Heavy Metal Contamination in Soil and Vegetables: A Review with Health Risk Assessments

Over the past 200 years, urbanization and industrialization have grown at a rapid rate, resulting in significant levels of air, water, and soil pollution. Because of their toxicity, persistence, and capacity to build up in the biota, trace metals in the environment continue to pose a hazard even if persistent organic contaminants and greenhouse gases have received more attention [1]. Cadmium (Cd), Chromium (Cr), mercury (Hg), arsenic (As), lead (Pb), and other metalloids having a density of greater than 5 g/cm 3 are examples of heavy metals. Besides polluting the environment, heavy metals can seriously injure humans by building up in bodily organs and other living things via the food chain [2, 3]. Exposure to heavy metals in soil primarily occurs via inhalation, oral consumption, and skin touch, constituting the three main routes of human contact [4,5]. The main causes of heavy-metal pollution include milling, industrialization, burning fossil fuels, agrochemicals, and mining which release a range of HHMs into agricultural soils and water bodies, including Pb, Cr, Co, Cd, Zn, As, Ni, Cu and Hg [6][7][8]. Because they include essential nutrients that humans need to survive, vegetables are a vital part of a typical diet. Additionally, they serve as protective meals by helping people prevent disorders. Vegetables may gather greater concentrations of hazardous heavy metal when cultivated near heavy metal pollution sources [9]. The accumulation of heavy metals in vegetables as well as crops that are edible in contaminated soils eventually poses a major risk to the human body as well as animal health as a result of the detrimental and irreversible effects of metals [10]. Few heavy metals such as Co [11], Mn [12], Zn [13], Cu [14] and Ni [15] are key components at certain levels in humans yet they become toxic when exposed to higher doses. Heavy metals pose significant health risks, particularly Pb along with Cd, which can cause damage for example malformations and bone fractures, along with hypertension, kidney failure, cardiovascular problems and other severe nervous system, immune system, lung and liver diseases [16][17][18]. Memory loss, heart problems, digestive disorders, and genotoxicity are among the negative consequences of mercury poisoning [19]. Many analytical methods and instruments use mercury (Hg), such as barometers and thermometers, blood pressure monitors with fluorescent illumination, amalgam for dental restoration, and fluorescent lighting [20]. Heavy metal accumulation at chronic level in the liver, kidney and bones of humans can result in kidney, cardiovascular, neurological, and bone illnesses. This buildup can be caused by long-term ingestion of high quantities of heavy metals through contaminated food. These days, as more people become aware of the health dangers linked with heavy metal exposure, risk assessment has become a global hot subject [21].

Plant food contamination can result from air pollution or by coming into touch with polluted soil. Heavy metals from various sources can deteriorate the soil, which is essential to the growth of food crops. Anthropogenic activities related to agriculture and industry are the main causes of soil pollution with heavy metals worldwide [22][23][24]. Soil-crop ecosystems are impacted by sewage from industries, irrigation, agricultural practices, and mining activities in highly populated and developing nations [25]. The use of specific inorganic and organic fertilisers, insecticides, herbicides, and organic manure that contain heavy metals can potentially boost heavy metal content in soils [26,27]. The swift advancement of industrialization around the globe has led to a notable rise in the likelihood of heavy metal pollution in the environment. Toxic materials build up in the soil, air, water as a consequence of rapid industrialization, chaotic urbanisation, and long-term heavy fertiliser and pesticide usage [28,29]. In part because of the higher concentrations of heavy metals and metalloids in leachates from municipal solid waste dumps, inadequate garbage disposal methods can also affect soil-crop systems [30]. The amount of heavy metals in food is directly impacted by soil contamination since samples of plant food taken from contaminated soils have been shown to accumulate these hazardous elements. [31][32][33]. Different origins of those heavy metals and their injurious proliferation on vegetables as well as crops are given in Table 1.

Table 1. Different origins of heavy metals and their toxic contamination of vegetable /crops

Heavy metal accumulation in soil is the most significant. Heavy metals, which include chromium (Cr), arsenic (As), copper (Cu), cadmium (Cd), mercury (Hg), zinc (Zn), nickel (Ni), and lead (Pb) are often found contaminants in soil environments. This kind of pollution is pervasive, longlasting, and physiologically hazardous in the soil environment [44]. A vital component of any ecosystem is soil. Because of its capacity to both absorb and emit, soil may get contaminated from various sources [45]. The weathering of rocks, volcanic activity, and erosion are the primary natural sources of the incorporation of heavy metals into soils. Anthropogenic sources include smelting, ore mining operations, landfills, industry effluents, electroplating, military training and warfare, use of phosphate fertiliser and pesticides, biosolids utilisation (such as fertilisers, animal manure, and municipal effluent), and atmospheric deposition [46,47]. The excess soil contamination scale is 16.1%, with the highest over-standard concentrations of heavy metals being Pb, Cd, Hg, As, and Cr at 1.50%, 7.0%, 1.60%, 2.70%, and 1.10%, respectively [48]. The overall state of the soil in agricultural settings can serve as a marker for heavy metal pollution brought on by farming practices [49]. The soil has already begun to absorb the majority of these heavy metals. On the contrary, long-term exposure to heavy metals can cause bone fractures and lung cancer in humans [50,51]. Remediation methods for heavy metal-polluted soil have been extensively studied. These methods include ex-situ remediation methods (soil washing, landfilling, vitrification, solidification) and in-situ methods (electrokinetic process of extraction, encapsulation, surface capping, chemical immobilisation, phytoremediation, soil flushing, and bioremediation) [52][53][54].

Notwithstanding the mass amount of heavy metals in aerial tissues and excluders, different plants as well as vegetables serve as accumulators and evidence of survival. Plants that successfully undergo biodegradation or biotransformation of pollutants into innocuous arrangements inside their tissues have higher rates of survival [62]. Plants accumulate hazardous heavy metals through various mechanisms, including absorption by root, xylem loading, heavy metal mobilisation, root-to-shoot transit, cellular compartmentation, and sequestration. The bioaccumulations of heavy metals in different organs of vegetables are given in Table 2. Almost all heavy metals are found in soil in an insoluble form which is not bioavailable to plants. Different root exudates that plants release can change the rhizosphere's pH along with enhancing the solubility of heavy metals, hence increasing their bioavailability [63,64].

Table 2. Bioaccumulation of heavy metals in different organs of vegetables

Vegetables absorb the heavy metals found in soil organic matter, and when they connect to short carbon atom chains, they transform into crucial cations [65]. In root hairs, heavy metals are actively transferred to the root cell plasma membrane from the apoplast or the soil. Upward movement, vacuoles, and barriers in the xylem as well as phloem are crucial for the translocation of the absorbed metals to the shoot [66]. The bioavailable metal penetrates the cell wall to access the root cells after being absorbed at the root surface. The two main pathways by which heavy metals are taken by roots are the symplastic pathway (active transport against electrochemical potential gradients and concentration across the plasma membrane) and the apoplastic pathway (passive diffusion) [67].

soil pH, dissolved cations and anions, chemical pesticides, and species of vegetation, fertilisers, and soil type all affect how much hazardous metal is taken up from the soil [62]. In the rhizosphere, bacteria generate DMA (dimethyl arsenic acid) and MMA (monomethyl arsenic acid), nevertheless, this is insufficient to initiate the methylation of As in plants [68]. As is catalyzed by specific enzymes, which includes Sadenosylmethionine-dependent arsenite methyltransferase, which is also crucial for the biogeochemical pathway of As [69]. Numerous metal transporters are involved in the mobilisation along with the transfer of metals from one compartment of cells and plant organs to another, including OsHMA2, OsHMA3, OsNramp5, Lsi1, NIP, Lsi2, MMA, HAC, ABC, DMA, and PT [70]. Daye, China [42] Commercial and municipality sewage water Ni, Zn, Pb, Cd Crops, Soil Oryza sativa Lenjan, Iran [43] Heavy metal ions have a tendency to combine with a variety of chelators, including organic acids, after they have entered root cells. These complexes, which include phosphate precipitate, sulphate, and carbonate, are subsequently immobilised in either the intracellular (symplastic compartments, including vacuoles) or extracellular (apoplastic cellular walls) [71]. Through the root symplasm, the metal ions contained inside the vacuoles may travel into the stele as well as the xylem stream [72]. Heavy metals are mostly accumulated in leaves by xylem-loading absorption, and in seeds and fruits through phloem-loading [73] are then transported by xylem vessels to the shoots. They are carried and dispersed throughout leaves by apoplast or symplast, where they are confined in extracellular compartments (cell walls) or plant vacuoles, preventing the buildup of free metal ions in the cytosol [74]. Metal ion transporters and complexing agents are two of the many molecules that mediate the uptake and translocation of heavy metals in plants. The absorption of heavy metal ions from soil depends on these specialised transporters, channel proteins, also known as H+-coupled carrier proteins, which are present in the plasma membrane of root cells. In addition to mediating the influx-efflux of metal translocation from roots to shoots, they are capable of moving certain metals across cellular membranes [75]. Several heavy metal ions, for example, Fe 2+ , Co 2+ , Cd 2+ , Mn 2+ , and Cu 2+ are also transported by the naturally resistant associated macrophage proteins (NRAMPs) [76,77]. In addition to metal ion transporters, amino acids as well as organic acids, which are complexing agents, function as metal ligands to contribute to the chelation of heavy metal ions. For instance, in the xylem, citrate also is a significant chelator for Ni and Fe [67,71,78]. The bioaccumulations of heavy metals in different organs of vegetables are presented in Table 2.

A human health risk assessment (HRA) is the method of determining the kind along probability of adverse health outcomes in individuals who could be subjected to chemicals or other potentially dangerous substances in the environment. An HRA is a type of health inquiry designed to evaluate a person's health risks and quality of life [16]. This section describes some of the parameters found in the literature used to calculate the HRA.

Food supply chain contamination is one of the noteworthy approaches that heavy metals penetrate human body. It is potentially dangerous to human beings and impedes the worldwide supply of food. The main causes of food chain contamination are inadequate effluent management, lax enforcement of standards and laws, a lack of research evidence on sustainable management, and low public consciousness. In the end, food safety is concealed by these constraints. This review has uncovered the root causes of heavy metal pollution, how they contaminate food chains, how plants carry metals, and the way our bodies absorb them. Transportation of heavy metals into plants, including fruits and vegetables, is mostly due to soil pollution. For this reason, attaining food safety and security on a global scale is hampered by the buildup of heavy metal(loid)s in agricultural soils. Although bioremediation is a potential natural remedy for heavy metal(loid) pollution, several problems require to be resolved before it can be used more widely. Accelerating global soil mapping and developing regional models capable of accurately predicting pollutant distributions and pinpointing contamination sources will be advantageous. Biosorbents offer appealing prospects as an inexpensive way to prevent pollution in the environment.