.

The invention uses mild ferric trichloride reagent to activate cadmium in soil into solution, aiming at the solution (low pH, low concentration cadmium, high concentration Zn and Ca, etc.) after ferric trichloride activation, firstly raising the pH of the solution, removing a large amount of competitive ions (generally higher than Cd 1-2 orders of magnitude) such as solution Zn, etc., then adding non-toxic reagent phosphate into the solution for precipitation reaction, zinc hydroxide, zinc phosphate, hydroxyapatite, etc. formed under low phosphorus concentration form coprecipitation with solution low concentration cadmium or adsorb low concentration cadmium or form cadmium phosphate precipitate, then solid-liquid separation is carried out, the separated solid phase substance can be used for resource recovery, and the separated solution passes through the soil treated by ferric trichloride, so that the pH value of the treated soil is increased, the pH value of the precipitated solution is increased, residual phosphorus and other substances in the solution are adsorbed, and the reuse and safe discharge of remediation water can be realized.

Example 1

The soil polluted by cadmium in certain area of Jiangxi Yingtan has the pH of 5.89 and the cadmium concentration of 21.8 mg/kg^-1Adding 35 mM ferric chloride/kg soil solution,adding the cadmium-polluted soil into the cadmium-polluted soil, adjusting the water content of the soil to be 100% of the soil weight, reacting until the pH of the system is stable, adding water, stirring and mixing, standing the soil particles for sedimentation, leading out an upper solution, adjusting the pH of the solution to be 7, and adding phosphate (P content, potassium phosphate) with the weight of 0.7 per thousand of the soil weight for sedimentation; and (3) carrying out solid-liquid separation on the precipitate, recovering the solid, and repeatedly returning the liquid to the soil for utilization or discharge, wherein the total removal rate is 68.3%, and the treated water can reach the II-grade surface water and the water quality standard of farmland irrigation.

Example 2

Meanwhile, in example 1, the soil polluted by cadmium in Guangxi Guilin soil has pH of 6.60 and cadmium concentration of 10.1 mg/kg^-155 mM ferric chloride/kg soil and 0.5 wt.% per mill phosphate (P content, potassium phosphate) are added, the soil cadmium removal rate is 75.1%, and the treated water can reach the II-grade surface water and the water quality standard of farmland irrigation.

Example 3

Meanwhile, in example 1, the soil polluted by cadmium in certain areas of Yunnan Yangjiang has the pH value of 6.80 and the cadmium concentration of 23.6 mg/kg ^-1100 mM ferric chloride/kg soil and 1 wt.% per mill phosphate (P content, potassium phosphate) are added, the cadmium removal rate of the soil reaches 82.5%, and the treated water can reach the II-grade surface water and the water quality standard of farmland irrigation.

Example 4

Meanwhile, in example 1, the soil polluted by cadmium in Yunan Yangjiang has a pH of 7.02 and a cadmium concentration of 31.1 mg/kg ^-1100 mM ferric chloride/kg soil and 1 wt.% per mill phosphate (P content, potassium phosphate) are added, the soil cadmium removal rate is 88.7%, and the treated water can reach the II-level surface water and the water quality standard of farmland irrigation.

Claims

1. A method for removing soil cadmium by using ferric trichloride and phosphate is characterized in that a solution of 100 mM ferric trichloride/kg soil is added into cadmium-polluted soil, the water content of the soil is adjusted to be 100% of the soil weight, the soil reacts until the pH of the system is stable, water is added for stirring and mixing, soil particles are placed for sedimentation, an upper layer solution is led out, the pH of the solution is adjusted to 7, and potassium phosphate of 1wt. -%, of the soil weight is added for sedimentation; and (4) carrying out solid-liquid separation on the precipitate, recovering the solid, and repeatedly returning the liquid to the soil for utilization or discharge.