The use of natural Mn oxide-containing wastes as a contaminated land remediation strategy and their effects on soil microbial functioning

Abstract

The viability of using natural manganese oxide (MnOx)-containing wastes as amendments for contaminated land remediation was examined. The success of MnOx as a viable strategy was determined via the impact that their addition had upon microbial soil functioning, in addition to their ability to immobilise and/or transform inorganic and organic contaminants within industrially polluted soils. Contaminated soils were obtained from two former industrial sites that are polluted with PAHs and metals. The intrinsic microbial functioning of these soils was assessed using a suite of microbial indicators reportedly sensitive to contamination [basal respiration (BR), potential nitrification (PNR), denitrification enzyme activity rates (DEA), microbial biomass carbon (MBC), metabolic quotient (qCO2), microbial quotient (qmic)]. The diversity and community structure of key populations related to microbial indicators were assessed using culture-independent community analysis [polymerase chain reaction (PCR)-denaturing gradient gel electrophoresis (DGGE)] to target 16S rRNA and functional genes (amoA, nirS/K). These measurements were combined with a suite of physico-chemical analyses [pH, total organic carbon (TOC), soil organic matter (SOM), moisture content (MC), water holding capacity (WHC), extractable metal and PAH concentrations] to provide a combined geochemical and molecular microbial approach. Contaminated soils were compared to a range of soils from land types defined as non-contaminated to provide a robust evaluation and define suitable microbial indicators for use in assessing soil microbial function in contaminated land and its remediation. Results showed that long term metal and PAH pollution had resulted in a microbial populations exhibiting extremely suppressed rates of BR and DEA, indicative of pollutants being bioavailable within the soil. Functional gene profiling revealed that inherent denitrifying and ammonia-oxidising community structures were significantly affected by contamination. Microbial functional processes of BR, DEA and PNR were determined to be superior indicators through their ease of use with standardised rapid and high throughput methods that could infer contaminant availability. A preliminary assessment of MnOx addition upon microbial soil functioning was investigated though 6 month microcosm trials employing BR, DEA and PNR as indictors. Microcosms employed two natural MnOx-containing wastes (mine tailings and coated sands) in a range of 0-30 % total MnOx, which were added to a low level metal contaminated soil. MnOx was found to stimulate PNR and DEA. Results implied that MnOx is not detrimental to soil microbial functioning and is capable of removing inhibitors of N-cycling, via a chemical rather than biological mechanism, ascribed to the immobilisation of bioavailable toxic metal ions by the MnOx. The potential of MnOx amendment as a viable remediation strategy was investigated through the use of 9 month outdoor lysimeter trials. Measurement of extractable PAHs, along with extractable and bioaccessible Pb and As in metal, PAH and mixed contaminated soils showed no positive effects of using a 10 % by weight MnOx-coated sand amendment for remediation. Analysis of soil microbial functional indicators (BR, DEA, PNR) showed that MnOx amendment had no detrimental effects upon the function of microbial populations in the aforementioned soils. Mn(II)-oxidising bacteria were isolated from contaminated soils and MnOx-containing wastes. This suggested that sustained and biologically enhanced redistribution of MnOx was possible in MnOx-amended soils, which may play an important role in pollutant transformation. This study provided the first demonstration of species within the genera Amycolatopsis, Sacchorothrix, Lentzea and Micromonospora as being capable of Mn(II) oxidation.