Accumulation of Toxic and Trace Metals in Agricultural Soil: A Review of Source and Chemistry in Ethiopia

: The main aim of this paper review is to know the extent of toxic and trace metals in some listed part Ethiopian farmland soil and its cause. Heavy metals are persistent in the agricultural soils (environments), they are non -termly and biological degradable. Among those toxic heavy metals, Pb, Cd, As, Hg, Cr, Co, Ni, Cu, Zn, Fe, Mn, are some of them. Every year’s large amount of industrial wastes, urban wastes, automobile emission, fertilizer, pesticide, wastewater irrigation and other agricultural activities are common in large agricultural farm land of Ethiopia. Those leads in to contamination of farmland with toxic heavy metals and other pollutants, The effect of those pollutants lower the quality of products finally they affect the health of consumers. According to different reports


Introduction
Heavy metal is a member of element that exhibits metallic properties. It includes transition metals, some metalloids, lanthanides and actinides. Many different definitions have been proposed-some based on density, some based on atomic number or atomic weight and some on chemical properties or toxicity [1]. Heavy metal pollution and contamination of the soil, water and air has been become common because of anthropogenic activities. Environmental pollution by heavy metals has increase due to industrial revolution, thereby causing serious ecological problems ecological problems [2,3]. Soil is dynamic body of natures and complex system they undergo physical, chemical, biological, geological and bio geophysical reactions [4].
Potentially toxic elements are often presented as "trace metals" or "heavy metals. Trace metals relate to the elements present in soils in trace amounts (normally described as 1 mol m −3 ). Heavy metals, a loosely defined group of elements, represent the elements that exhibit metallic properties with atomic mass over 20 (excluding the alkali metals) and specific gravity more than 5 [5]. According to some researchers, heavy metals are the elements with an atomic density >6 gcm −3 (except for As, B, and Se) [6].
Uncontrolled agriculture is difficult phenomenon, they give useful, and harmful effect on environment. [7]. Environmental Pollution is a big problem because of its adverse effect on human health, plans, and animals. [8]. Among environmental pollution, the heavy metals toxicity have received special attention globally and become an environmental problem of worldwide concern due to neurotoxin, carcinogenic and several other impacts arising from their consumption even at lower contents [9]. Therefore, the present review was conducted with an aim to assess the heavy metals accumulation potential on agricultural soil in Ethiopia, it sources, and chemistry reviewed.

Source of Heavy Metals in Agricultural Soils
Heavy metals are very harmful because of their non-biodegradable nature, long biological half-lives and their potential to accumulate in different body parts. Most of the heavy metals are extremely toxic because of their insolubility in water. Even low concentrations of the heavy metals have damaging effects to man and animals because there is no good mechanism for their elimination from the body systems. Nowadays, heavy metals are ever-present because of their excessive use in industrial applications. Wastewater contains substantial amounts of toxic heavy metals, which create problems [10]. Excessive accumulation of heavy metals in agricultural soils through wastewater irrigation may not only result in soil contamination, but also affect the food quality and safety [11]. Knowing the source of heavy metals in agricultural soils are important in order to know levels of contamination, for taking of proper treatment and keeping sustainable agricultural development.

Natural Source
The first source of toxic heavy metals in agricultural soils are parent materials from which the soil are derived, but influence of parent materials on heavy metal concentration and forms of heavy metals in agricultural soil is modified to varying degrees by pedogenetic processes [12]. In different areas heavy metals in soils derived mainly from pedogenetic parent materials, the levels of heavy metals status was affected by several factors among them moisture and management patterns. According to report conducted Li in 2008 soil aqua regia soluble fraction of Co, Ni, Pb and Zn were highly correlated with soil aluminum and iron. These elements were associated with indigenous clay minerals in the soil high in Al and Fe [13,14].

Wastewater
In many part of the world, urban wastewater is used for irrigation water for production different crops mostly vegetables. Wastewater mostly contains many nutrients like heavy metals, organic compounds and pathogens that may have a negative impact on the environment and human [15]. Most of the studies in Ethiopia shows that the use of heavy metals contaminated wastewater for irrigation over long period of time increases the heavy metal content of the soil above safe limits [16,17]. In the end, increasing the heavy metal content in soil also increases the uptake of heavy metals by plants depending upon the soil type, plant growth stages and plant species [18,19]. The soil type, plant growth stages and plant species [18,19].

Fertilizers
Agricultural activity was the major human influence on the agricultural soil. Some agricultural soils are deficient in metals (such as Co, Cu, Fe, Mn, Mo, Ni, and Zn) that are vital for healthy plant growth, and crops may be supplied with these as an addition to the soil or as a foliar spray. Large quantities of fertilizers are regularly added to soils in intensive farming systems to provide adequate N, P, and K necessary for crop growth. Application of certain fertilizers inadvertently adds potentially toxic elements to the soil. The raw materials and components used for the production of fertilizers contain trace amounts of heavy metals (e.g., Cd, Pb, Hg, As, Ni, Cu, Mn, V) as impurities, which, after continued fertilizer application, may significantly increase their content in the soil and the transfer to the food chain [20][21][22].

Pesticides
Several common pesticides used extensively in agricultural activities mostly in horticulture production they contained substantial concentration of heavy metals. For instance in the recent past, about 10% of the chemicals have approved for use as insecticides and fungicides in UK were based on compounds which contain Cu, Hg, Mn, Pb, or Zn. Although in Australia, timbers have been preserved with combination of copper, chromium and arsenic (CCA) and there are now many derelict sites where soil concentration of these elements greatly exceed background concentrations [23].

Biosolids and Manures
Biosolids (sewage sludge) are primarily organic solid products, produced by wastewater treatment processes that can be beneficially recycled [24]. The application of numerous biosolids (livestock manures, compost, municipal sewage sludge) in to agricultural soils inadvertently leads to the accumulation of heavy metals such as Arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, molybdenum, zinc, titanium and antimony in the agricultural soil [25].

Toxic and Trace Metals in Different Farmland Soil of Ethiopia
Agricultural soil pollution by heavy metals has cerise global problems [26,27]; especially for developing countries like Ethiopia where waste disposal is a big problem. Although heavy metals occur naturally in soil, human activities such as agricultures, urbanization, industrialization, and other activities are gradually rise the levels of heavy metal in soil [28].
Because of urbanization and industrialization, the disposal of wastes and the use of agrochemicals the pollution levels of heavy metals reached alarming situation in Ethiopia [29][30][31][32].
At the resent time, agricultural soil of Ethiopia was becoming polluted with different trace and toxic heavy metals, especially in urban centers. The dominant heavy metal pollution sources in Ethiopian agricultural soils are irrigation with stream/river and industrial effluents, and application of agrochemicals [29,33,34].
According to Deribachew groups in 2015, assessment of heavy metals (chromium, cobalt, cadmium and lead) concentration in soil of Haramaya universities farm site, around Haramaya, Awadey, and Harer cities farm lands were determined, almost all in the sites the concentration of those metals are above the recommended levels set by different organizations [35]. In similar studies, Bahiru groups 2019 and Bahiru and Teju, 2019 are reported that concentration of heavy metals (Cr, Cd, Pb, Zn, Cu and Zn) in the soil around eastern industry zone, Dukem central Ethiopia have the recommended levels set by WHO, USEPA, CMH ( [29,30]. The concentration of those heavy metals and other several studies were presented in Table 1.

Suggestion
Heavy metal remediation technologies were required for agricultural soil, in order to secure human health from different toxic heavy metals caused diseases.

Conclusion
Heavy metals are persistent in the agricultural soils (environments), they are non -termly and biological degradable. Among those toxic heavy metals, Pb, Cd, As, Hg, Cr, Co, Ni, Cu, Zn, Fe, Mn, are some of them. Huge amount of city, household and other disposal of municipals and industrial wastes, mining activities, application of fertilizer and pesticides for agriculture and automobile emission have main effect for continuous accumulation of heavy metals in agricultural soils. The concentration of heavy metals in different farming land in Ethiopia according to different reports show above the value recommended by WHO, FAO and other organization limit set for agricultural soils. The investigation of heavy metals in agricultural soils Ethiopia are low, it need different assessment especially around cities, industry zones, floriculture farms and other possibly suspected areas, and also environmentally friend and economically safe heavy metal remediation techniques are required especially in suspected areas.