How to effectively control and mitigate the pollution and hazard of heavy metals in soils is an increasingly serious international problem, which is especially prominent in our country. In particular, it is difficult to find a low-cost, broad-spectrum and practical technical method for treating large-scale heavy-metal polluted farmland soils. Besides, it is especially difficult for treating heavy metal composite pollutants in farmland. For example, arsenic and cadmium composite pollutants, compared to a single pollutant, result in more complex environmental effects due to their interaction, and are more difficult to be treated. Increasing the pH of soil can effectively reduce the bioavailability of cadmium in soil and the content of cadmium in rice, but may increase the activity of As in soil. Moreover, flooding can reduce the content of Cd in rice, but increase the content of As in rice. Therefore, how to control the soil process and manage water and fertilizer under the condition of As/Cd composite pollution is very complicated.
In recent years, the technology of passivating heavy metal pollutants in farmland has received more and more attention. A soil-friendly conditioner is applied to passivate heavy metals in polluted soil and reduce the absorption of heavy metals by crops, thus realizing simultaneous control and use of the agricultural soil polluted by heavy metals, which is a new economical and safe idea of prevention and control of heavy metal pollution in soil. The soil heavy-metal passivation technology has relatively low investment, high repair efficiency, and simple operation; it has superiority for repairing soils polluted by heavy metals at a medium/low level, and can meet the current demand of our country for controlling heavy metal pollution of farmland soil and ensuring safety of agricultural products. Commonly used soil heavy-metal passivators include the following substances: lime, calcium carbonate, fly ash and other alkaline substances; hydroxyapatite, ground phosphorite, calcium hydrogen phosphate and other phosphates; natural or modified zeolite, bentonite and other minerals; blast furnace slag, slag and other silicon-containing fertilizers; and peat, farmyard manure, green manure, bio-organic carbon and other organic fertilizers. These passivators often have a good passivation effect on heavy metal pollution in soil. However, the heavy metal pollution of soil is often composite pollution resulted from coexistence of two or more kinds of metals. Different heavy metal ions have different physical and chemical properties, and their migration in the soil and their environmental behavior are also quite different. Using a single soil passivator to repair multi-metal polluted soils is often challenging, and it is difficult to find a single substance that can reduce the mobility of all heavy metal ions. At present, therefore, the passivation of soil is mainly focused on passivation of a single heavy metal element, and composite additives or a variety of repair methods are usually simultaneously used for multi-metal composite pollution. If a multi-functional heavy-metal passivation material that can passivate multiple pollutants at the same time can be prepared, it will inevitably reduce the repair cost and improve the repair efficiency.
The toxicity and bioavailability of heavy metals in soil are not only related to the total amount of heavy metals, but also mainly affected by the physical and chemical properties of the soil. Iron oxides in soil are the key factors that control the morphological transformation and bioavailability of heavy metals in the soil. Iron is the most important redox active element in red soils (Wang et al., 2009) with a geochemical abundance of 5.1%, ranking fourth (Zhao Qiguo, 2002); iron is trapped in the surface of soil particles mainly in the form of free iron oxides, having high geochemical activity; it directly affects many soil processes (Borch et al., 2010). Iron oxides and other iron minerals, due to their very large specific surface area, chemical activity and morphological transformation, have higher adsorption capacity for a large number of heavy metals and oxygen-containing anions (such as PO43−, AsO43−, CrO42−, etc.), and are often used as treatment agents for wastewater polluted by arsenic, chromium and other heavy metals. However, because iron and its oxides are easy to aggregate, they combine with the elements Mg, P, Ca and S in soils and affect their chemical properties, which hinders the application of iron and its oxides in the repair of heavy-metal polluted soils.
Sulfur in soils plays an important role in controlling the activity and bioavailability of heavy metals. SO42− entering the soil is quickly reduced to S2− under anaerobic conditions, with S2− forming sulfides with metal ions to stabilize heavy metals; SO42− generated by organosulfur mineralization of the soil and SO42− entering the soil due to atmospheric deposition and fertilization are rapidly reduced to S2− under anaerobic conditions, with S2− forming sulfides with metal ions. Metal sulfides in anaerobic soils are stable and insoluble, and have a significant impact on the concentration of heavy metal ions in soil pore water. In paddy fields, therefore, the application of sulfur-containing fertilizers, especially at the rice seedling stage, plays a very important role in stabilizing heavy metals in soils. However, when sulfur is oxidized in the soil to form SO42−, a lot of H+ is generated, resulting in the activation of heavy metals. Therefore, the application of sulfur-containing fertilizers alone has the risk of reactivating heavy metals in the later stage of rice growth. If the sulfur-containing fertilizers are applied together with other soil conditioners to avoid oxidation again, the passivation effect and application scope of sulfur-containing passivators for heavy metal cadmium will be inevitably increased.
Silicate fertilizer can reduce the availability of heavy metals in soil and inhibit the absorption and accumulation of heavy metals by crops. In recent years, studies have shown that silicon can increase resistance of plants to heavy metal toxicity, and is easy to use and cheap, thus having attracted people's attention. The current research has shown that the application of silicon fertilizer can increase the resistance of plants such as rice to heavy metals such as manganese, iron, cadmium and aluminum, and reduce the absorption and accumulation of these heavy metals by plants such as rice. Although the total content of Si in soil is very high, Si exists in the form of silicate; the silicon that plant can absorb and use is monosilicic acid (Si(OH)4), and so the content of effective silicon in soil is often very low. The content of silicic acid in soil solution is typically 0.1-0.6 mM (Epstein, 1994). With the development of intensified agriculture, long-term continuous cropping of crops (especially gramineous plants such as rice) will result in continuous absorption of the effective Si from the soil, leading to a reduction of crops. Due to the strong desiliconization in the red soil area, the content of the effective silicon in the soil tends to be lower. Therefore, the role of silicon fertilizer in agricultural production has drawn more and more attention. However, most of the current silicon fertilizers come from blast furnace slag, silicon-bearing ores and the like, and the silicon in these silicon fertilizers is less effective. Silicic acid and other silicon-containing salts, once applied to the soil, are also easily fixed by soil minerals.
Biochar is a new type of material, and is a product of pyrolysis of biomass under hypoxic conditions. Biochar, whose particles are small, finely distributed, and light in weight, is a black porous solid, and is mainly composed of carbon, oxygen and other elements, with the carbon content therein generally above 70%. Biochar can be prepared from a wide range of raw materials, such as sawdust, straw, industrial organic waste, municipal sludge, palm filaments, coconut filaments, etc. Biochar is characterized by loose porous structure, large specific surface area, a lot of negative charge on the surface and so on, which make biochar have good adsorption properties. As a soil structure improver or a repair agent for soil pollution, biochar can improve the pH value of acidic soil and increase the cation exchange capacity, thus adsorbing the pollutants and heavy metals in the soil, reducing the accumulation of Cd, Pb and Zn in the crop body, accelerating the microbial metabolism and improving the microbial biomass of the soil, thereby enhancing the soil fertility, increasing the output of rice and other crops, and improving the quality of agricultural products. However, mobility of As increases with the pH of soil and As tends to bond to oxides and hydroxides of Fe, Al and Mn that have anion exchange sites in the soil, which mean that adding biochar to the soil does not necessarily control the bioavailability of As. Zheng et al. found that when biochar was applied to paddy soil polluted by heavy metals, biochar promoted the formation of iron film in paddy soil, which affected the migration of Cd, Zn, Pb and As in the soil, with the concentrations of Cd, Zn and Pb in rice roots decreased by 98%, 83% and 72%, respectively, while the concentration of As increased by 327%. Most farmland soil pollution is heavy-metal composite pollution, which poses a challenge to passivation of heavy metals in farmland by biochar.
The above iron oxides, sulfates, silicates and biochar have been widely used in heavy metal passivation of soil. They each have a good passivation effect on certain heavy-metal polluted soil under specific soil conditions. For example, iron oxides have a good effect on the passivation of arsenic-polluted soil, biochar has a special effect on the passivation of cadmium-polluted acid soil, and sulfates and silicates have good passivation effects on Cd or Pb pollution of acid soil. However, under the condition of heavy metal composite pollution of soils, the application of one of the above-mentioned conditioners alone often cannot achieve the goal of simultaneously passivating various heavy metals, and two or more passivators need to be applied at the same time. This causes not only inconvenience to the application, but also the phenomenon that iron oxides, sulfates and silicates tend to react chemically, resulting in loss or decrease of the passivation effect.