Heavy Metals’ Poisoning in Farm Animals

Abstract

Heavy metals are metallic elements with a high density compared to water that are found in trace amounts in a variety of matrices. Mercury poisoning can cause brain damage, skin damage, and increase risk of cancer development. Mercury-poisoned animals cannot produce meat, liver, or kidneys fit for human consumption. Heavy metals can cause cell dysfunction and toxicity by attaching to protein sites and displacing the original metals from their native binding sites. Reducing input/output ratio of heavy metals in animals should be the main objective of effective solutions. Electro-remediation involves running an electric current through liquid manure to cause metal ions to precipitate on an electrode.

1. Introduction

Metals are naturally occurring elements of the earth’s crust, and through wind and water-induced erosion, they are dispersed as powders or dissolved into rivers [1]. However, compared to human activity, these natural processes release fewer metals into the environment. These substances spread widely in the environment, which causes them to move up the food chain. Heavy metals are metallic elements with a high density compared to water that are found in trace amounts in a variety of matrices [2]. Examples include Fe, Co, Cu, Mn, Mo, Se, Zn, Cr, and As. Since heavy metals can cause toxicity at low levels, their weight and toxicity are connected [3]. In order to maintain certain biochemical and physiological processes in humans, animals, plants, and other organisms, certain metals are necessary [3]. These trace elements, sometimes known as microelements, include cobalt (Co), copper (Cu), chromium (Cr), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), and zinc (Zn). Their nutritional requirements are typically minimal. Although they have varying bioavailability, they are present in a variety of matrices in trace amounts (ppb or ppm). The term “heavy metals” refers to metallic elements with an atomic number greater than 20 and possessing metallic characteristics [4]. There are trace elements, or elements necessary for the proper growth, development, and operation of living creatures, among them (such as copper, zinc, chromium, and iron) (including cadmium, lead, and mercury) [5]. However, they all share the trait of being poisonous and extremely deadly for humans, animals, and plants beyond a specific level [3]. Toxic heavy metals are generally determined by the level of pollution, but they can also be toxic depending on the species and age of the organism, the route taken to enter the body, the chemical’s structure, how it interacts with other metals, or the body’s physiological state [6]. Metals enter the body through the respiratory or gastrointestinal systems, where they are concentrated and stored before being carried by the blood to the tissues and organs [7]. According to European Union (EU) Regulation 1881/2006, essential trace elements are typically added to animal feed as nutritional supplements to enhance health and maximize output. However, prolonged exposure to these substances at higher concentrations has been associated with cellular or systemic problems and may be a source of pollution [8]. Other metals, such as As, Cd, Pb, and Hg, are regarded as pollutants and undesirable chemicals in animal feed (Regulation 2002/32/EC) since they lack biologically recognized roles [5]. Additionally, the preceding public health hazards—As, Cd, Cr, Pb, and Hg—show a high toxicity since they can cause organ damage even at low exposure levels [8]. According to their biological significance, heavy metals can be divided into four main groups: necessary, non-essential, less toxic, and highly toxic heavy metals (Table 1) [9]. Heavy metals can be both beneficial and harmful to the organism.

Group of heavy metals	Examples
Macronutrient elements	Cobalt, iron
Micronutrient elements	Copper, nickel, chromium, iron, manganese, molybdenum
Highly toxic elements	Mercury, cadmium, lead, silver, gold, palladium, bismuth, arsenic, platinum, selenium, tin, zinc
Precious elements	Platinum, silver, gold, palladium, ruthenium
Radio nuclides	Uranium, thorium, radium, cerium, praseodymium

Table 1.

Classification of heavy metals with examples.

2. Sources of heavy metal pollution

According to the FAO, the following are the sources of heavy metals: mining effluents, industrial effluents, domestic effluents, urban stormwater, leaching of metals from solid waste dumps and garbage, metal inputs from rural areas, batteries, pigments, paints, glass, fertilizers, textiles, dental and cosmetic products, atmospheric sources, and petroleum industrial activities [9]. More causes of heavy metal contamination include the preparation of nuclear fuels, the smelting of copper, and the electroplating of chromium and cadmium [10, 11]. Tiny particles of cadmium, lead, and zinc that have been liberated allow these dangerous metals to float on the wind and land on top of soil or edible plants [12]. PVC goods, color pigments, various alloys, and today’s most frequently rechargeable nickel-cadmium batteries all use cadmium compounds as stabilizers [13]. Cadmium metal is mostly utilized as an anticorrosion substance and also be found in phosphate fertilizers as a contaminant [14]. Agricultural sewage sludge and fertilizer application are two anthropogenic sources of cadmium that can contaminate soil and increase crop cadmium uptake [15]. All living creatures’ primary source of heavy metals is food [16]. In general, methylmercury exposure in fish is the main way that living things are exposed to it, along with dental amalgam [11, 16]. The electrochemical method used to produce chlorine uses mercury as an electrode, and the chlor-alkali business is a significant source of mercury use in this process [17]. Methylmercury, an extremely stable form of organic mercury, is present and builds up in the food chain [18, 19].

3. General effects of heavy metals

Heavy metals have been used for a variety of purposes since before recorded history, and they have been vital to the advancement and prosperity of civilizations. Animal populations may be subjected to dangerous amounts of heavy metals at levels very close to those experienced by humans [20, 21]. A substantial number of animals found in heavily metal-polluted areas ingest metal-affected grasses, feed materials, vegetables, and rice plants in addition to contaminated drinking water, which is another potential source of exposure to heavy metals [1, 17]. Heavy metals are widely used and naturally found in the environment, which exposes both humans and animals to them to varying degrees [1, 5]. In addition, because metals are ubiquitous and have been there throughout life’s evolution, organisms must contend with them because they may be harmful [22]. Heavy metal traces, including copper (Cu), cobalt (Co), manganese (Mn), iron (Fe), and zinc (Zn), are necessary for a variety of vital physiological processes, including the regulation and operation of several enzyme systems, oxygen and electron transport, hormone synthesis, antioxidant defense, immunity, and fertility [1, 5]. In addition to negatively affecting growth and physiological processes, a lack of essential metals can also make non-essential heavy metals more hazardous [1, 5]. However, even necessary metals will turn hazardous with prolonged contact [14]. Lead (Pb), cadmium (Cd), and mercury (Hg) are examples of toxic heavy metals that are dangerous even at very low doses and have no known biological benefits [23]. Non-essential toxicant metals frequently imitate essential metals to enter the body and potentially disturb important cellular processes [24]. The bioaccumulation of hazardous metals can also be explained by this. Furthermore, because detoxification systems cannot break down an atomic species into a subcomponent with lower toxicity, the elemental nature of metals influences their biotransformation and toxicity [25]. Since metals are elements, their indestructibility and bioaccumulation together lead to a significant concern for metal as a toxicant [14, 22]. The level of exposure, type of heavy metal and its form, age, sex, physiological and nutritional health of the exposed animal, and method of poisoning all affect how toxic heavy metals are to animals [26].

The majority of metals are concentrated in the liver and kidney, along with other essential organs, where they can have toxic or non-toxic effects such as oxidative stress, immunotoxicity, cardiotoxicity, teratogenicity, enzyme inhibition, birth abnormalities, and endocrine disruption [27]. Due to the wide range of chemical characteristics and toxic endpoints, the precise chemical basis of metal toxicity is poorly understood, and there is no universal mechanism for all dangerous metals [22]. It is true that heavy metals cannot degrade into other substances since they are elements, their forms, however, can be converted to free metal ions, which will modify their biological availability, activity, and toxicity [28]. Metals in their ionic form can interact with biological systems and toxicological targets in a variety of ways, making them chemically very reactive [14]. This can lead to a variety of toxic effects and damage to different organs, such as the kidney, nervous system, liver, respiratory system, endocrine and reproductive systems, and gastrointestinal tract (Table 2) [11, 25, 27]. Targets for heavy metals typically include biological molecules, macromolecules, membranes, or organelles, and interactions between free metal ions and these targets are what cause toxicity [29]. Hazardous metals commonly act by inhibiting enzymes, subcellular organelles, interactions with DNA that cause mutagenesis and cancer, covalent alteration of proteins, displacement of other essential metals-dependent proteins, and the production of free radicals [28, 29]. The toxic metals are grouped into four based on their toxic effects: metals (copper and iron) acting as Fenton reaction catalysts and contributing to the production of free radicals and oxidative stress; metals (nickel, cadmium, and chromium) linked to cancer; metals (aluminum, lead, and tin) linked to neurotoxicity; and generally toxic metals like mercury [30]. However, a growing body of research indicates that most heavy metals can cause oxidative stress in a variety of animal species, including buffalo [31]. This can have an impact on the oxidative stress quotient [25]. Free radical overproduction and oxidative stress damage biomolecules, subcellular structures, and even entire cells, such as neurons, which not only compromise immunity but can result in a variety of illnesses [30]. Oxidative stress has a significant impact on farm animals’ ability to produce and reproduce, and it may result in significant financial losses [30, 31]. The endocrine systems of animals may be disrupted by toxic heavy metals, which can also affect animal reproduction and productivity [27]. Heavy metal-induced co-selection of antibiotic resistance genes (ARGs) has emerged as a new environmental concern as there is mounting evidence that heavy metals might affect antibiotic resistance [32]. It has been discovered that the presence of heavy metals in the environment, such as arsenic, copper, and zinc, even at low levels increases bacterial resistance to tetracycline [33, 34, 35]. Due to their usage in feed and exposure to heavy metal contamination in the environment, livestock and the systems used to produce them are seen as a major source of heavy metals [9]. As a result, the environment around cattle may be contaminated with heavy metals and antibiotics, which could increase the fast-expanding worry over antibiotic resistance [35]. The vulnerabilities of both humans and animals as a result of compound resistance are highlighted by the confirmed positive link between heavy metal resistance and coexisting methicillin-resistant Staphylococcus aureus (MRSA) [35, 36]. Mercury-poisoned animals cannot produce meat, liver, or kidneys fit for human consumption [18]. Depending on the type of mercury poisoning, milk might also be dangerous [37].

3.1 Ruminants

Cattle, particularly young calves, are more susceptible to heavy metals [25, 37]. Owing to their natural curiosity, licking habits, and indiscriminate eating habits, cattle can ingest lead-bearing objects present in their environment as domestic, industrial, or agricultural waste and suffer from accidental acute lead poisoning [37]. Contamination of vegetation and pastures nearby secondary lead smelters (battery recycling units) and lead-zinc smelters [17] was the source of acute lead toxicity in cattle and buffaloes and subclinical toxicity in goats affecting the essential trace mineral profile [27]. The liver and kidneys of the fetus of a lead-poisoned pregnant heifer contained 0.425 and 4.84 ppm of lead, respectively, which were 72% and 84% of the lead concentrations in the respective organs of the dam [25, 37]. Most findings indicate a comparatively higher tolerance in sheep and goats to toxic metals like lead and cadmium than cattle [25, 38, 39, 40]. Sheep excrete higher concentrations of lead, chromium, and nickel in their excrement than cows [41]. This may be a reason for the comparatively higher lead concentrations in the milk of ewes than cows [42]. Sheep mostly show subacute toxicity that simulates signs of lead toxicity in adult cattle [37, 41, 42]. Goats, though comparatively more tolerant to lead (chronic toxicity dose of 400 mg per kg body weight) than cattle and sheep, can also exhibit cumulative lethal toxicity with predominant signs of CNS involvement following long-term exposure to lead [25, 27]. Cattle might accidentally absorb lead-containing things from their environment, such as household, industrial, or agricultural trash, due to their natural curiosity, licking tendencies, and indiscriminate feeding habits. This can result in acute lead poisoning [37, 43]. Acute lead toxicity in cattle and buffaloes, as well as subclinical toxicity in goats affecting essential trace mineral profiles, was caused by contamination of vegetation and pastures near secondary lead smelters (battery recycling units) and lead-zinc smelters [44, 45]. The majority of research shows that sheep and goats are more tolerant than cattle to hazardous metals like lead and cadmium. Compared to cows, sheep produce more lead, chromium, and nickel in their excretions [27, 39, 46]. This could explain why sheep milk has somewhat higher lead amounts than cow milk [45]. Most sheep exhibit subacute toxicity, which mimics adult cattle’s lead toxicity symptoms [37, 45]. Even though they are more tolerant of lead than cattle and sheep (chronic toxicity dose: 400 mg per kg body weight), goats can nevertheless develop cumulative fatal toxicity, with CNS involvement being the main symptom after prolonged lead exposure [27, 47]. Despite the fact that unintentional acute or chronic poisoning from organic mercury or inorganic compounds can occur in domestic animals, mercury poisoning is uncommon [48, 49, 50]. It’s possible that phenyl-mercury, which is present in the treated grains as organic mercury, is a more frequent cause of chronic cumulative poisoning [50, 51]. Only when massive amounts of grains are fed to cattle over long periods of time does clinical disease develop. Accidental administration of mercury-containing medications and licking or cutaneous absorption of mercuric oxide-containing skin dressings can also result in sporadic occurrences of poisoning in horses [37, 52]. Mercury poisoning is rare in domestic animals, but accidental acute or chronic poisoning can occur following ingestion of organic mercury or inorganic compounds [53]. Oral ingestion of organic mercury present in the form of phenyl-mercury in the treated grains may be a more common source of cumulative chronic poisoning [47]. However, a ration containing up to 10% of treated grains was not harmful; even feeding a single large amount of grain was incapable of causing toxicity in ruminants [49]. The clinical illness may occur in cattle only when large amounts of grains are fed for long periods [51]. Sporadic cases of poisoning in horses can occur due to accidental administration of drugs containing mercury and licking or cutaneous absorption of skin dressings containing mercuric oxide [27, 37]. In a study, mercury levels were found to be significantly higher in the blood (7.410 g/kg), milk (4.750 g/kg), and urine (2.80 g/kg) of nursing cows raised within a 5-kilometer radius of a thermal power plant [37]. The exposed cows’ hemoglobin levels were significantly lower, and their blood urea nitrogen, serum creatinine, albumin, and serum glutamate oxaloacetate transaminase values were all higher, showing effects of mercury on animal health [54, 55]. The study came to the conclusion that long-term exposure of the cows to fly ash mercury may have an effect on the human population, either directly or indirectly through the food chain [55]. While chronic administration of inorganic mercuric chloride (0.8 gm/kg body weight for 14 weeks) in horses caused mercury toxicity, toxic effects in sheep can be evident with an intake of 17.4 mg/kg body weight [37]. Due to the frequent discharge of cadmium and lead together from many industrial sources, the clinical symptoms of spontaneous poisoning commonly combine the two metals [48]. Cadmium levels in feed greater than 50 mg/kg dry matter are linked to toxicity in cattle and sheep [56]. Large amounts of cadmium accidentally consumed can harm the liver and induce acute nephrotoxicity [57]. In animals, chronic intake is linked to metal accumulation, particularly in the kidneys, liver, lungs, and testes [25, 58]. Inappetence, weakness, weight loss, poor hoof keratinization, dry, brittle horns, matting of the hair, and keratosis are a few examples of clinical symptoms in cattle [59]. Significant necropsy abnormalities included degenerative alterations in most organs and hyperkeratosis of the stomach epithelium [37, 59]. Cattle and buffaloes from an industrial area have been observed to have vascular degeneration and necrotic alterations in the liver, kidneys, and lungs and frequently have tissue cadmium levels above 2 ppm [25, 58, 60]. Anemia, nephropathy, and bone demineralization were the results of experimentally poisoning sheep with 2.5 mg of cadmium per kg of body weight [27, 59, 61]. Congenital flaws, stillbirths, and abortion are further potential impacts [61]. The ruminant species that is most vulnerable to chronic copper poisoning is the sheep, and poisoning cases in sheep have been reported all over the world [62, 63]. Contrarily, cattle were thought to be significantly more tolerant of copper accumulation in the past, and up until recently, copper poisoning in cattle was very rare [64]. On the other hand, copper poisoning and the mortality it causes are on the rise everywhere in the world, especially in dairy cattle [64, 65]. Since ruminants have poor homeostatic control over copper absorption, which makes them more sensitive, they have evolved mechanisms for storing excess copper in the liver by decreasing copper in the liver [64, 66]. However, when exposed to copper levels greater than those required for health, they are unable to manage their excretion skills and could become copper toxic [66]. Acute poisoning may happen from consuming 20–100 mg of copper per kilogram of body weight in sheep and young calves and 200–800 mg of copper per kilogram of body weight in mature cattle [67]. Chronic copper poisoning in sheep may occur with daily consumption of 3.5 mg of copper per kg when their grazing pasture contains 15–20 ppm copper (DM) and low levels of molybdenum [25, 68, 69]. Goats have substantially higher copper requirements (15–25 mg per kg, DM) than sheep (640 mg per kg, DM) and can tolerate a far higher dietary copper intake than sheep [69]. Goats’ great tolerance to copper may be due to low hepatic absorption [67]. Goats may be able to tolerate a higher concentration of the copper antagonist molybdenum compared to sheep and cattle [66]. The clinical symptoms of acute copper poisoning that are most frequently seen are anorexia, stomach pain, diarrhea, dehydration, unsteadiness, salivation, and collapse before death, which typically occurs within 24 hours. Animals that survive acquire icterus and dysentery [70]. Calves that survive for three or more days are known to have ascites, hydrothorax, hemoglobinuria, head pushing, opisthotonus, aimless roaming, bruxism, circling, and ataxia [71]. A haemolytic disease is chronic poisoning [72]. Sheep that are affected exhibit anorexia, thirst, sadness, jaundice, haemolytic anemia, and hemoglobinuria, as well as accelerated breathing and heart rate [73]. Sheep also exhibit nervous indications such as sadness, blindness, and tetraparesis [25, 74].

3.2 Poultry

Among animal products, eggs are also a possible source of heavy metal contamination. The transferability of heavy metals between hens raised in improved cages and those raised outdoors and found that the extensive soil contamination with these pollutants resulted in free-range eggs having greater heavy metal concentrations than conventionally produced eggs [1, 75, 76]. A study revealed that Cd levels were 0.018 vs. 0.023 ppm in the free-range group, essential Cu levels were 2.591 vs. 2.734 ppm, and essential Zn levels were 5.386 vs. 5.522 ppm in improved cages [1, 76]. By attaching to protein sites and displacing the original metals from their native binding sites, heavy metals can lead to cell dysfunction and toxicity [77, 78]. Additionally, the binding of heavy metals to macromolecules like DNA and nuclear proteins causes oxidative distress [7, 79]. Albumin, the most prevalent protein in plasma, binds to their ions [77]. According to a study, [79] they attach to the free sulfhydryl group of terminal cysteine residues and to histidine residues, which disrupt mitosis, cell respiration, and cell enzymes, especially when arsenic is present [80]. In chicken meat, higher quantities of cadmium have been found [78, 81]. Exposure to cadmium can cause oxidative stress and change the antioxidant enzyme activity in the erythrocytes of adult poultry birds [82]. Cadmium primarily builds up in the proximal tubular cells, where it damages bones or interferes with kidney function to promote bone mineralization [83]. As metallothionein, cadmium binds to proteins that are high in cysteine [84]. As they have similar oxidation states, it can take the place of the zinc in metallothionein and prevent it from acting as a cell scavenger of free radicals, producing hepatotoxicity in the liver and circulating to the kidney, accumulating in the renal tissue, and causing nephrotoxicity [85, 86, 87]. Poisoning by lead can occur in poultry [88]. A report from a study revealed that, lead in the feed can significantly stunt chicken growth and decrease blood Delta-aminolevulinic acid dehydratase levels [89]. In hens, the bone has the highest concentrations of deposited lead, followed by the kidney, liver, and skeletal muscle [90]. Majority of investigations revealed that the most often found heavy metals in chicken liver were arsenic, cadmium, mercury, and lead residues. Due to its ability to bind to and deactivate vital enzymes, lead is the most hazardous element [1, 7, 25, 37]. The liver and kidneys bio-transform arsenic, and the methylated metabolites are distributed throughout the body [91]. The amount of accumulated element in the organs varies on the duration of exposure, the amount of ingested element, the animal’s age, and breed [92, 93]. Since these effects are dose-dependent, prolonged exposure to heavy metals through contaminated feed is likely to result in more harmful changes to tissues [93].

3.3 Fish

Fish may be a source of heavy metal exposure for people because it contains important proteins and n-3 polyunsaturated fatty acids [94]. Fish living in contaminated waters have a tendency to accumulate heavy metals from the petroleum industry, air sources, and cosmetics in their tissues [11]. In general, accumulation activities depend on the amount of metal present, the length of exposure, the method used to prepare the metal for absorption, the environment (water, nuclear fuels, chromium and cadmium electroplating, temperature, pH, hardness, salinity), and intrinsic factors such as fish age and feeding habits [92, 93]. Rubber tire dust on the road surface contains cadmium, lead, and zinc [95]. Different fish tissues have a predilection for these tiny sizes [96]. Majority of these particles allow the harmful metals to rise on the wind, where they will primarily concentrate in the liver, kidney, and gills [97]. They may also be ingested or transferred to the soil or edible plants. Heavy metal build-up in fish tissues (Table 3) is mostly influenced by their concentrations in the water, in food, or in commercial feed [98]. The visceral tissues of fish, such as the liver, kidney, and intestines, which are typically discarded throughout the production procedures, tend to accumulate more metals than the muscles when compared to the other tissues in fish [1, 97, 99]. Significant correlations between lipid levels and concentrations of critical Cu and Zn [100] in two species of farmed fish (pompano and snapper). According to the research, lipid content may play a significant role in controlling the bioaccumulation of certain metals. Although copper is a necessary metal for both types of fish, its toxicity causes damage to the fish’s gills, liver, and kidneys, which can result in death [101]. Lead has physiological and biochemical effects by acting as a mimicking agent to replace necessary metabolic components like calcium, iron, and zinc [102]. For example, it directly inhibits the action of sulfhydryl group-rich protein enzymes as well as zinc and iron in the production of heme [103]. Lead binds to a variety of transport proteins, including calcium-ATPase, calmodulin, transferrin, metallothionein, and metallothionein [104].

Heavy metals	Sources	Side effects	References
Arsenic	Dipping and spraying of animal to control ecto-parasites	Skin damage, circulatory problem, and increase risk of cancer development	[14, 25]
Lead	Industrial polluted waste, Highway crops	Major organ like Liver, Kidney, Brain Damage.	[11, 25, 27]
Cadmium	Mining, smelting, and manufacturing of batteries, pigments, stabilizers, and alloys	Lungs, Kidney, Bone Damage	[18, 27]
Chromium	Tannery facilities, chromate, ferrochrome and chrome pigment production, stainless steel welding.	Kidney damage	[18]

Table 2.

Sources and toxicological side-effects of some heavy metals.

Heavy metal	Concentration, mg/kg
Co	0.015
Cd	0.030
Pb	0.050
Fe	0.300
Cu	1.000

Table 3.

Levels of some heavy metals in fishes as reported in WHO.

4. Analytical methods of detection in animal production

4.1 Determination of heavy metals in fish species of the Mediterranean Sea (Libyan coastline) using atomic absorption spectrometry

The toxicity of heavy metals when their concentration exceeds the allowable limit has made their presence in our environment a major source of worry [105]. The World Health Organization (WHO) has established the approved values of the Co, Cd, Pb, Fe, and Cu concentrations in fish, which are shown in Table 3. The document presents the Co concentrations in various tissues. The concentration of this metal was found to range from 0.570 mg/kg to 44.693 mg/kg, meaning that the levels of Co in all tissues that were investigated were higher than those that had been previously reported.

4.2 A titrimetric method for the quantitative estimation of lead in biological materials

As a result of Fischer’s 1929 announcement of the exceptional affinity of dithizone (diphenylthiocarbazone) solutions for lead [106], other laboratories have been inspired to look for useful micro-methods that may be used to detect lead in biological materials.

4.3 Ion chromatographic and voltammetric determination of heavy metals in soils. Comparison with atomic emission spectroscopy

The aim is to compare different analytical techniques, aspectroscopic, an electrochemical and a chromatographicone to determine total heavy metals concentrations in soils and to establish a correlation between these three techniques even though soil solutions are complex matrices providing a lot of interference problems [107]. Atomic emission spectroscopy is the method of reference for heavy metals concentrations analysis.

4.4 Speciation of heavy metal binding non-protein thiols in Agropyronelongatum by size-exclusion HPLC: ICP-MS

In order to quantify the major of heavy metals (Pb, Cu, Cd, Co, Zn and Ni), three ionic separation column systems were evaluated [108]: (1) a cationic column (HPIC-CS2, Dionex) tested with two eluents (10 mMoxalic acid–7.5 mM citric acid; and 40 mM D-tartaric acid–12 mM citric acid); (2) an anionic column (HPIC-AS4, Dionex) evaluated with 25 mM oxalic acid as eluent; and (3) a bifunctional ion-exchange column (Ionpac CS5, Dionex) which was also tested with two eluents (6 mM pyridine, 2,6-dicarboxylic acid; and 50 mM oxalic acid/95 mM lithium hydroxide

5. Strategies to control heavy metal pollution

In order to minimize soil pollution issues and heavy metal contamination, efforts must be made to close nutrient cycles on farms by recycling nutrients in livestock manure [109]. As a result, numerous studies have been conducted to discuss methods for reducing heavy metal levels in soil and livestock waste [110, 111, 112]. Reducing the input/output ratio of heavy metals in animals should be the main objective of effective solutions [113]. For this reason, several multidisciplinary strategies should be taken into consideration to reduce animal intake, excretion in feces, and concentration in manure [1, 114, 115]. Since nutrients found in manure or in compounds come from the portion of feed that is not absorbed by the animals, adjusting the diet may be an effective strategy to influence the amount of manure generated as well as its composition [1, 116]. A formulated diet is required to increase the efficiency of nutrient retention by animals and decrease their excretion in feces [117]. For instance, using commercial amino acids to minimize nitrogen excretion in pigs and poultry is a very effective method. As a result, imports of feedstuffs high in protein, including soybean meal, are significantly reduced at the farm level. In several species, it has been proven effective to add enzymes to the meal to increase the biological availability of some particular nutrients [118]. Animal nutrition is a key factor in controlling nutrient flows on livestock farms [119]. Utilizing trace mineral supplements may help stop the “waste.” Alternative innovative compounds to antibiotics as well as to Zn and Cu should be used to control enteric diseases, and the maximum permitted level should not be thought of as the ideal level for animal requirements [120]. There is a need for various technologies to remove the content of heavy metals from contaminated soil and water in agricultural land because an excessive amount of heavy metal output from manures can still seep into the soil and water [121]. Different manure treatment methods have been researched and can be used in the field to lower the heavy metal output from animals [112].

Metal concentrations can be lowered using electro-remediation, which involves running an electric current through liquid manure to cause metal ions to precipitate on an electrode [122]. The technique has not been tested on farms yet, so it’s unlikely to be economical at this time. In order to reduce the environmental impact and guarantee high nutrient efficiency, the recycling loop of manure back into food production should, from a whole-farm viewpoint, be as brief as feasible [123]. A group of researchers claim that phytoremediation, a straightforward clean-up technique, has the potential to get rid of metals from agricultural land by using plants that accumulate significant amounts of heavy metal contamination [124] . The discovery that plants might metabolize harmful pesticides led to the development of this technique a few decades ago [125]. It is seen by the communities as an acceptable, efficient, and novel technology that is also cost-effective. The term “phytoremediation” refers to a group of methods that use plants and related bacteria to remove contaminants from matrices by transfer, confinement, accumulation, or dissipation [126]. Its cost-effectiveness and potential to limit the exposure of the polluted substrate to people, animals, and the environment are both facilitated by the fact that phytoremediation is typically carried out in situ [124]. The four types of phytoremediation are phytoextraction, phytofiltration, phytostabilization, and phytovolatilization, depending on the circumstances, the extent of clean-up necessary, the plants used, and the contaminants [127]. Since nutrients found in manure or in compounds come from the portion of feed that is not absorbed by the animals, adjusting the diet may be an effective strategy to influence the amount of manure generated as well as its composition [124]. Phytoremediation is a straightforward clean-up technique that makes use of plants that accumulate significant levels of heavy metal pollutants and offers hope for the removal of metals from agricultural land. The discovery that plants might metabolize harmful pesticides led to the development of this technique a few decades ago [125]. It is seen by the communities as an acceptable, efficient, and novel technology that is also cost-effective. The ideal plants for heavy metal removal ought to possess the following traits:

1. rapid growth;

2. a deeply branched and widely dispersed root system;

3. good climatic and environmental adaptation;

4. ease of cultivation and harvest;

5. production of more above-ground biomass;

6. resistance to pathogens and pests;

7. increased accumulation of the desired heavy metals from soil; and

8. translocation of the accumulated heavy metals from the soil.

A formulated diet is required that decreases the efficiency of nutrient retention by animals, increases their excretion in for instance, using commercial amino acids to minimize nitrogen excretion in pigs and poultry is a very effective method [117]. Alternative innovative compounds to antibiotics as well as to Zn and Cu should be used to control enteric diseases, and the maximum permitted level should not be thought of as the ideal level for animal requirements [128].

Heavy metals are absorbed, precipitated, and concentrated by plant or seed roots that have been raised in aerated water [129]. In order to stop contaminants from migrating to groundwater or entering the food chain, phytostabilization is employed to minimize their mobility and bioavailability in the environment [130]. Phytovolatilization is the process by which pollutants are taken up by plants from the soil, transformed into a volatile state, and then released into the atmosphere [131]. The primary and most effective method for removing heavy metals and metalloids from contaminated soils or water is phytoextraction [132]. The bioavailability of metals in soil, which is affected by a variety of factors such as chemical composition, pH, geochemical properties of metals, environmental variables, and agricultural soil management, has a significant impact on phytoremediation effectiveness [133]. Bioavailability can be increased by lowering the pH of the soil, using fertilizers, soil microorganisms, and root exudates, and adding chelating agents [134]. In order to accomplish the homeostasis of agriculture with natural habitats and to maintain balanced production systems, it is crucial to control environmental losses and the spread of toxins from livestock manure [1]. The unintentional discharge of farm waste into water has led to outbreaks of dangerous infections, even though it is unlawful to spread manure close to surface water and on frozen land, as it is in the majority of European nations [135]. Technologies and approaches are consequently required to manage these environmental issues.

6. Conclusions

Heavy metals are shown in the commercial agricultural sector as both mineral fertilizers and contaminants/unwanted chemicals. Although there are set extensive regulations to prevent their pollution, their distribution at various levels makes it impossible to completely avoid the presence of heavy metals in the environment and the food chain. Controlling the animal input may be a useful tactic for lowering the dangers to human health from consuming items with animal origins and from manure’s environmental contamination. It is possible to change the diets of animals in order to lower the potential amount of minerals and nutrients that are not absorbed and end up in the manure. Effective solutions against heavy metals must take into account the intricate linkages between rural activities, the vast range of farming practices, the soil, and climatic variables. It should be advised to use additives more precisely in order to prevent contaminating the environment.