Controlling Heavy Metal Pollution: Strategies And Solutions

Heavy metal pollution is a growing problem, and it is becoming a great concern due to the adverse effects it is causing around the world. These inorganic pollutants are being discarded in our waters, soils and into the atmosphere due to the rapidly growing agriculture and metal industries, improper waste disposal, fertilizers and pesticides.

Heavy metals are defined as metallic elements that have a relatively high density compared to water. They are considered toxic to the environment and humans. Some of the heavy metals include cadmium, chromium, copper, mercury, lead, zinc, arsenic, boron and the platinum group metals.

The toxicity of heavy metals depends on several factors including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals.

Arsenic, cadmium, chromium, lead, and mercury are considered to be the most toxic heavy metals. They are also classified as human carcinogens (known or probable) according to the U.S. Environmental Protection Agency, and the International Agency for Research on Cancer.

The main pathways of exposure include ingestion of contaminated food; inhalation from the atmosphere, drinking contaminated water; and due to skin contact from agriculture, pharmaceutical, manufacturing, residential and industrial areas.

The toxicity of heavy metals can be reduced through phytoremediation, which involves the use of hyperaccumulator plants to remove heavy metals from the soil, sediments and waters.

Phytoremediation

The process of phytoremediation involves several steps:

• Mobilization of heavy metals in the rhizosphere
• Uptake of heavy metals by plant roots
• Translocation of heavy metal ions from roots to aerial parts of the plant
• Sequestration and compartmentation of heavy metal ions in plant tissues

The efficiency of phytoremediation relies on several factors, including plant selection, plant performance, heavy metal bioavailability, soil, and rhizosphere properties.

There are different categories of phytoremediation, including:

• Phytostabilization: Using metal-tolerant plant species to immobilize heavy metals belowground and decrease their bioavailability, thereby preventing their migration into the ecosystem and reducing the likelihood of metals entering into the food chain.
• Phytoextraction: Using plants to extract and remove heavy metals from soil or water, and translocate and accumulate those contaminants in their aboveground biomass.
• Phytovolatilization: Using plants to absorb heavy metal from soil and release them into the atmosphere as volatile compounds.
• Phytofiltration: Using hydroponically cultured plants to absorb or adsorb heavy metal ions from groundwater and aqueous waste.
• Phytodegradation: Using plants and associated microorganisms to break down organic pollutants.

The selection of appropriate plant species is crucial for effective phytoremediation. To fulfill the requirement of highly effective phytostabilization, plants should be tolerant to the heavy metal conditions and have a dense rooting system.

Genetic engineering has emerged as a powerful tool to modify plants with desired traits such as fast growth, high biomass production, high heavy metal tolerance, and accumulation. This can be achieved by transferring metal hyperaccumulation traits to high biomass plants or introducing hyperaccumulation traits to fast-growing, high-biomass plants.

Phytostabilization and phytoextraction

Phytoremediation is a promising approach for revegetating heavy metal-polluted land. It involves the use of plants to restore polluted environments. Phytoremediation mechanisms include phytoextraction, phytostabilization, phytovolatilization, rhizofiltration, phytodegradation, and rhizodegradation.

Phytostabilization is a technique that uses metal-tolerant plant species to immobilize heavy metals belowground, reducing their bioavailability and preventing their migration into the ecosystem. This process decreases the likelihood of metals entering the food chain. Phytostabilization can occur through various mechanisms, including the precipitation of heavy metals, reduction in metal valence in the rhizosphere, absorption, and sequestration within root tissues, or adsorption onto root cell walls. It is important to note that phytostabilization only temporarily contains heavy metals, and these metals remain below ground.

On the other hand, phytoextraction is a permanent solution for removing heavy metals from polluted soil. It involves the use of plants to take up contaminants from soil or water and translocate and accumulate them in their aboveground biomass. Plants that can accumulate heavy metals are cultivated on polluted sites, and the metal-enriched biomass above the ground is collected, resulting in the removal of soil pollutants. Phytoextraction is the most important phytoremediation technique for reclaiming heavy metals from polluted soil and is more suitable for commercial applications.

To improve the efficiency of phytostabilization and phytoextraction, various strategies have been explored, including genetic engineering, microbe-assisted, and chelate-assisted approaches. These techniques offer promising solutions for revegetating and remediating heavy metal-polluted environments.

Intercropping

One example of intercropping for heavy metal phytoremediation is the combination of Moso bamboo and Sedum plumbizincicola. Moso bamboo is a fast-growing, high-biomass species with a high tolerance for heavy metals, while Sedum plumbizincicola is a Cd/Zn hyperaccumulator. When intercropped, these two species can increase the removal efficiency of heavy metals from polluted soils.

Another example of intercropping for heavy metal phytoremediation is the combination of gramineous pasture ryegrass and leguminous forage alfalfa. Intercropping these two species can increase plant biomass, reduce the accumulation of heavy metals, and improve the resistance of plants to heavy metals. It achieves this by reducing oxidative damage and increasing enzymatic antioxidant activities.

Overall, intercropping plays a vital role in phytoremediation and can be a promising strategy for controlling heavy metal pollution.

Organic farming

Heavy metal contamination in soil is a serious issue that organic farmers need to address. While some heavy metals occur naturally in the soil, they become a problem when they are present in high concentrations or when synthetic heavy metals accumulate in processed waste streams.

Organic farmers must navigate the risks and rewards of every decision they make, such as the potential benefits of a new product versus the risks of contamination. One example of a contaminant that organic farmers must consider is chicken manure. While it provides high levels of nitrogen and phosphorus, it can also be contaminated with heavy metals due to conventional feed additives containing arsenic. Similarly, phosphate rock, a valuable nutrient source, may include mineral deposits with high levels of arsenic and cadmium. Composting methods can break down certain pesticides, but heavy metals are elemental contaminants that persist during the composting process.

To keep heavy metals out of organic soil, farmers can refer to resources provided by organizations like the Rodale Institute and the Organic Materials Review Institute (OMRI). The OMRI has developed thresholds for potential contamination of input materials used in organic production, taking into account the typical loading rate of identified high-risk materials. For instance, compost, manure, and mulch, which are applied at higher load rates, have lower thresholds than mined minerals and ash, which are generally applied at lower rates.

Additionally, certifiers require organic farmers to identify management practices that address heavy metal contamination as part of the Organic System Plan. Farmers who have completed a HACCP (Hazard Analysis and Critical Control Points) assessment or other risk assessment plans may also implement heavy metal testing as a method of control. Certifiers can request additional testing if potential contamination is suspected. By following these guidelines and staying informed about potential sources of heavy metal contamination, organic farmers can take proactive steps to minimize the presence of heavy metals in their soil and promote sustainable agricultural practices.

Chelation

• DMSA chelation therapy increased lead excretion on average by a factor of 12 and rapidly reversed lead-related symptoms (largely neurological and gastrointestinal) in a case series of 17 lead-poisoned adults; these authors also reviewed effectiveness of DMSA.
• The same group reported a case of a jeweler with extensive neurological symptoms of mercury poisoning, reversed with DMPS treatment.
• A trial of oral DMPS therapy in the Philippines provided two weeks of treatment in a community highly exposed to mercury used for artisanal gold mining. Most participants experienced multiple significant neurological improvements.
• The effective use of chelation in patients with lower levels of accumulation of toxic elements is not as widely recognized, but positive trials are being reported.
• In a randomized, double-blind controlled trial conducted by Adams et al., reductions in measures of the severity of autism were associated with the difference in urinary excretion of toxic metals before and following treatment with DMSA, demonstrating both a significant positive association between the severity of autism and the body burden of toxic metals, and efficacy of reduction of this body burden in improving symptoms.

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