Magnetic susceptibility and trace element distribution in compost size fractions

Magnetic susceptibility can be used for assessing anthropogenic pollution in solid matrices, including soils and composts. This work studies the distribution of trace elements and magnetic susceptibility in the different size fractions of six composts, for the development of measures aimed at reducing compost pollution at the production stage. The results showed that magnetic susceptibility decreased with increasing particle size in all composts, and the same was true for most trace element concentrations. Magnetic susceptibility was significantly correlated with Fe, as well as with Cu, Zn, Pb, Cr and Ni, which proves the relationship between the presence of ferric particles and trace element contamination in compost. Our results suggest that the association of trace elements and magnetic susceptibility is a characteristic feature in municipal solid waste composts.


introduction
The application of organic amendments such as compost to agricultural land has a number of beneficial effects including restoring soil fertility by increasing the soil organic matter content, supplying plant nutrients and improving the soil physical condition (Hargreaves et al. 2008;Diacono and Montemurro 2010).However, composts may contain high concentrations of potentially toxic trace elements such as Cu, Zn or Cr, in particular municipal solid waste (MSW) composts (Smith 2009).The presence of pollutants in this type of compost is mostly related to the inadequate separation of biodegradable fractions from metal-rich non-degradable or inert materials when the mechanically-sorted organic fraction is used for their production instead of the separately collected waste (Haug 1993).
As a consequence of their potential adverse effects on plant growth and the associated environmental risks, trace element contents in MSW compost are the main restriction to their agronomic use, and considerable efforts are currently devoted to the study of their concentrations and distribution.
One of the techniques that have recently gained attention in the field of pollution assessment is the measurement of magnetic susceptibility, due to the simplicity of measurement and the fact that it is a non-destructive technique.Magnetic susceptibility is the degree of magnetization of a material in response to an applied magnetic field, and it is a typical feature of soil iron oxides (Thompson and Oldfield 1986).Its utilization in the environmental field is based in the fact that many inorganic pollutants are associated by sorption processes with magnetic particles such as iron oxides, and so it can be used as a proxy for detecting anthropogenic pollution in solid matrices including soils and composts (Knab et al. 2001;Wang and Qin 2005;Spiteri et al. 2005;Yoshida et al. 2003).In a previous work (Paradelo et al. 2009), we found significant positive correlations between magnetic susceptibility and trace element concentrations in composts from different origins, and we demonstrated their relationship with ferric particles when the application of a magnet reduced simultaneously the magnetic susceptibility and trace element content.The objective of this work is to further explore this link by studying the distribution of trace element concentrations and magnetic susceptibility in the different size fractions of six composts.This knowledge could lead to the proposal of new measures for trace element correction during compost production.

Material and Methods
The composition of five municipal solid waste composts and a manure vermicompost, most of which was used for our previous study (Paradelo et al. 2009), was investigated.These were the following: MSWC1 is a compost obtained by anaerobic fermentation of the biodegradable fraction of municipal solid waste (MSW) separated before collection, followed by an aerobic composting step to stabilize the incompletely digested residue.MSWC2 and MSWC3 are aerobic MSW composts obtained from the source-separated biodegradable fraction of MSW; all MSW composts were provided by industrial composting facilities located in A Coruña (Spain).MSGW is a commercial compost obtained from the sourceseparated biodegradable fraction of MSW mixed with green waste, and MGSS is compost from municipal garden trimmings mixed with sewage sludge; they both were supplied by an industrial composting facility located in Catalunya (Spain).MV is a mixed manure vermicompost supplied by a local producer in Galicia (Spain), and it was included in the set for comparison purposes.For the analysis of the composts, the Spanish version of the European methods for the characterization of soil amendments and substrates (AENOR 2001a(AENOR , 2001b(AENOR , 2001c) ) was followed.The general properties of the composts are shown in Table 1.All the composts were alkaline (pH values 7.3-9.2) and presented similar TOC contents (217-298 g kg -1 ), whereas salinity was the highest for the three composts produced from unblended MSW (MSWC1-MSWC3).
For the size distribution study, four size fractions were separated after sieving 400 g of air-dried compost through the following meshes: 0.5, 2, and 5 mm.Each fraction was weighed, ground in an agate mortar (< 500 μm), and analysed for total Fe, Cu, Zn, Pb, Cr and Ni after wet digestion with aqua regia (AENOR 2002).The specific mass magnetic susceptibility of the fractions was measured using a magnetic susceptibility meter MS2 linked to a MS2B dual frequency sensor (Bartington Instruments Ltd.); the measures were performed at low frequency.
[ Magnetic susceptibility and trace eleMent distribution in coMpost size fractions ]

results
The trace element distributions among compost size fractions ( In general, magnetic susceptibility decreased with increasing particle size, as shown in Figure 1.This trend was very clear for the MSW composts, for which the highest magnetic susceptibility values were found in the < 0.5-mm fraction, followed by the 0.5-2-mm fraction, and then the 2-5 and 5-mm fractions.The trend was less clear in the composts MGSS and MSGW, which include green waste in their composition, although the size fractions under 2 mm still presented higher magnetic susceptibility values than the coarser fractions.It must be noted that there is a marked enrichment in the 5-mm fraction of the compost MGSS, probably due to the unexpected presence of an iron particle in the sample, which is confirmed by the high Fe, Ni and Cr contents of this fraction.Finally, no differences in magnetic susceptibility between size fractions were observed in the manure compost.These results suggest that the association of trace elements and magnetic susceptibility is a characteristic feature of municipal solid waste composts.a the amount of this fraction in MSWC1 was not sufficient for the determinations.The correlation between trace element concentration and magnetic susceptibility does not necessarily mean that all trace elements are exclusively associated with ferric particles in the final composts.This may be true for Cr and Ni, which are present in Fe alloys, as confirmed by the low solubility of these elements in the composts (Paradelo et al. 2011).For the other elements, the correlations suggest that metallic particles are their main origin and source.These elements enter the compost pile at the beginning of composting, associated with ferric particles and/or oxides, and afterwards some of them (Cu, Pb, Zn) can migrate due to changes in the physicochemical conditions during the composting process, thereby changing their distribution within the compost fractions (in particular in the organic fraction).
In consequence, the fraction associated with metallic particles in the finished composts could be lower than at the beginning of the process, even though high correlations with magnetic susceptibility remain, showing their common origin.The migration of elements from the original source points out the difficulties existing for reducing trace element concentrations in compost by mechanical procedures, particularly when they are associated with fine compost particles, and emphasizes the importance of an adequate separate collection in order to minimise metal concentration in MSW composts.
For practical purposes, the association of trace elements with metallic particles opens the door to a potential utilization of magnetic separators in combination with mechanical screening as a method for reducing metal contamination in compost at the production stage.

Figure 2 .
Figure 2. Relationships between trace element contents and magnetic susceptibility (MS) in the compost size fractions. 2

Table 1 .
Properties of the composts.EC: electric conductivity; OC: total organic carbon

Table 2 .
Trace element concentrations in composts' size fractions