Table 1.

Different sources and effects of heavy metals and their mechanism of toxicity.

Metals	Sources	Effects	Mechanisms of heavy metal toxicity	Permissible limits (mg/L)	References
Chromium	Metal plating, electroplating, leather, mining, galvanometry, and dye production	Normocytic, hypochromic anemia, leukopenia, disturbing the vegetable yield and its quality to humans (i) Reduction in root growth, leaf (ii) Inhibition of seed germination (iii) Reduction of protein content in algae and photosynthetic pigments	Reactions between Cr6+ and biological reductants like thiols and ascorbate	0.5	[10, 23, 24]
Lead	Industrial sources, mining, plumbing, and fuels	Mental retardation in children, lung, and kidney damage Disturbs various plant physiological processes (i) Fastens the production of reactive oxygen species (ROS) (ii) Lipid membrane damage	An imbalance between the production of free radicals and the generation of antioxidants to detoxify the reactive intermediates (i) Replacing other divalent cations like Ca2+, Mg2+, and Fe2+ and monovalent cations like Na+	0.001	[4, 24]
Cadmium	Electroplating, fertilizers, mineral processing, and battery manufacturing	Kidney damage, cancer, gastrointestinal disorder Influencing the enzymatic systems of cells and oxidative stress and inducing a nutritional deficiency in plants	(i) Binding to cysteine-rich protein such as metallothioneins (ii) Binding with cysteine, glutamate, histidine, and aspartate ligands	0.003	[4, 10, 24]
Arsenic	Mining by-product, pesticides, chemical waste, and fossil fuel burning	Internal cancer, skin lesions bronchitis, dermatitis, and death (i) Acute poisoning (ii) Affect the quality of surface water	(i) Biotransformation of harmful inorganic arsenic compounds get methylated by bacteria, algae, fungi, and humans to give monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)	0.01	[4, 10, 24]
Mercury	Batteries, paper industry, metallurgy industries, chemical manufacturing, and mining, coal	Damage to the central nervous system, protoplasm poisoning, increased heart rate (i) Microtubule destruction, mitochondrial damage, lipid peroxidation, and accumulation of neurotoxic molecules (ii) Malfunctioning of nerves, kidneys, and muscles	(i) Binding to freely available thiols as the stability constants (ii) Attachment to the selenohydryl and sulfhydryl groups	0.001	[4, 6, 10, 24]
Zinc	Refineries, brass manufacture, metal plating, and plumbing	Damage to the nervous membrane, corrosive effect on the skin	(i) Generating reactive oxygen species (ii) Activation of the mitogen-activated protein kinase pathway	5.0	[7, 25]
Manganese	Mining, industrial waste, acid mine drainage, welding, and fuel addition	Damage to the central nervous system	(i) It is added to gasoline as methylcyclopentadienyl manganese tricarbonyl (MMT)	0.04	[4, 7, 26]
Copper	Copper and brass plating, mining, metal industries, and copper-ammonium rayon industries	Liver and kidney damage inducing DNA strand breaks and oxidation of bases via oxygen-free radicals	(i) Reacting with several biomolecules (ii) Participating in the formation of reactive oxygen species (ROS)	1.5 mg/l	[10, 24, 26]
Nickel	Nickel- or chromium-plated taps, bore-hole equipment	Skin sensitizer, dermatitis, and prenatal mortality	(i) Replacing the essential metal of metalloproteins (ii) Binding to catalytic residues of nonmetalloenzymes	0.020	[26, 27]
Cobalt	Aircraft engines, magnets, grinding and cutting tools, artificial hip and knee joints, glass, ceramics, and paints	Congestive heart failure, dermatitis, liver and kidney effects, nausea, vomiting, diarrhea, bleeding, and coma	Generating superoxides Generating free radical		[26, 28]