Effect of Combined Foliar Spray on Heavy Metals Accumulation in Facility Fruit Vegetables

Huiwei ZHAO Jumei LI Xueping SHI Yiming LIU Chuan LU Shuo SUN

Abstract With the continuous development of society， the development of agricultural economy is also accelerating. Meanwhile， a large amount of sludge and waste materials enter the farmland system， and the state of soil heavy metal pollution is becoming more and more serious. In order to ensure food security and the health of people's lives， a large number of experts and scholars have begun to look for remediation methods for heavy metal contaminated soil. At present， the use of mineral passivators in the remediation technology of heavy metal contaminated soil is a new and healthy recovery method， and has received extensive attention.

Key words Passivator; Heavy metal; Remediation technology

With urbanization and industrialization in China developing rapidly and the large amount of sludge and waste discharged from cities entering the surrounding agricultural ecosystem， the pollution of heavy metals in the soil has become increasingly severe. Moreover， China's large population has created a fierce contradiction between the demand for food and high-quality agricultural land resources. Therefore， how can we effectively solve the problem of soil pollution by heavy metals in China has been urgent. Many scholars and experts have conducted a lot of research and found that the labor and cost of land remediation are too high， and the cycle of phytoremediation is too long. For the soil contaminated by heavy metals， we need effective remediation measures that can guarantee the safe production of crops， and a large number of studies have shown that the effect of chemical remediation is significant[1]. The principle of chemical passivating agent remediation is： by adding passivators to the soil， the activated heavy metals are turned into a stable form， avoiding the migration and biological reutilization of heavy metals， so as to realize the remediation of heavy metal contaminated soil[2]. The current chemical passivation remediation technology has many advantages， mainly including fast remediation rate and convenient and simple operation， which is conducive to the large-scale promotion and use in moderate to light heavy metal contaminated farmland[3-4]. At present， the main passivation materials used include a variety of minerals， biochar， organic matter， etc.， and different passivators have certain differences in response to different pollutants， different soil types and pollution levels[4]. In the process of farmland use， adding a passivator to the soil can improve certain physical and chemical properties of the soil and replenish nutrients. Meanwhile， it can also convert active exchangeable heavy metals into relatively stable chemical forms such as organic binding state and residue state by means of adsorption， precipitation， complexation， ion exchange and redox between elements， so as to reduce the mobility and bioavailability of heavy metals[5].

Comprehensive analysis shows that the remediation of heavy metal contaminated soil will be a long-term and arduous task. While considering the cost of materials， attention should be paid to its environmental friendliness. At present， the material with relatively low price and best compatibility with the environment is minerals， especially non-metallic minerals， which have been frequently used as passivators for soil heavy metals in recent years and have achieved good results[6]. Minerals themselves have certain particularities. Used as a passivating agent， they can significantly enhance the affinity between the soil and heavy metal ions， improve the soil's ability to adsorb and fix heavy metals， and reduce the secondary release of heavy metals. Dolomite in minerals has a good remediation effect against heavy metal pollution in acid soils， and has a significant reduction effect on available Zn， Cd and Pb[7]. For vegetable soil， sepiolite can be used to reduce the exchangeable Cd content by about 23.1%-41.2%， and the Cd contents of the edible parts of leaf lettuce， rape and radish grown in vegetable fields can be reduced by 51.8%， 47.0% and 24.9% to the largest extent[8]. However， the application of only one mineral passivator can easily lead to accelerated decomposition of organic matter in the soil and difficult accumulation of humus in the soil， and long-term addition can also cause symptoms such as soil compaction and lack of micronutrients[5]. Facing these problems， researchers need to make relevant research on the defects of chemical passivation remediation technology in actual production. The combined remediation technology of organic fertilizer and passivator can improve soil fertility， promote the normal growth of crops， improve the remediation efficiency， and increase the stability of soil heavy metal passivation remediation， thereby producing huge economic and environmental benefits[9]. Studies have also shown that the combined remediation of sepiolite passivator and organic fertilizer significantly reduces the accumulation of Cd in the roots and leaves of pakchoi[10].

Types of Mineral Passivators

The types of mineral passivators commonly used in heavy metal contaminated soil remediation technology mainly refer to the following three categories： organic type， inorganic type and combination of the two. At present， most researchers at home and abroad are mainly devoted to the research of more specific mineral deactivators. The first category is naturally occurring mineral passivators， the most important of which are dolomite， zeolite， sepiolite， gypsum， limestone， montmorillonite and other inorganic passivators. The second category is mainly some industrial solid waste materials， mainly including phosphogypsum， fly ash， steel slag， municipal sludge， desulfurized gypsum， etc. Adding a certain amount of passivating agent to contaminated soil can provide certain nutrients to the soil， which is beneficial to improve the physical and chemical properties of the soil， and these passivators themselves have some special characteristics， such as physical adsorption， chemical complexation， exchange of different types of ions， and chemical reactions including oxidation-reduction， which can transform active heavy metal ions in the soil into a stable organic binding state or residual state， and ultimately reduce the penetration of heavy metals in the soil into the groundwater or transfer to plants.

Heavy Metal Remediation Mechanism of Mineral Passivators

The basic principle of passivator heavy metal remediation technology： Under certain conditions， a passivation agent can form a dense oxide film （passivation film） on the metal surface through a certain physical or chemical reaction through its own particularity， and this film can effectively enhance the corrosion resistance of the metal. There are obvious differences in the passivating agents used by different types of metals. For example， concentrated nitric acid can promote the passivation of iron and aluminum， but for other types of metals， concentrated nitric acid may not have this effect. At present， the more serious and large area of soil pollution is heavy metal pollution， which is a relatively serious problem facing soil pollution in China. The main causes of soil heavy metal pollution， especially the pollution of farmland， are the spraying of pesticides， the precipitation of heavy metal elements floating in the atmosphere， and the wanton discharge of rainwater and polluted water bodies. Facing the serious problem of heavy metals polluting the soil and further affecting human health， many researchers at home and abroad have proposed different remediation methods and treatment techniques， mainly including engineering methods， physical and chemical methods， and biological methods.

Heavy Metal Remediation Technology of Mineral Passivators

There are many types of natural mineral passivators， and most of these minerals contain rich calcium oxide， calcium carbonate， calcium and magnesium compounds and other substances. Such compounds can improve soil pH and increase the content of clay particles in the soil texture， the content of organic matter in the soil and the negative charge on the surface of soil particles， which is beneficial to the formation of strong affinity between the soil and heavy metal ions， strengthens the soil's absorption of heavy metals， and reduces the secondary release of heavy metal elements in the soil， and meanwhile， hydroxide ions can produce precipitation with some heavy metals[11]. Ca2+ and Mg2+ in minerals have an antagonistic effect with heavy metal ions in the soil， reducing the absorption and utilization of heavy metals by plants[12]. In addition， clay minerals， an important component of soil colloids， have special significance for the remediation of heavy metal contaminated soils. They can make minerals more stable， increase the comparative area and ion exchange capacity， and have a strong adsorption capacity. Therefore， such minerals mainly fix heavy metals in the soil through methods such as ion exchange， adsorption and replacement， reducing their activity and reducing their migration and transformation in the environment.

The reuse of industrial solid wastes can effectively reduce waste stacking and discharge. As the main waste in the coal burning process， fly ash contains a large amount of oxides such as Si， Al， Fe and Ca， and has adsorption sites on the surface， so heavy metal ions can be removed by methods such as chemical adsorption， ion exchange and precipitation.

The Significance of Mineral Passivators for Heavy Metal Remediation

At present， there are many studies on the remediation of heavy metal pollution by mineral passivation， and most of them focus on the use of one or more minerals to reduce the migration and bioavailability of heavy metals in the soil. However， in practical applications， there are few studies on the effects of organic fertilizers on heavy metal passivation and remediation effect and stability， and the use of good agronomic measures in passivation remediation can not only promote the improvement of remediation efficiency， but also increase the stability of soil heavy metal purification and remediation. Therefore， studying the effects of organic fertilizers on the passivation remediation effect and stability， minimizing the adverse effect of organic fertilizer application on the passivation remediation efficiency as much as possible， maximizing the synergistic strengthening effect of organic fertilizers on the passivation remediation effect， and improving the passivation remediation effect and stability， have extremely important theoretical value and practical significance to further promote the research and application of heavy metal contaminated farmland soil.

References

[1] HU HQ， HUANG YZ， HUANG QY， et al. Research progress of heavy metals chemical immobilization in farmland[J]. Journal of Plant Nutrition and Fertilizer， 2017， 23（6）： 1676-1685.（in Chinese）

[2] WANG YX， SUN YB， XU YM， et al. Enhancement of chicken manure on the immobilization remediation of cadmium contaminated vegetable soil and enzyme activity using sepiolite[J]. Environmental Science， 2016（35）： 159-169.（in Chinese）

[3] LIANG Y， WANG XC， CAO XD. Immobilization of heavy metals in contaminated soils with phosphate-， carbonate-， and silicate-based amendments： A review[J]. Environmental Chemistry， 2012（31）： 16-25.（in Chinese）

[4] SUN JF， WANG X. Research and application progress of soil conditioners[J]. Soil and Fertilizer Sciences in China， 2013（1）： 5-11.（in Chinese）

[5] CHEN GN. Mechanism and remediation effect of mineral amendment on heavy metals contaminated red soil[D]. Guangzhou： South China University of Technology， 2017.（in Chinese）

[6] DAI R， ZHENG SL， JIA JL， et al. Research progress of non-metallic mineral environmental materials[J]. China Non-metallic minerals Industry， 2009（3）： 3-10.（in Chinese）

[7] VONDR KOV S， HEJCMAN M， SZKOV J. Effect of quick lime and dolomite application on mobility of elements （Cd， Zn， Pb， As， Fe， and Mn） in contaminated soils[J]. Pol J Environ Stud， 2013， 22（2）： 577-589.

[8] LIANG XF， XU YM， WANG L， et al. In-situ immobilization of cadmium and lead in a contaminated agricultural field by adding natural clays combined with phosphate fertilizer[J]. Acta Scientiae Circumstantiae， 2011， 31（5）： 1011-1018.（in Chinese）

[9] WANG L， XU YM， LIANG XF， et al. Effects of biochar and chicken manure on cadmium uptake by rape with low cadmium accumulation[J]. China Environmental Science， 2014， 34（11）： 2851-2858.（in Chinese）

[10] WANG YX， SUN YB， XU YM， et al. Enhancement of chicken manure on the immobilization remediation of cadmium contaminated vegetable soil and enzyme activity using sepiolite[J]. Environmental Chemistry， 2016， 35（1）： 159-169.（in Chinese）

[11] CHEN Z， TIE BQ， LIU XL， et al. Impact of optimized agronomic regulation management on cadmium absorption and accumulation by late rice[J]. Journal of Agro-Environment Science， 2013， 32（7）：1302-1308. （in Chinese）

[12] GRAFE M， NACHTEGAAL M， SPARKS DL. Formation of metal-arsenate precipitates at the goethite-water interface[J]. Environmental Science &Technology， 2004， 38（24）： 6561-6570.