Table 2.

Removal of heavy metals with conventional methods

Conventional method	Adsorbent	Heavy metal	Observation	Efficiency	Advantages	Disadvantages	Reference
Adsorption	Graphene oxide-based microbots	Lead(II)	Cleaned water from 1000 ppb down to below 50 ppb in 60 min	95%	A wide range of heavy metals are removed. More removal efficiency. High specific surface area.	Expensive. Sludge production. Regeneration is not possible. Adsorbent decides the metal removal efficiency.	[30]
	Oxidized activated carbon	Copper(II)	Adsorption capacity increased with a pH range of 3.0–6.0	91.30%			[31]
	SiO2-Carbon nanotube	Mercury(II)	Endothermic process, mercury removal increased with increase in temperature	98%			[32]
	Polypyrrole-based activated carbon	Lead(II)	Highest adsorption at pH 5.5, followed chemisorption pathway.	81.80%			[33]
	Geopolymer from dolochar ash	Cobalt(II), nickel(II), cadmium(II), and lead(II)	The process was spontaneous and endothermic. Maximum removal at pH, temperature, and initial metal ion concentration were 7.8, 343 K, and 10 ppm.	98–99%			[34]
Air stripping		Nickel ammoniacomplexion	Optimal parameters pH 11, the temperature of 60°C, and an airflow rate of 0.12 m3/h	Nickel and ammonia were less than 0.2 mg/L and 2 mg/L	Low cost. Reliable technique.	Not suitable for a wide range of pollutants. Bulk pollutants could not be removed.	[35]
		Mercury	Air stripping with chemical reduction treats a large volume of water.	94% Decrease in mercury level during the injection.			[36]
Coagulation	Ferric chloride and alum	Arsenic	Not effectively remove As from the municipal wastewater to <2.00 μg/L	Reduced total recoverable arsenic from 2.84 and 8.61 μg/L	Dewatering, microbial inactivation, and sludge settling properties.	More sludge is produced. Requirement of chemicals.	[37]
	Humic-like component of terrestrial origin	Copper(II)	Enhanced removal efficiency by intermolecular bridging between the pollutant and humic component of molecular range 100 kDa0.45 μm.				[38]
	Iron electrode	Chromium(IV)	Sinusoidal alternating current reduces energy consumption and enhances removal efficiency.	99.73% and the residual Cr(VI) in the effluent was <0.1 mgdm−3			[39]
Chemical precipitation	Cu-EDTAdecomplexation	Copper	Cu ions were precipitated as Cu2(OH)2CO3, CuCO3, Cu(OH)2, and CuO.	68.30%	Low investment. Facile process.	More sludge is produced containing metals. High sludge and maintenance cost.	[40]
	Magnesium hydroxy carbonate	Oxovanadium(IV), chromium(III), and iron(III)	Removal efficiencies of heavy metals were increased with the dose of magnesium hydroxy carbonate (.30 g for 50 mL)	99.90%			[41]
Electrochemical	Graphene oxide electrode	Copper, cadmium, and lead	The high density of surface functional groups to assist the electrodeposition by the graphene oxide electrode	>99.9%	Pure metals can be recovered. No chemicals requirement. Rapid technique.	High capital and running costs. Generation of by-products.	[42]
		Zinc (Zn), nickel (Ni), and copper (Cu)	Electrochemical better than nanofiltration	99.81%, 99.99%, and 99.98%			[43]
Ion exchange	Li1.9MoS2	Mercury(II), lead(II), cadmium(II), and zinc(II)	Lithium-intercalated layered metal chalcogenides experience exfoliation when treated with water	580 mg of mercury/g	A wide range of heavy metals are removed. Appreciable regeneration and pH tolerance.	High capital and running costs. Only selective metals are removed.	[44]
	Carboxylic weak acids	Copper(II), iron(II), lead(II), and zinc(II)	The complexing nature of carboxylic weak acids stabilize metal ions in solutions generating broader functional pH regions for metal extraction.	Extraction >85%-99%			[45]
Membrane	Ceramic supported graphene oxide (GO)/Attapulgite (ATP)	Copper(II), nickel(II), lead(II), and cadmium(II)	The use of aluminum oxide substrate increased stability and extended usage of membrane	Rejection efficiency 99–100%	High efficiency toward metal selected. Less chemical consumption. Simple design that occupies less space.	Expensive. Fouling of membrane. Flow rates are less. Sludge production.	[46]
	Layered cellulose-based nanocomposite membrane	Silver, copper(II), iron(II), and iron(III)	The high affinity of the membrane toward metal ions.	86–100%			[47]