『Abstract
Organic carbon introduction in the soil to initiate remedial
measures, nitrate infiltration due to agricultural practices or
sulphate intrusion owing to industrial usage can influence the
redox conditions and pH, thus affecting the mobility of heavy
metals in soil and groundwater. This study reports the fate of
Zn and Cd in sandy aquifers under a variety of plausible in-situ
redox conditions that were induced by introduction of carbon and
various electron acceptors in column experiments. Up to 100％ Zn
and Cd removal (from the liquid phase) was observed in all the
four columns, however the mechanisms were different. Metal removal
in column K1 (containing sulphate), was attributed to biological
sulphate reduction and subsequent metal precipitation (as sulphides).
In the presence of both nitrate and sulphate (K2), the former
dominated the process, precipitating the heavy metals as hydroxides
and/or carbonates. In the presence of sulphate, nitrate and supplemental
iron (Fe(OH)3) (K3), metal removal was also
due to precipitation as hydroxides and/or carbonates. In abiotic
column, K4, (with supplemental iron (Fe(OH)3),
but no nitrate), cation exchange with soil led to metal removal.
The results obtained were modeled using the reactive transport
model PHREEQC-2 to elucidate governing processes and to evaluate
scenarios of organic carbon, sulphate and nitrate inputs.
Keywords: Heavy metals; Zinc; Cadmium; Sulphate reduction; nitrate
reduction; Redox processes; Geochemical modeling』
1. Introduction
2. materials and methods
2.1. Sample collection
2.2. Column design and packing
2.3. Influent composition for different columns
2.4. Analytical methods
2.5. Reactive transport modeling
2.5.1. Transport calculations
2.5.2. Modeling sulphate reduction and metal precipitation in
the presence of acetate (K1)
2.5.3. Modeling nitrate & sulphate reduction and metal precipitation
in the presence of acetate (K2)
2.5.4. Modeling sorption of Zn and Cd in abiotic column (K4)
3. Results
3.1. Zn and Cd removal profile and pH variation in columns
3.1.1. Sulphate concentration
3.1.2. Nitrate concentration
3.1.3. Acetate concentration
3.1.4. Iron concentration
4. Discussion
4.1. Processes occurring in the column and interpretation
with biogeochemical modeling
4.1.1. Iron and sulphate reduction
4.1.2. Nitrate reduction in presence of sulphate
4.1.3. Nitrate reduction in presence of sulphate and iron
4..1.4. Adsorption-surface complexation and cation exchange
4.2. Evaluation of different scenarios
5. Conclusions
Acknowledgments
Appendix A. Supplementary material
References