**4.1 Effects on soil**

In industrialized countries, the issue of heavy metals such as copper, nickel, cadmium, zinc, lead, and thallium (etc.) in the soil is of critical relevance [38]. Soil microbial functions, such as respiration rate and enzyme activity, are important markers of soil contamination, and are typically impacted by an increase in metal concentration. Heavy metals have been shown to negatively impact soil's biological, physicochemical, and biochemical properties [39–41]. Heavy metal contamination can affect the size, composition, and activity of the microbial population, which can then have knock-on impacts on a variety of plant quality and yield indices [42]. Because of this, heavy metals are thought to be a significant contributor to soil pollution.

Toxic effects of heavy metals on soil biota include interference with vital microbial functions and a subsequent decrease in the diversity and abundance of soil microbes. As stated by Shun-Hong et al. [43], heavy metals can alter the composition of soil microbes, which in turn affects soil enzyme activity. But prolonged exposure to heavy metals can boost bacterial tolerance, which is useful for cleaning up polluted

### *Abattoirs: The Hidden Sources of Plants' Heavy Metals and Other Pollutants in Lagos, Nigeria DOI: http://dx.doi.org/10.5772/intechopen.110339*

environments [44]. Heavy metals led to a decline in the bacterial population. The abundance and variety of bacteria living in polluted soils may have suffered because of this shift. The chemical affinities of enzymes in the soil system for various metals have been shown to have an impact on their respective activity, as described by Karaca et al. [45]. For instance, cadmium's (Cd) rapid mobility and low affinity for soil colloids makes it more hazardous to enzymes than lead (Pb). Lead (Pb) greatly reduces the activities of urease, catalase, invertase, and acid phosphatase; copper (Cu) lowers-glucosidase activity more than cellulose activity. Protease, urease, alkaline phosphatase, and arylsulfatase activities are negatively impacted by cadmium contamination, whereas invertase is unaffected [44]. As with enzymes, the sensitivity of different soil organisms to different metals varies.

### **4.2 Effects on plants**

Some heavy metals, including arsenic (As), cadmium (Cd), mercury (Hg), lead (Pb), or selenium (Se), are not necessary for plant growth because they do not perform any recognized physiological function in plants, although others, like iron (Fe), copper (Cu), cobalt (Co), and zinc (Zn), are [46]. Chlorosis, stunted development, and decreased production are just some of the detrimental consequences that heavy metals can have on plants. Heavy metals can inhibit nitrogen fixation in plants, alter plant metabolism, and impair nutrient uptake. Temperature, pH, organic matter, moisture, and nutrient availability are just few of the variables that affect the uptake and accumulation of heavy metals in plant tissue. Root absorption is a significant pathway for heavy metals to enter the food chain and potentially affect animals and humans [46, 47]. Plant species and the efficiency of plant uptake of metals determine the accumulation of heavy metals in plants [48]. Lead (Pb) in soils can have a negative effect on plant productivity, and even trace amounts of lead can impede photosynthesis, cell division, and water uptake. Dark green leaves, elder leaves wilting, reduced growth, and brown, short roots are all possible manifestations of toxic effects [49].

### **4.3 Effects on aquatic environment**

Extreme oxidative stress could be caused by even trace levels of heavy metals in aquatic species. Consequently, these contaminants are crucial to study in the field of ecotoxicology. Moreover, metals are frequently not degraded by microbes and hence persist in the marine environment indefinitely [50]. However, heavy metal contamination of a river could have catastrophic consequences for the aquatic ecosystem, reducing diversity of aquatic creatures and upsetting the delicate balance of the aquatic environment [51].

Particulate matter emitted into aquatic systems typically contains heavy metals, which settle and become a part of sediments. When it comes to metals and other pollutants in water, surface sediment is the most significant reservoir or sink. Aquatic macrophytes and other species with deep root systems can absorb sediment-bound contaminants [52]. Heavy metals could enter the food chain when an aquatic organism accumulates them. Many of the heavy metals used by carnivores like humans are found in marine life. The presence of fish makes this much more significant, as it could cause biomagnifications [51]. Increased formation of reactive oxygen species (ROS) due to the presence of heavy metals in aquatic systems is harmful to fish and other aquatic creatures [50]. Heavy metals are just one example of the environmental pollutants that could poison fish. Consequences for public

health from these contaminants could be devastating. It is vital to be mindful of the sorts of fish you eat because of the potential health risks associated with heavy metal intake [53]. Mercury (Hg) is a significant contaminant due to the damage it could do to marine life and the health problems it can create for humans. Bacteria in watery sediments methylate organic mercury, forming a highly poisonous chemical compound known as methylmercury. Methylmercury makes up almost all the mercury found in fish muscles [53].

### **4.4 Effects on human health**

Heavy metals in soil could be taken up by plants, and then by animals farther down the food chain, which could have serious consequences for human health. Growing plants in soil contaminated with heavy metals, such as that found near slaughterhouses, poses a threat since plant tissues can acquire these toxins [46]. When heavy metals are not broken down in the body, they accumulate in fat and muscle and become poisonous [54]. Negative impacts on human health are seen over long periods of time due to this buildup [48].

Toxic cadmium (Cd) has a specific gravity 8.65 times that of water, making it a heavy metal. Liver, placenta, kidneys, lungs, brain, and bones are particularly vulnerable to Cd poisoning [54]. It has been challenging to link morbidity and mortality to Cd′s environmental exposures, even though exposure to Cd has been linked to a wide range of clinical conditions, including anosmia, cardiac failure cancers, cerebrovascular infarction, emphysema, osteoporosis, proteinuria, and cataract formation in the eyes [55].

When administered orally, zinc (Zn) is quite secure. Overexposure to Zn could cause systemic dysfunctions that limit growth and reproduction. Zinc poisoning has been associated with gastrointestinal symptoms, hemorrhagic cystitis, icterus (yellow mucus membrane), hepatic failure, renal failure, and anemia [56].

Copper (Cu) is a component of metalloenzymes where it could donate or take electrons, making it a critical element for mammalian nutrition. Diet and drinking water are the two most common routes of Cu exposure for people. Ingestion by mistake is the most common cause of acute Cu poisoning, while certain persons may be more vulnerable due to genetics or illness [57]. Mucosal irritation and corrosion, extensive capillary damage, hepatic and renal damage, central nervous system irritation, and depression may result from excessive Cu intake in humans. Necrotic abnormalities in the liver and kidney are also possible, in addition to severe gastrointestinal discomfort. When exposed to Ni, people could experience a wide range of symptoms, from skin irritation to problems with their lungs, nervous system, and mucous membranes [58].

Humans are extremely vulnerable to lead's (Pb) harmful effects on their physiology and nervous systems. Kidney, reproductive system, liver, and brain malfunction are possible outcomes of acute lead poisoning [59]. Even in trace amounts, Pb is the most dangerous element [60]. The synthesis of hemoglobin is inhibited by lead poisoning, and the cardiovascular system and the central nervous system (CNS) and the peripheral nervous system (PNS) are both damaged acutely and chronically (PNS). Anemia, exhaustion, gastrointestinal issues, and a lack of oxygen are some more long-term consequences. Low birth weights, hypertension, and muscle and joint pain are just some of the problems that lead exposure can bring [56, 59].

A strong oxidizing agent, caustic, soluble in alkaline and mildly acidic water, poisonous, and a possible carcinogen, chromium-Cr (VI) is harmful to plant and animal

### *Abattoirs: The Hidden Sources of Plants' Heavy Metals and Other Pollutants in Lagos, Nigeria DOI: http://dx.doi.org/10.5772/intechopen.110339*

life [43, 61, 62]. Cr (VI) toxicity results from the fact that it can oxidize biological molecules despite being able to diffuse across cell membranes [61].

Mercury is poisonous and has no recognized biological or physiological function in humans. Inorganic mercury is linked to spontaneous abortion, congenital deformity, and gastrointestinal diseases (such as corrosive esophagitis and hematochezia) [56].
