An assessment for each method is conducted. The methods discussed in this review are classified into adsorption-, membrane-, chemical-, electric-, and photocatalytic-based treatments. Moreover, it is essential to choose the most applicable method based on the removal efficiency, chemicals added/adsorbents, initial concentration, optimal treated pH value, and other operating conditions. Therefore, the present review comprehensively and critically discusses the available technologies to expel heavy metal ions from wastewater efficiently. A complete picture of the heavy metals removal methods from wastewater resources has not been drawn yet. These studies stood on the advantages and disadvantages of a specific method for wastewater treatment, including heavy metal removal. Recent studies have focused on a particular method for heavy metal ions removal, such as electrocoagulation (EC), adsorption using synthetic and natural adsorbents, magnetic field implementation, advanced oxidation processes, membranes, etc. are commonly available in wastewater and need to be removed. The aforementioned metals and others such as silver (Ag), iron (Fe), manganese (Mn), molybdenum (Mo), boron (B), calcium (Ca), antimony (Sb), cobalt (Co), etc. Table 1 summarizes some heavy metals, focusing on their major sources, health effects, and the permitted quantity in drinking water. Although these heavy metals can be detected in traces however, they are still hazardous. The most popular heavy metals are lead (Pb), zinc (Zn), mercury (Hg), nickel (Ni), cadmium (Cd), copper (Cu), chromium (Cr), and arsenic (As). The heavy metals are non-biodegradable 1 and could be carcinogenic 2, 3, 4, 5, 6 thus, the presence of these metals in water by improper amounts could result in critical health issues to living organisms.
The heavy metal contaminated wastewater finds its way into the environment, threatening human health and the ecosystem. The presence of heavy metals in wastewater has been increasing with the growth of industry and human activities, e.g., plating and electroplating industry, batteries, pesticides, mining industry, rayon industry, metal rinse processes, tanning industry, fluidized bed bioreactors, textile industry, metal smelting, petrochemicals, paper manufacturing, and electrolysis applications. Future research studies should focus on eco-friendly, cost-effective, and sustainable materials and methods. More attention should be drawn to using real wastewaters rather than synthetic ones when investigating heavy metals removal. Electric- and photocatalytic-based methods are still less mature. Electrical-based methods were also reported to be efficient however, industrial-scale separation is needed in addition to tackling the issue of large-volume sludge formation.
However, pre-treatment and periodic cleaning of membranes incur additional costs. Fouling and scaling inhibition could lead to further improvement in membrane separation. Even though the chemical and membrane methods are practical, the large-volume sludge formation and post-treatment requirements are vital issues that need to be solved for chemical techniques. The major obstacles of the adsorption methods are the ability to remove different ion types concurrently, high retention time, and cycling stability of adsorbents. Generally, it is noticed that most of the recent studies have focused on adsorption techniques. Besides, the key findings of the previous studies reported in the literature are summarized. This paper comprehensively and critically reviews and discusses these methods in terms of used agents/adsorbents, removal efficiency, operating conditions, and the pros and cons of each method. These methods could be classified into adsorption-, membrane-, chemical-, electric-, and photocatalytic-based treatments.
Different reported methods were devoted to heavy metal ions removal from various wastewater sources. Removal of heavy metal ions from wastewater is of prime importance for a clean environment and human health.
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