Metals become contaminants when their amounts in soil, reach levels that are toxic to human life, plants or other biological organisms. This creates a need for soil remediation. Remediation is the process of reducing the toxicity of contaminants or ridding the soil completely of toxic contaminants. Heavy metals (for example, arsenic, cadmium, lead and zinc) fall under the group of inorganic chemical contaminants (5). These metals are released into the soil from diverse industrial activities including mining, metal works and refineries. Environmentalist and soil scientist employ different ways of restoring metal contaminated soils. These methods are listed below.
1. Physical remediation: Some techniques here include replacing contaminated soils with clean soils or use of thermal desorption (use of heat to volatilize contaminants) (1,2).
2. Chemical remediation: Examples of chemical remediation methods include- chemical leaching (using chemical reagents, fluids or gas to wash down contaminants); chemical fixation, electrokinetic remediation, vitrify technology (see bibliography 1 and 4 for an exhaustive review) (1).
3. Biological remediation: Biological remediation involves the use of biological organisms to restore the soil. Some examples are phytoremediation, land farming, bioaugmentation, biostimulation and the use of organic materials (such as compost, wood ash, and saw dust).
This article discusses the use of compost to restore soils contaminated with heavy metals.
Compost for bioremediation of metal contaminated soils
Compost is made by mixing organic materials such as animal manure or crop residues under conditions favourable for microbial decomposition. It is a significant soil additive, used to organically improve soil fertility and quality. Compost is rich with micro-organisms which have the remarkable ability to break down organic contaminants to smaller forms or forms which they can utilise for metabolism (5).
On the other hand, inorganic contaminants such as metals cannot be broken down by composting instead they are transformed into a lesser bioavailable form (9). The humic substances and iron oxides present in compost binds to metals in soil and limit the availability of these metals for plant uptake (9).
Compost is a cost effective and natural way of restoring soils. It is environmentally friendly and more likely to gain public acceptance. Some examples of successful compost remediation studies are:
1. Research by Dr Rufus Chaney of the US department of Agriculture In Bowie, Maryland USA showed compost reduced the toxicity of lead contaminated soils (6)
2. In spain, an experiment with Chenopodium album L (a weedy annual plant also known as lambs quarter) grown on heavy metal contaminated soil; showed addition of compost to soil reduced concentrations of Pb, Zn, Fe, and Mn, in the plant. In the same study they found that cow manure reduced concentrations of Zn and Mn by 91% and 95% when compared with soils that did not receive manure (7).
3. A study by researchers in Britain showed application of compost or fresh organic waste reduced the accumulation of Cu, Pb, Zn, and As in plants grown on metal polluted mine soils (8).
Some limitations to the use of compost for bioremediation
1. Compost does not degrade inorganic contaminants such as heavy metals it only reduces their availability for plant uptake (5,9).
2. Compost itself may release some metallic contaminants (9). For example, poultry manure can contain considerable amounts of metals or metalloid such as As, Co, Cu, Fe, Mn, Se, Zn which is susceptible to run-off when the manure is applied to the soil. These elements are often added to poultry feeds to improve their productivity (10). Sewage sludge which is sometimes a component of compost may also contain metallic elements (9,11). However, research has shown that treatment of manure with aluminium sulphate can reduce run-off of metallic elements; in addition co-composting of sewage sludge with lime can reduce the availability of potentially harmful elements (10,11,12).
Bibliography and further reading
1. Yao et al (2012) Review on remediation technologies of soil contaminated by heavy metals. Procedia Environmental Sciences 16: 722-729.
2. Smith et al (2001) Thermal desorption treatment of contaminated soils in a novel batch thermal reactor. Industrial and Engineering Chemistry Research 40: 5421–5430.
3. Shorouq et al (2015) Acid leaching of heavy metals from contaminated soil collected from Jeddah, Saudi Arabia: kinetic and thermodynamics studies. International Soil and Water Conservation Research 3: 196-208.
4. Wuana et al (2011) Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation. SRN Ecology-
5. Coker C (2006) Environmental Remediation By Composting- https://www.biocycle.net/2006/12/14/environmental-remediation-by-composting/
6. Innovative uses of compost bioremediation and pollution prevention- https://www.epa.gov/sites/production/files/2015-08/documents/bioremed.pdf
7. Walker et al (2004) Contrasting effects of manure and compost on soil pH, heavy metal availability and growth of Chenopodium album L. in a soil contaminated by pyritic mine waste. Chemosphere 57:215-224.
8. Tandy et al (2009) Remediation of metal polluted mine soil with compost: Co-composting versus incorporation. Environmental Pollution 157:690-697.
9. Barker, AV and Bryson, GM (2002) Bioremediation of heavy metals and organic toxicants by composting. The Scientific World Journal 2: 407–420.
10. Tufft, LS and Nichols, CF (1991) The effects of stress, Escherichia coli, dietary EDTA, and their interaction on tissue trace elements in chicks. Poultry Science 70:2439–2449.
11. Fang, M and Wong, JWC (1999) Effects of lime amendment on availability of heavy metals and maturation in sewage sludge composting. Environmental Pollution 106: 83–89.
12. Moore et al (1998) Decreasing metal runoff from poultry litter with aluminum sulfate. Journal of Environmental Quality 27: 92–99.