## 2 Food Security: A Global Challenge in View of Soil Pollutants and Heavy Metal Toxicants
### Heavy Metal Sources in Soil-Crop Ecosystems  
It is evident from numerous studies that the source of heavy metals in the agricultural ecosystem is due to industrial pollution, sewage sludge, phosphate-based fertilizers, metal pesticides, and contaminated water [9; 25; 26; 27; 28; 29]. In addition to the mentioned sources of heavy metal pollution, it is also essential to understand the impact of anthropogenic pollutants, as they pose a risk to the health of human beings through dietary intake of contaminated food crops which become polluted either through soil or direct atmospheric deposition over the surface of plant leaves [17; 30]. Waste effluents discharged from industries and automobiles accumulate in soil, thereby causing contamination in the food chains [9; 31; 32]. It is also worth noting that coal-based power plants pose a significant threat with respect to Hg soil pollution. In this regard, [PERSON] et al. [24; 33] stated the adverse effect on human health caused by consuming lettuce and water spinach grown in Hg-contaminated soil area. Likewise, many strong sources present a grim outlook for global food safety. Figure 2 shows the ecosystem of heavy metal contamination. Heavy metals emitted from automobile and industrial smelters are transported to distant locations, i.e., up to several kilometers away from their sources by wet or dry deposition [34]. These metals accumulate in plant leaves through foliar transmission after the deposition of atmospheric particles above the surface of the leaf [34]. [PERSON] and [PERSON] [35] proposed that particles can penetrate the leaf tissue through the pores present on the leaf cuticle and inside stomata. Similar to root uptake, foliar uptake of heavy metals may also arise in a dose-dependent manner. Nonetheless, one of the leading contributors of heavy metal penetration in agricultural soil also comes from sewage and sludge pollution. To address the concern of rising wastewater produced from domestic and industrial processes as a result of the growing human population, a reliable strategy and stable treatment systems are urgently needed. With the rapid growth of agriculture in the past decade, overuse of water resources, especially for agriculture, has become a problematic issue which threatens to intensify the effects of climate change. Agriculture constitutes by far the largest use of water in all countries [36]. In order to solve this problem, it has been found that wastewater reuse is a feasible option to minimize anthropogenic impact [37]. Figure 2: Ecosystem of heavy metal contamination. Furthermore, raw wastewater reuse in agriculture is a valuable tool for developing countries to minimize usage of freshwater while also addressing the challenge of rising food production in water-scar areas. The benefits of reusing water in agriculture are numerous and unquestionable, i.e., it saves significant quantities of freshwater which may be assigned for other essential purposes; it provides nutrients that may replace chemical fertilizers, thus helping to increase soil fertility and crop yield and reduce production costs; it allows for the expansion of agricultural land in dry areas; it is a relatively cheap disposal method for raw wastewater; and it can help to prevent contamination of surface water. In addition, it has been demonstrated that the health risks associated with raw wastewater usage in agriculture are satisfactory if its biological content meets specified standards [38; 39]. Having elaborated on the positive side of raw wastewater, there are also possible negative effects, such as soil salinization and groundwater contamination of metals and organic compounds. It should be noted that these are only theoretical at this point. Furthermore, the use of untreated water for agricultural production is potentially dangerous due to its microbial substance, especially bacteria, viruses, and parasites, which can cause a number of diseases because many of them can be sustained for longer periods in the environment. This in turn tends to affect the quality of the soil.