Case Study: A Review of Heavy Metal Contamination in the Soil

In 2014 Su et al. reviewed the impact of heavy metal soil contamination. This article is the basis of the second case study of the week.

Industrialization has led to an increase of heavy metal contamination in the soil, including mercury (Hg), cadmium (Cd), lead (Pb), chromium (Cr), and arsenic (As), etc. When a soil is contaminated with heavy metals it is difficult to be remediated.

Soil contamination can affect people both directly and indirectly, through the consumption of contaminated plants and animals. Heavy metals are highly hazardous to the environment and organisms because they can be enriched through the food chain. It has only been in recent years that the scientific community and politics have began to consider the health of soil through measures to reduce pollution.

Heavy Metals in Soil

The paper by Su et al. (2014) lists the characteristics, sources and harmfulness of heavy metal contamination in the soil. Heavy metal contamination poses a serious threat to almost every country. It is very dangerous since it is hard to detect and remediation is expensive and long.

Heavy metal contamination may have different origins: atmospheric deposition; sewage irrigation; improper stacking of the industrial solid waste; mining activities; the use of pesticides and fertilizers; as well as energy, transport, metallurgy and production of construction materials. Automotive transport is responsible for Arsenic (As) and Nickel (Ni) contamination. Heavy metals may contaminate fields and crops by irrigative sewage. Moreover, fertilizers and pesticides are an important source of heavy metal contamination.

Heavy metals may affect soils by impacting on microorganisms and enzymatic activity; or by poisoning the plants. They may also affect human health both through the inhalation of gases and dust particles and by ingestion of contaminated food causing serious illnesses.

The paper describes the current situation of the soil heavy metal contamination in the world. Although with important differences, heavy metals pollute both urban and agricultural soils. Copper (Cu), lead (Pb), Nickel (Ni), and Cadmium (Cd) are present in both soils while Chromium (Cr) is more commonly diffused in urban soils and Arsenic (As), Mercury (Hg), Dichlorodiphenyltrichloroethane (DDT) and Polycyclic Aromatic Hydrocarbons (PAHs) are an issue for agricultural soils.

Remediation Strategies

The review by Su et al. (2014) also explains the possible remediation strategies of heavy metals in contaminated soils.

The most diffuse remediation methods are engineering or bioremediation methods.

Engineering methods involve:

1) Replacement of contaminated soil, soil removal and soil isolation, which is used for seriously contamination in small areas.

2) Electrokinetic remediation is a new economically effective technology which performs well in soil with low permeability.

3) Soil leaching i.e. washing the soil with specific reagents and subsequent removal of the heavy metal complex and soluble irons adsorbed on the solid phase particles. This allows separation of the heavy metals from the soil. Heavy metals are then recycled from extracting solution.

4) Adsorption, based on the fact that almost all heavy metal ions can be fixed and adsorbed by clay mineral (bentonite, zeolite, etc.), a steel slag, furnace slag, etc.

Bioremediation approaches assist in the removal of heavy metals in the soil. This process can be done by plants (phytoremediation), microbes (which may be improved by using biotechnological manipulations) and animals (e.g. maggots, earthworms, etc.) which degrade the target pollutants.

What we would like you to do

Please share your thoughts in the following questions in the comments section below:

• Are you surprised urban and agricultural soils have different levels and types of heavy metal contamination?

• Were you aware of the different remediation strategies to address soils contaminated with heavy metals?

• Do you think it is necessary to care and mitigate against heavy metals in our soils?

WARNING: Please note that there are images that appear in the paper by Su et al (2014) that are not suitable for any learners under the age of 16.