Hazardous Heavy Metals and Metalloids

Hazardous Heavy Metals and Metalloids in the Environment: Sources, Effects, and Removal Techniques

• Heavy metals are naturally occurring chemical elements in the environment that are needed for better biological cell functions in low concentrations or potentially toxic elements in high concentrations.
• Anthropogenic activities such as modern agriculture, mechanization with fossil fuel, industrialization, mining and production process, geogenic, pharmaceutical, and forest burning contribute to increasing heavy metals and metalloids concentrations in the ecosystems.
• Heavy metals and metalloids constantly flow in the ecosystems throughout the year by wind mass blow and water current in rain seasons into the lagoons, streams, lakes, rivers until seas and oceans.
• Hazardous heavy metals and metalloids continuously circulate in the environment due to residential source sewage, sludge, and wastewater used for crop production, contaminating healthy food and causing several metabolite disorders, mainly cancer diseases.
• Advanced techniques were experienced to remove several metals and metalloids in wastewater, such as chemical precipitation, ion exchange, coagulation-flocculation, flotation, electrochemical, and nanotechnologies.
• Since 1990, several studies have verified hazardous heavy metals removal in wastewater based on processes that adopt adsorption, membrane filtration like osmosis and nanofiltration membrane for copper and cadmium, polysulfone technologies adsorption membranes for lead (II), graphene-based microbots for capture, transfer and removal of lead (II), carbon-nanoparticle for remove manganese (VII), lead (II), and chromium (VI) in natural water, (nano)filtration for lead (II), nickel (II), arsenic, chromium, and cadmium, polyethersulfone nanocomposite membrane for copper (II), the hybrid membrane of nanoparticle of Fe-Ag/carbon nanotube/polyethersulphone to remove chromium (VI), organic polyethersulfone and inorganic Fe–Mn binary oxide to eliminate arsenic (III), waste fiber based poly(amidoxime) ligand to remove copper and ion, dried pomegranate peel to remove copper, cadmium, and zinc, coupling infiltration-percolation process in urban wastewater effluents and adsorption on activated carbon to remove zinc, lead, copper, and cadmium.
• Chemical elements treatment by competition was reported in zinc decreasing cadmium in soil for wheat grains, bacterial species such as Bacillus subtilis and Enterobacter cloacae reduce chromium (IV), and Pseudomonas putida decreases chromium (VI) to less toxic chromium III, as well as using plants to promote stabilization, extraction, volatilization, and filtration reducing hazardous metals and metalloids contamination in soil.
• Despite several efforts in designing and developing advanced studies and technologies to reduce hazardous heavy metals and metalloids in wastewater and arable soil, some challenges are faced, such as industrial low cost production and acquisition, material unstable and