In: Application of microbial Products to Promote Electro- dialytic Remediation of Heavy Metal Contaminated Soil
Conclusions
combining EDR and heterotrophic leaching of soil fines in suspension is rejected. In contrast enhancement of EDR with nitric acid shows promising results at current densities increased beyond what is feasible by addition of only distilled water.
Batch extraction of Pb from contaminated soil-fines by sulfuric acid is negligible even at very low pH, probably due to the low solubility of lead-sulphate. In accordance with this, EDR of soil-fines in suspension is impeded by preceding sulfur amendment to induce autotrophic leaching as well as by suspension in sulfuric acid.
In contrast, Zn-removal is enhanced by both treatments.
The process of EDR of Pb-contaminated soil fines can be divided into four phases: In phase (1) the soil buffer capacity is eliminated by the production of hydrogen-ions at the surface of the anion-exchange membrane where water-splitting takes place. The dissolution of soil-carbonates results in complete extraction of Ca, partial extraction of Mg and K, and a corresponding loss of soil mass. During phase (2) a sharp pH-decrease takes place along with increased conductivity. During this phase Pb is removed at a high rate, and a significant fraction of the Pb is dissolved in the soil-solution. In phase 3) pH stabilizes at 1-2, while the conductivity continues to increase and the voltage between working electrodes decreases. During this phase Pb is extracted at a lower rate. In phase 4) extraction of Pb and most soil-cations ceases, and the primary transport is that of hydrogen-ions. The overall order of removal-rate among soil-cations found is: Ca > Mn > Mg > K > (Al and Fe).
Among toxic, contaminating elements the overall order of removal rate from various soils is: Cd > Zn > Cu > As > Pb Ni > Cr(III) > Hg. All elements except Hg are amenable to remediation with maximum removals obtained as follows: 79% for As, 92% for Cd, 55% for Cr, 96% for Cu, 52% for Ni, 53% for Pb and 88% for Zn.
During remediation As (III) is oxidized to As(V), succeeded by removal of anionic As(V). In contrast, no oxidation of Cr(III) takes place during the remediation, and Cr removal is in general low. Among soil-types, EDR in suspension is more efficient from soil-fines low in organic matter and carbonate in accordance with the observations made with traditional, stationary EDR.
Among the studied options, application of microbial products to promote EDR of Pb-contaminated soil-fines does not seem feasible, while the combination of EDR and heterotrophic leaching showed potential for other toxic metals. EDR of the residual sludge after soil-wash seems to be a more promising technology for treatment of soil contaminated by Pb. In addition the method is suitable for removal of most other toxic elements enabling a simple treatment of soils affected by several contaminants. The value of the method relies upon the general validity of some observations made in this work, which should therefore be verified: (1) Pb is generally concentrated in the fine fraction of contaminated soils; (2) Pb bound in the coarser fractions is less mobile than Pb in the fine fraction; (3) the immobile Pb in the coarser fractions is concentrated in single grains, which may be separated from the soil by a density separation process during soil washing; (4) a higher current density may be applied in the suspended setup compared to the stationary method (probably due to the limited concentration polarization). The technology provides a solution for one of the most challenging obstacle to implementation of commercial soil-wash technology. The results of the present work suggest that it could be beneficiary to apply the treatment as a number of reactors in series, where the initial reactor works at the highest possible removal rate, and the final reactor works at the target heavy metal-concentration at an increased current density. Nitric acid addition is recommended in situations where the removal rate is of higher importance than energy expenditure and chemical consumptions. Increased changes in soil characteristics by addition of nitric
Conclusions acid could however be expected, and should be investigated when relevant for any succeeding application of the remediated soil fines. In addition, it is recommended to terminate remediation as soon as the extraction of the relevant contaminating elements ceases in order to limit the dissolution of Fe and Al-minerals. For Cr and in particular Hg, the effect of soil-conditioning with oxidizing/complexing agents should be tested, although general conclusions on the treatability of Hg-contaminated soil-fines are recommended to be made only after investigation of additional Hg-contaminated soils. Several potential applications of soil-fines after remediation exist depending on their characteristics, whish could be controlled by appropriate process management.
In order to establish a complete evaluation of any potential applications of the soil-fines after remediation, this investigation should be complemented by investigations of the fate of phosphate, nitrate, chloride and organic matter as well as of the mineralogical condition of the fines after remediation. In order to be able to handle mixed contaminations, the fate of common organic contaminants, like PAH’s, during EDR in suspension should also be established. A second main obstacle for implementation of commercial soil-washing is the laborious dewatering of the sludge.
As changes in the sedimentation-velocity of the soil-fines during EDR was observed during the present work, it is further recommended to investigate the dewater-ability of the soil-fines before and after treatment to establish an understanding of the most optimal treatment sequence. Based on the practical experience obtained while working with the method, a few additional issues are recommended be considered prior to upscaling of the technology to bench/pilot scale. Firstly, the effect of the ion-exchange membranes needs to be elucidated, as these are expensive, and should only be employed for treatment of a low-value product such as soil if strictly necessary.
Secondly it is suggested to investigate the feasibility of various optimization-options:
(1) employment of pH-static control of the catholyte, (2) run the process with constant voltage in stead of constant current, (3) employ pulsed electric fields, and (4) use tap-water as electrolytes.
Conclusions
Appendix I
Appendix I
Abbreviations and Symbols
AAS Flame Atomic
Absorption Spectrometry
Ac Acetate
aj hydrated ionic radius of specie j [m]
Am. Citr. Ammonium Citrate AN ANion-exchange
membrane atm atmosphere
BCR Bureau Communautaire de Reference
c/c0 normalized concentration c0 initial concentration cb ion-concentration in bulk
solution CA Citric Acid CAS Chrome Azurol S CAT CATion-exchange
membrane CCA Chromated Copper
Arsenate CEC Cation Exchange
Capacity
CFU Colony Forming Unit ci final concentration in soil
slice i
cj concentration of specie j CV-AAS Cold Vapor Atomic
Absorption Spectrometry ºC degrees Celsius
(c) Crystalline
D Diffusion coefficient [m2/s]
Dj Diffusion coefficient of specie j [m2/s]
DC Direct Current DFO-B desferrioxamine-B
di distance of soil slice i from the anode-end DFO-M desferrioxamine-M DFOMTA
N-(2,3-dihydroxy-4- (methylamido)benzoyl)-desferrioxamine-B DW Distilled Water e charge of an electron
1.602 x 10-19C EDDS[s,s]
ethylenediamine-disuccinic acid EDR ElectroDialytic Remediation EDTA
Ethylenediamine-tetraacetic Acid EC Electrical Conductivity EKR ElectroKinetic
Remediation
EPA Environment Protection Agency
EU Expandable Undefined Exp Experiment
F Faraday constant 96,485 C/mol
GF-AAS Graphite Furnace Atomic Absorption Spectrometry G.L. Governmental assigned
Limits
ha. hektar (100m x 100m)
HM Heavy Metal
ICP-MS Inductively Coupled Plasma Mass-spectometry ilum limiting current IQ Intelligence Quotient J current density [mA/cm2] Jje Electroosmotic flux of
specie j [mg/(m2s)]
Appendix I
Jjm Electromigrative flux of specie j [mg/(m2s)]
Jopt optimal current density [mA/cm2]
k Boltzmann constant 1.381 x 10-23 J/K KC Potassium Citrate Kd Equilibrium adsorption
coefficient [L/kg]
ke electroosmotic permeability [mg/(m s V)]
Ksp solubility constant
L Length
li length of soil slice i L/S Liquid to Solid ration
M Molar
M1 1st moment of contaminant M0 0th moment of
contaminant
MA Malic Acid
Me Metal
meq milli-equivalents Mm
metal-mobilization-length
MSW Municipal Solid Waste MSWI Municipal Solid Waste
Incineration N Neutral conditions NA Nitric Acid OECD Organisation for
Economic Co-operation and Development OM Organic Matter PbB blood-Pb-level pe -log[e-] pH -log[H+]
pK -logK, K = equilibrium constant
PAH Polycyclic Aromatic Hydrocarbons PEM Poul E Meier PNEC Predicted No-Effect
Concentration PTWI Provisional Tolerable
Weekly Intake PVC Polyvinyl Chloride R gas constant
8.31451 J/(K mol)
r2 correlation coefficient rpm rounds pr. minute SEM-EDX Scanning Electron
Microscope-Energy Dispersive X-ray
Si Silicium
sp specie
spike contaminate artificially Std. dev. Standard deviation
T temperature
tbl transport number of counter-ions in boundary layer
tm transport number of counter-ions in membrane
Po2 Partial pressure of O2
SCC Soil Cut-of Criteria
Sn Tin
SQC Soil Quality Criteria uj ionic mobility of specie j
in free solution [m2/(s V)]
uj* ionic mobility of specie j in soil pores [m2/(s V)]
WHO World Health Organization xe tortuousity XRD X-Ray Diffraction
z valence
zj valence of ion j η viscosity [kg/(m s)]
boundary layer thickness
Appendix II