The remediation of industrially contaminated soil
The remediation of two contaminated soils in the Tyne and Wear Metropolitan district was examined. These were a sediment dredged from the river bed at Dunston Coal Staiths on the River Tyne (downstream from Derwenthaugh coke work site) and coke work-contaminated soil from the Derwenthaugh site, Blaydon, Nr. Newcastle-upon-Tyne. The river Tyne dredgings were of a very fine material (70% silt; 24% clay) with high water retention capacity. Levels of (EDTA available) Zn (490mg/kg), total Cd (7.5mg/kg) and total Pb (510mg/kg) were above the Department of Environment's (1987) threshold values for soil contaminants. Barley (Hordeuin vulgare L. cv Kym) sown in the drcdgings in ten outdoor plots (Irn x 0.5m), grew very poorly (yield = 2.4g dry wt. /plant, compared with that on an uncontaminatedc. ontrol soil (7.4g dry wt./ plant). The barley exhibited all the classic signs of metal phytotoxicity despite the addition of fcrtiliscr and organic waste (straw and spent mushroom compost). When lime was added to raise the pH of the dredgings in the plots to over pH 7.1, the growth rate and the yield of barley improved significantly (yield = 6.8g dry wt. /plant). Levels of available Zn, Cd, Pb and Cu in the limed dredgings were now lower than in the unlimed dredgings. Copper and zinc levels in leaves of barley raised on the limed material were lower than levels in barley grown on unlimed dredgings. There was no significant difference in yield or growth rate between the different plots of dredgings in which organic supplementation parameters were varied. In conclusion, pH was the dominant factor in the remediation of the heavy metal phytotoxicity in the dredged material. Gas chromatography/mass spectrophotometry analysis showed the principal contaminants of the coke works soil to be organic. The soil was heavily contaminated with coal tars (19.0%) consisting of a complex mixture of aliphatic, polycyclic and aromatic compounds including phenols (160mg/kg). Viable counts of the soil microflora, on selective media, showed the presence of bacteria capable of degrading phenol and several of its alkylated homologues and thiocyanate, which was converted to ammonia and used as aN source. The coke works soil was treated on a laboratory scale using microbially based clean-up methods. Soil was incubated in glass jars under laboratory conditions. Nu trients (yeast extract) and microbial biomass (a mixed culture, previously isolated and enriched by growth on cresol and thiocyanate, but capable of oxidising a wide range of alkylated phenols), were inoculated into the contaminated soil. The addition of such biomass (106 organisms /g soil) led to a marked improvement in the rate of phenolic degradation in the soil (26% loss in'22 weeks, compared with 9% in the untreated control. ). Degradation rates decreased after 14 days but a repeated application of biomass (106 organisms/g soil) caused further phenolic loss (47% total loss). Cresol (100mg/kg) subsequently added to the bacterial ly-amended soil disappeared within 7 days, showing that the biomass amendment was still biochemically very active. These findings demonstrate the importance and the effectiveness of two different treatment methods in the rcmediation of contaminated soil.