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David Bradley
Composting is perhaps not the most pleasant of subjects to discuss but there is a lot of it going on around the world and given the right conditions it could help solve the landfill crises that are facing many countries and perhaps assist with the bioremediation of brownfield sites contaminated with heavy metals, writes David Bradley.

According to Gillian Greenway and Qi Jun Song of the Centre for Waste and Pollution Research at the University of Hull composting is one of the more efficient and environment-friendly ways to dispose of solid waste, Indeed, it has tremendous advantages over conventional landfill disposal, among them the potential for production of useful horticultural fertiliser, heat and methane generation. If you have ever taken a look inside your garden compost bin, you already suspect that composting is actually a rather complex process — a complicated hybrid of biological and straight chemical activity. It is highly dependent on temperature, the ratio of different materials, the overall volume involved, microbial populations and other such not-so-readily controllable factors. Indeed, explain Greenway and Song in a forthcoming issue of the Journal of Environmental Monitoring, "Intensive microbial activity and the detailed mechanisms of the process have yet to be fully understood."

One factor, into which the analytical scientists can provide some insight, however, is in obtaining metal ion information since the interaction between various metal ions within compost strongly influence and are influenced by the microbial populations. Some metals can make good compost while others can severely reduce its quality. On the other hand, the microbial denizens within compost might be exploited to bioremediate land contaminated with certain metal species.

Three-step metal speciation

Greenway and Song have applied a systematic approach to metal speciation in compost using the standardised three-step method for determining metals in soils and sediments, which was developed by the European Commission's Standards, Measurement and Testing Programme. The primary result of their analysis reveals that in general metal ions are less readily extracted for analysis as the composting process proceeds. This, they explain, implies that the composting process, at least on the large-scale municipal composting sites they have studied, must be redistributing the metals from 'more labile forms to more fixed forms'. While this hints at the potential of composting for reducing metal contamination by absorbing metals into a composting layer, which can then be retrieved, the metal redistribution certainly depends on the type of compost being used.

For the agricultural use of compost, where it can not only supply valuable nutrients but also act to suppress crop diseases in a way that generic fertiliser simply cannot, there is a definite legal requirement to understand a compost's metal profile. Previous researchers had demonstrated that municipal solid waste compost simply results in the accumulation of nickel, unwanted lead, cadmium, and chromium in the soil, and in the vegetable products being grown on that soil.

“There are broadly three types of compost in terms of their metal profiles.”

The team found that there are broadly three types of compost in terms of their metal profiles. The first group contains copper and lead. These two metals have low extractability in the first two steps of the analytical procedure and decreases further as composting goes on. The result is that they are generally obtained only as complexes of organic materials. Zinc, arsenic, cobalt and cadmium, are placed in a second group as the team found them all to be readily available throughout each stage of the analysis and indeed they show similar partitioning behaviour in terms of what reagent system will extract them in the first place. Their availability remains fairly steady throughout the composting process, perhaps hinting that they are not so heavily involved in the chemical formation of end products. Greenway points out that copper and lead are more easily combined with humic substances but not necessarily more involved in the microbiological processes. The third group bears chromium and nickel, the final two metals speciated by the team for this particular study. The way in which Cr and Ni are partitioned depends on the initial levels of these metals in the virgin compost or else the actual materials being composted.

What lies in the compost

Greenway and Song point out that some of the differences in metals retrieved from compost for analysis can be due to the analytical procedures themselves but in general they have shown that, as one might have suspected, the source of the compost itself is a major factor in the levels of metals measured. It may perhaps have seemed obvious that the source of the quality of a compost would be determined by environmental and endogenous factors that would affect the levels of metals in it but Greenway and Song have now demonstrated that this is indeed so. "To better understand this behaviour more physicochemical and biological data need to be obtained," says Greenway.

There might be a solution to controlling those heavy metals that, as revealed by Greenway and Song, always remain labile. The addition of clay in the form of bauxite refining residue (red mud) was one suggestion that has been made. And, as far as Ho Goen and Qiao Liang of the Institute for Environmental Science at Murdoch University in Western Australia demonstrated red mud did reduce metal mobility and the potential hazard of a simulated compost of grass clippings and sawdust spiked with chromium solution. They suggested that most metals ions in municipal solid waste were not usually bound to organic matter, so believe adding red mud before composting could fix free metal ions before they could be incorporated into the organic fraction. "This would fit in with our results, our next step is to look at how compost can be used in remediation," says Greenway.

“The results need to be kept churning so the development of compost as both a viable waste disposal method and a potential bioremediant can be seen.”

"This work is part of a larger project to characterise chemical and microbiological processes and products from composting farm, green, domestic and other organic wastes," Greenway told Catalyst. "DNA-based nucleic acid probe technology is being used to identify and enumerate the microbial species and it is hoped that using the results we can identify management practices for optimising the microbiology and chemistry of the composting process and also find new applications for compost, including bioremediation," she continued. In terms of large-scale composting municipal sites tend to avoid mixed streams to reduce contamination and while electrokinetic approaches are being investigated for the removal of metal ions they may not remove fixed metals. The researchers need to keep churning over the results so we can see the development of compost as both a viable waste disposal method and as a potential bioremediant.

Reference: Gillian M Greenway & Qi Jun Song. Heavy metal speciation in the composting process. J Environ Monit 2002, (4):Advance Article DOI: 10.1039/b110608m. 
 
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