Which methods are used to remove heavy metals from wastewater?
Heavy metals such as arsenic, copper, cadmium, chromium, nickel, zinc, lead, and mercury are among the main contaminants of fresh water reservoirs because of their toxic, non-biodegradable, and persistent nature. Heavy metal pollution, including that found in the air, water, soil, and biosphere, is primarily brought about by industrial expansion. Because they are highly soluble in aquatic environments, heavy metals are readily absorbed by fish and plants. As a result, they could enter the human body through the food chain. To reduce the levels of heavy metals in water and wastewater, numerous techniques have been developed and put to use. Membrane filtration, ion-exchange, adsorption, chemical precipitation, nanotechnology treatments, electrochemical reactions, and improved oxidation techniques are some of these technologies.
In this blog, the treatment methods as well as their mechanisms are discussed briefly.
Treatment for heavy metal removal:
Water that has been contaminated by heavy metals requires multiple steps of wastewater treatment. From the contaminated water to the final treated water, it uses a number of techniques. The primary, secondary, and tertiary treatment procedures are typically combined in the water treatment units.
Primary Water Treatment Technology:
The process of purifying water begins with primary treatment. Then, additional treatments are used. In general, this treatment's efficiency is substantially lower than that of tertiary treatment. These less complex and more affordable technologies allow us to eliminate significant volumes of pollutants.
Microfiltration offers the widest range of pore sizes among membrane filtration techniques (100–1000 nm). Heavy metal ions are eliminated via microfiltration. Additionally, it filters germs, algae, and TDS. Tertiary heavy metal purification procedures use water that has undergone microfiltration treatment.
· Chemi?cal Precipitation
An essential adsorptive mechanism for the elimination of heavy metals is precipitation. It depends on the metal's solubility product, the wastewater's pH, the amount of metal ions present, and the concentration of pertinent anions. Compared to other technologies, it is simple and inexpensive. Simple sedimentation is then used to get rid of them. Large sludge production and secondary contamination are major drawbacks.
· Che?mical coagulation
Fine solid wastewater particles are destabilized by chemical coagulation, which is always employed after chemical precipitation so that they can aggregate during chemical flocculation. Since the particles are often negatively charged, they cannot aggregate into bigger groupings or settle. Positively charged coagulants are added during chemical coagulation, which lowers the charge of the negatively charged particles and allows them to form larger groups. When an anionic flocculant is added to the mixture, it combines with the mixture's negatively charged components to bind the particles into bigger groups, which can then be removed by filtration or sedimentation. Chemical coagulation is one of the less expensive methods for removing heavy metals from industrial effluent, along with chemical precipitation. Since it is not acid-resistant, treating wastewater with a high acid concentration is not a good use for it. Additionally, the sediment contains heavy metals that require additional processing, including neutralization or dehydration.
Secondary Water Treatment Technologies:
Microorganisms that are present in nature are used for secondary water treatments. These organisms transform contaminants into less complex materials. The treatment is separated into two main groups:
· Anaerobic Treatment
Anaerobic treatment is an energy-efficient method. Here, bacteria convert wastewater contaminants into biogas. It happens without oxygen present. Methane-rich biogas is produced as a result of this process, and it is used as a sustainable energy source.
· Aerobic Treatment
In the presence of oxygen, microorganisms turn contaminants into carbon dioxide. Because aerobic microbes require oxygen, air must be pumped through the tanks on a regular basis. Aerobic bacteria feed on the waste present in the water source when forced air from an air blower or compressor is combined with the wastewater.
Tertiary Water Treatment Technologies:
The final stage of wastewater treatment is tertiary water treatment. After that, we can either recycle it or release it into the environment.
· Advanced oxidation and chemical oxidation
Chemical oxidation includes adding an oxidizing agent to the wastewater, which causes electrons to transfer from the oxidant to the contaminants, modifying their structurally and making them less harmful. Chemical oxidation, however, could result in hazardous byproducts. Advanced oxidation can assist in removing any harmful byproducts of chemical oxidation through procedures like steam stripping, air stripping, or activated carbon adsorption. Heavy metal wastewater including organic molecules is always pretreated using chemical oxidation and advanced oxidation.
The majority of commonly used tertiary water treatment processes are based on crystallisation technology. Thermal procedures, like distillation, use enough energy to heat the dirty water to boiling before allowing it to vaporise. Some of the water turns into steam as a result. Condensing the process steam into pure water is the last stage.
· Ion exchange
Another popular heavy metal removal technique is ion exchange, which can lower the levels of nickel, mercury, lead, cadmium, chromium, and copper in water. Heavy metal ions are easily exchanged for innocuous ions like chloride when water flows through an ion exchange resin, making the discharge water suitable for drinking and irrigation.
Through the use of intermediary solid resins, ion exchange moves one or more ionic species from one liquid phase to another. To do this, a group of ions—the target ions—present in a solution are swapped with a different set of ions—IX resin or IXR—present in a solid phase. Target ions—which could be harmful, valuable, or impure—are thereby focused on the resin, and non-harmful, valuable, or impure ions are released from the resin to replace them in the solution. The ion exchange procedure is often designed to be reversible as well. As a result, it is anticipated that the used resins will be recycled. As a result, it is anticipated that the used resins will be recycled.
Adsorption is a different approach to removing heavy metals from wastewater. Adsorption is primarily a mass transfer process where a substance is moved from the liquid phase to the surface of a solid and is then chemically and physically bonded. To remove heavy metals from wastewater, a variety of low-cost adsorbents made from agricultural waste, an industrial by-product, natural materials, or modified biopolymers have recently been created and used. Adsorption for the treatment of industrial wastewater may need to be chemically modified to improve the removal of heavy metals.
· Membrane filtration
The effective removal of suspended particles, organic compounds, and inorganic pollutants such heavy metals from wastewater via membrane filtration has drawn a lot of attention. Depending on the size of the particle that can be retained, there are many types of membrane filtration. Additionally, membranes can be made from ceramics like silicon carbide or polymeric materials. The silicon carbide ceramic membranes are the best for removing heavy metals because they are very durable, adding to their thermal and chemical strength, making them excellent for the harsh settings where heavy metals are frequently found in wastewater.
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