Inorganic nanoadsorbents for the removal of heavy metals from water

Pollution from toxic heavy metals in water and soil poses a serious threat to human health. Despite the fact that much work has been done to report heavy metal pollution around the world, there are few review articles on how to deal with this problem. Search in the Open Scientific Journal of the Royal Society examines inorganic nanoparticles and defines the selection criteria as suitable nanoadsorbents to remove heavy metals from water.

To study: Pollution by heavy metals and the role of inorganic nanomaterials in environmental sanitation. Image Credit: Silent Corners / Shutterstock.com

Various inorganic nanoparticles, such as metal, metal oxide and metal sulphide nanoparticles, have indeed been used successfully as nanoadsorbents to clean water with high pollution by heavy metals.

Pollutants as a global problem

Pollution of water and soil is a global problem. Toxic metals, including mercury (Hg), lead (Pb), cadmium (Cd) and arsenic (As), are extremely toxic and pose a major health hazard to humans and other domestic animals, even in small amounts in water and soil.

Mercury has harmful effects on the human kidneys, brain and nervous system, while lead contamination has been linked to neurodevelopmental problems, and cadmium is a carcinogen. [1,2]. Skin blemishes, malignant skin tumors, and bladder and lung cancer are all caused by consuming arsenic.

The development of illegal gold panning and mining activities using mercury-gold smelting technologies has been described as a major source of mercury contamination in most developing countries. Lead and cadmium are commonly released into the environment from the manufacture of batteries, metals, plastics and fertilizers, as well as from mining. In subterranean fluids containing sulphide and sediment layers, arsenic pollution is common.

Common pollutant

The most common types of lead in the environment are inorganic lead (Pb2+) compounds. Organic lead compounds that were once linked to leaded gasoline and paints have indeed been phased out. A significant amount of research on the amounts of heavy metal contamination in the environment has been extensively documented, laying the groundwork for the creation of restorative sequestration solutions.

Synthesis routes for nanoadsorbents: (a) reduction, (b) co-precipitation, (c) sol-gel methods and (d) hot injection.

Synthesis routes for nanoadsorbents: (a) reduction, (b) co-precipitation, (vs) sol-gel and (D) hot injection methods. Image Credit: Mensah, M., et al, Royal Society Publications

Activated carbon, zeolite, silica gel, activated alumina, natural clay (kaolinite, bentonite, illite, stevensite and rectorite) and biomass are the most commonly used adsorbents for the sequestration of heavy metals.

Adsorption products for contamination

These adsorption products are suitable. However, they lack selection and have a low affinity for the target heavy metals. Adsorbents can be created to have improved qualities over ordinary bulk material adsorbents with the advent of nanoscience, which incorporates methods to reduce the dimensions of materials at the nanoscale (1-100nm).

Nanoscale adsorbents have improved selectivity, capacity and affinity for heavy metal contaminants, while providing faster and more efficient adsorption processes.

Nanoscale adsorbents also have a high surface area to volume ratio, surface structural characteristics designed for a specific application, short diffusion path or no resistance to internal diffusion, high porosity, improved structural characteristics and catalytic capacities.

Nanomaterials as adsorbents

Nanomaterials are nanoscale substances with better performance than bulk materials, which are called emergent characteristics. Due to the size effect caused by quantum confinement, nanoscale materials can be exploited as effective adsorbents in environmental cleaning operations.

Inorganic, carbon-based nanomaterials, polymers, nanocomposites, and biomaterials are all examples of nanomaterials. Inorganic nanoparticles have been widely used in environmental cleaning.

Iron oxides are the most commonly used magnetic metal oxides. Because they can be quickly separated from solution using an external magnet, they have a wide range of applications as nanosorbents. Metal sulfides have a greater affinity for heavy metal ions than metal oxides, but they have received less attention as nanoadsorbents for the removal of heavy metals from water.

Contaminants chemisorb on the surface of the nanoadsorbent, which implies that chemical bonds are created between the nanoadsorbent and the pollutant. The desorption process for a good nanoadsorbent should be simple. The most desirable adsorbents are those which have a high recycle or reclamation potential.

Selective adsorption of heavy metal ions in a solution based on Pearson's theory of hard and soft acids and bases using inorganic nanoparticle (a) the thiol-functional nanoparticle selectively adsorbs Hg2 + and (b) the functional nanoparticle. carboxylate selectively adsorbs Pb2 + and Cd2 +.

Selective adsorption of heavy metal ions in a solution based on Pearson’s theory of hard and soft acids and bases using inorganic nanoparticles (a) Thiol functionalized nanoparticle selectively adsorbs Hg2+ and (b) Carboxylate functionalized nanoparticle selectively adsorbs Pb2+ and cd2+. Image Credit: Mensah, M., et al, Royal Society Publications

Inorganic nanoadsorbent materials have been proven to help improve the environment. Over 100 mg l1 of heavy metal pollution has been removed. Certain nanoadsorbents have the competence to implement roles other than adsorption, such as amalgamation. Gold can fuse with mercury, but it must be in the reduced Hg0 state for this to happen. Not all nanoadsorbents are capable of forming stable amalgams with the pollutant of interest. In most cases, mercury contamination of water is primarily caused by Hg2+ ions.

Metal nanoparticles, such as copper, silver, tin and aluminum, which form a stable amalgam with mercury, are attractive and potential nanoadsorbents for the sequestration of mercury contamination. Although iron is an excellent magnetic absorber, it does not create a stable amalgam with mercury. However, by doping nanoparticles like nZVI with metals that amalgamate with mercury, a more permanent amalgam can be formed, ensuring the successful development of a more efficient and permanent nanoadsorbent for Hg.2+ removal of contaminants from water.

The references

Mensah, M., et al. Pollution by heavy metals and the role of inorganic nanomaterials in environmental sanitation. Publications of the Royal Society. Posted: October 13, 2021 https://royalsocietypublishing.org/doi/10.1098/rsos.201485

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