Background, Aim and Scope:
NOVELTY:
At present is quite difficult to evaluate the compost quality due to absence of valid general EU legislative document. Legislations of EU Member States are quit various concerning HM amounts in compost. There was performed experiment of HM speciation in soil-SSC mixtures simulating soil fertilization with aerobically digested product of sewage sludge in this atricle. Calculated factors of HM availability showed risky increase of Pb availability. This finding suggests for more stringent limitation concerning Pb for SSC usage in land application and should be concerned when preparing document of biowaste management.
ABSTRACT
Heavy metal (HM) mobility in soil depends on the HM species in it. Therefore, knowledge of HM speciation in soil allows the prediction of HM impact on the environment. HM species in the soil depend on the metal chemical origin, soil texture and other factors, also on the origin and level of the soil contamination. Recently, problem of the organic waste utilization is of great importance as amounts of these recyclable materials continually increases. One of the possible ways of decision making is the use of processed organic wastes for the agricultural needs. But one of the most serious restrictions is HM transfer from organic wastes to the soil. Therefore, prediction of HM mobility in soil and their migration in the environment is the essential thing when using organic wastes in agriculture. Zn, Cu and Pb speciation was performed according to the modified Tessier‘s (1979) methodology in two different (sandy and clay) soils with the background HM amounts as well as in the soils amended with aerobically digested sewage sludge compost (SSC) in order to find out species of the investigated HM as well as to predict their potential mobility and availability.
Materials and Methods:
Zn, Cu and Pb speciation was performed according to the modified Tessier‘s (1979) methodology in two different (sandy and clay) soils with the background HM amounts as well as in the soil samples amended with aerobically digested SSC in order to find out predominant species of the investigated HM as well as to predict their potential availability.
The modified method of sequential extraction initially proposed by Tessier et. al. (1979) is designed for the HM speciation into five species, where HM mobility decreases in the order: F1 – exchangeable HM (extracted with 1M MgCl2 at initial pH 7 and room temperature), F2 – carbonate-bound HM (extracted with 1M CH3COONa buffered to pH 5, at room temperature), F3 – Fe/Mn oxide-bound HM (extracted with 0,04 M NH2OH•HCl at initial pH 2, at 96oC), F4 – organic matter-complexed or sulphide-bound HM (extracted with 0,02 M HNO3 and 30% (v/v) H2O2 at ratio 1:1 and initial pH 2, at 85oC) and F5 – the residual HM (digested with HNO3, HF and HCl mixture). After digestion HM amounts in solution were determined by atomic absorption spectrometry (AAS “Hitachi”). SSC mixtures with uncontaminated soils of different texture (clay and sandy) were used to simulate the land application of SSC (in ratios 20:1, 10:1 and 5:1). Changes of Zn, Cu, and Pb content within species weekly during the period of 7 weeks in soil-SSC mixtures were investigated and compared with their speciation in pure soil as well as in the SSC. HM availability calculated as ratio (F1+F2)/F5.
Results:
In the SSC more HM were in the mobile species compared to the soil. Also, in the sandy soil more HM were in the mobile species than in the clay soil.
Zinc generally occurred in the mobile species (F1 and F3) and changes of the Zn speciation were insignificant during the experiment.
Pb mostly transferred to the insoluble compounds (F5) in the SSC-soil mixtures. Also, increasing Pb amount in the mobile species (F1 and F2) throw the time of reaction was observed.
Copper in the soil-SSC mixtures tended to form compounds of low mobility, such as organic complexes and sulphides (F4) as well as immobile forms (F5). Initially, the mobile Cu species (F1 and F2) increased in the soils amended with SSC but after 3-4 weeks their decreased again.
In the soil fertilized with SSC the Zn potential availability calculated as a ratio (F1+F2)/F5 increased with increasing SSC amount but it was much lower than in the SSC. Cu potential availability increased only in the sandy soil - SSC mixture within the ratio 5:1 while SSC hadn’t influence on Cu potential availability in the clay soil. Pb potential availability increased markedly under the SSC influence, especially in the sandy soil.
Discussion:
Speciation of the HM strongly depended on the metal chemical origin. According to the potential mobility HM were ranked in the following order: Zn>Pb>Cu.
Zinc in all cases was mobile (F1 and F3) and SSC hadn't influence on changes of the Zn speciation in fertilized soil.
Pb transformation to the insoluble compounds (F5) in the SSC-soil mixtures confirms that Pb is extremely immobile in the soil. However, the observed increasing Pb amount in the mobile species (F1 and F2) throw the time show its’ risky trend to become more mobile under the anthropogenic influence.
The soil had great specific adsorption capacity to immobilize Cu of the anthropogenic origin.
Potential HM availability in the soil fertilized with SSC increased compared to the soils and ranked in the order Zn > Pb > Cu. The SSC increased Zn and Cu potential availability mostly in the sandy soil and with greater SSC ratio in it while Pb potential availability increased in all cases in spite of only moderate Pb mobility in SSC itself.
Conclusions:
The Zn mobility as well as environmental impact is greater to compare with Cu and Pb while environmental impact of both, the Cu and Pb is similar but variable depending on soil texture and level of the contamination. The effect on the shift of the metal mobility and potential bio-availability was greater in sandy SSC-amended soils than in clay soils and increased with increasing SSC amount. Due to increasing Pb potential availability the usage of SSC for land fertilization should be strictly regulated regarding Pb amounts.
Recommendations and
Perspectives:
The influence of SSC on Cu and Zn mobility as well as potential availability was more significant only in the case of sandy soil with higher SSC ratio. Nevertheless this waste product of anthropogenic origin increased Pb mobility in all cases in spite of only moderate Pb mobility in SSC itself. Therefore, aerobic processing of sewage sludge must be strictly regulated especially regarding Pb amounts as well as SSC ratios must be in control regarding HM amounts when using it for on-land application.
Further we are going to perform experiments on the HM immobilization and removal as well as on the improvement of technology of sewage sludge compost production.
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