Anaerobic sediment potential acidification and metal release risk assessment by chemical characterization and batch re-suspension experiences

M. P. DI NANNO, G. CURUTCHET AND S. RATTO

JOURNAL OF SOILS AND SEDIMENTS (PRINT)

Verlagsgruppe Hunting Jehle Rehm GmbH

Año: 2007 vol. 7 p. 187 - 194

1439-0108

Abstract Background, Aim and Scope: Sediments act as a sink for toxic substances (heavy metals, organic pollutants) and consequently dredged materials often contain pollutants above safe limits. In polluted anaerobic sediments the presence of sulphides and redox potential changes creates a favorable condition for sulphide oxidation to sulphate together with potential toxic metal release. The oxidation reaction is catalyzed by several microorganisms. Some clean up measures such as dredging can initiate the process. The aim of the present work is to assess the acidification and metal release risk in case of sediment dredging and to compare two different acid base account techniques with the re-suspension results. The oxidation mechanism by means of innoculation with an Acidithiobacillus ferrooxidans strain was also evaluated. Background, Aim and Scope: Sediments act as a sink for toxic substances (heavy metals, organic pollutants) and consequently dredged materials often contain pollutants above safe limits. In polluted anaerobic sediments the presence of sulphides and redox potential changes creates a favorable condition for sulphide oxidation to sulphate together with potential toxic metal release. The oxidation reaction is catalyzed by several microorganisms. Some clean up measures such as dredging can initiate the process. The aim of the present work is to assess the acidification and metal release risk in case of sediment dredging and to compare two different acid base account techniques with the re-suspension results. The oxidation mechanism by means of innoculation with an Acidithiobacillus ferrooxidans strain was also evaluated. Background, Aim and Scope: Sediments act as a sink for toxic substances (heavy metals, organic pollutants) and consequently dredged materials often contain pollutants above safe limits. In polluted anaerobic sediments the presence of sulphides and redox potential changes creates a favorable condition for sulphide oxidation to sulphate together with potential toxic metal release. The oxidation reaction is catalyzed by several microorganisms. Some clean up measures such as dredging can initiate the process. The aim of the present work is to assess the acidification and metal release risk in case of sediment dredging and to compare two different acid base account techniques with the re-suspension results. The oxidation mechanism by means of innoculation with an Acidithiobacillus ferrooxidans strain was also evaluated. Background, Aim and Scope: Sediments act as a sink for toxic substances (heavy metals, organic pollutants) and consequently dredged materials often contain pollutants above safe limits. In polluted anaerobic sediments the presence of sulphides and redox potential changes creates a favorable condition for sulphide oxidation to sulphate together with potential toxic metal release. The oxidation reaction is catalyzed by several microorganisms. Some clean up measures such as dredging can initiate the process. The aim of the present work is to assess the acidification and metal release risk in case of sediment dredging and to compare two different acid base account techniques with the re-suspension results. The oxidation mechanism by means of innoculation with an Acidithiobacillus ferrooxidans strain was also evaluated. Background, Aim and Scope: Sediments act as a sink for toxic substances (heavy metals, organic pollutants) and consequently dredged materials often contain pollutants above safe limits. In polluted anaerobic sediments the presence of sulphides and redox potential changes creates a favorable condition for sulphide oxidation to sulphate together with potential toxic metal release. The oxidation reaction is catalyzed by several microorganisms. Some clean up measures such as dredging can initiate the process. The aim of the present work is to assess the acidification and metal release risk in case of sediment dredging and to compare two different acid base account techniques with the re-suspension results. The oxidation mechanism by means of innoculation with an Acidithiobacillus ferrooxidans strain was also evaluated. Methodology: The sediments were chemically characterized (pH; organic matter –Cox-; acid volatile sulphides –AVS-; total sulphur; moisture; Cr, Cu and Zn aqua regia contents). A metal sequential extraction procedure- Community Bureau of Reference, BCR technique- was applied to calculate the Acid Producing Potential (APP) and Acid Consuming Capacity (ACC) of the sediment samples trough Fe, Ca and SO4-2 measurements. Acid base account was also performed by the Sobek methodology (Acid producing potential –AP- calculated with total sulphur and neutralization potential -NP- by titration of the remaining acid after a reaction period with the sample). Fresh sediments were placed in agitated shake flasks and samples were taken at different times to evaluate pH, SO4-2 and Cr, Cu, Zn and Fe+2 concentration. Some of the systems were inoculated with an Acidithiobacillus ferrooxidans strain to assess the biological catalysis on sulphide oxidation. Results: Sediment chemical characterization showed high Cox (5.4-10.6%), total sulphur (0.36-0.86%) and equivalent CaCO3 percentages (4.5-8%). pH was neutral-alkaline for all the samples. AVS content was high except for sample 5. The acid base account obtained with the two methods gave different results for the acid generating risk of the samples. pH measurement in the agitated shake flasks indicated a decrease of 0.4 to 3.1 pH units. In all the systems sulphate concentration increased (2100-2200 mg L-1 to 2500-3000 mg L-1) and that increment was positively correlated with the initial total sulphur content of the samples in the inoculated flasks. Cu and Cr in solution were not detected in most of the sampling occasions (<0.5 mg Cu L-1 and <0.5 mg Cr L-1). Zn reached high concentrations (up to 11.8 mg L-1). For every system- except sediment 1- the lowest pH registered was similar between inoculated and control systems. The inoculation effect was mostly evidenced in the systems by a higher sulphate release compared to the control systems. Discussion: The BCR method categorized all the samples like potentially acid generating material. The Sobek method using NPR (NP/AP) criteria classified sample 3 like a possible acid generator and samples 1, 2 and 5 with a low acid generation potential. Despite this, all the samples acidified the media in the kinetic tests in at least one of the conditions employed in this work. It would seem that NPR and NNP (NP-AP) risk classification criteria shouldn’t be directly used with anaerobic sediments. Appropriate classification levels for sediments should be developed considering the different sulphide reactivity between rocks and sediments. Sediment oxidation can cause acidification which is partially explained by sulphide oxidation. In the studied samples we found positive correlation between sulphate increase in solution after oxidation and total sulphur content in the inoculated systems. Significant amounts of Zn could be released to solution while Cr and Cu remained insoluble despite the pH decrease observed. The low Cu and Cr mobility could be explained by the very low solubility of their hydroxides and high affinity for organic matter and iron oxides/hydroxides that might form during sediment oxidation. Dredged sediment management and disposal should be carefully planed. Conclusions: All the sediment samples in the laboratory re-suspension experiments acidified. However Sobek acid base account with both risk classification criteria (NNP, NPR) was not able to predict the samples behavior accurately as the BCR derived base account did. The inoculation effect was mostly associated to a higher sulphate release and not to lower pH due to acid base equilibrium. Recommendations and Perspectives: Appropriate risk classification levels for sediments should be developed considering the different sulphide reactivity between rocks and sediments. Keywords: dredged sediments; re-suspension; Acidithiobacillus ferrooxidans; acid base account; anaerobic sediments; acid volatile sulphides