PbOH +PbSO4
2 Materials and Methods
Effect of Soil Type
oxide-bound and organic Pb was removed. It has also been demonstrated that remediation is more effective (in terms of % Pb removal) with highly contaminated soils than with soils with only slightly elevated Pb-concentrations (Jeong and Kang, 1997; Chung and Kang, 1999). This finding is probably due to the increased likelihood of finding a larger fraction of mobile charged Pb-ions in highly contaminated soils. In heavily contaminated soils, the capacity of sites for strong bonding is more likely to be exceeded and a larger fraction of the Pb may be present as water-soluble, exchangeable or carbonate-bound (Jensen et al., 2006). Speciation was found to be of primary importance when part of the Pb in a soil contaminated by chlor-alkali industry was observed to be moving towards the anode (Suer et al., 2003).
The presence of sulfur in the soil and formation of the negative complex Pb(SO4)2
2-was used to explain this behavior, and implying a complex interaction between Pb, soil and co-contaminating compounds during remediation.
This purpose of this work is to advance our understanding of Pb-contaminated soil remediation by the process of electrodialysis, or EDR. The emphasis of this work is on elucidating the influence of soil properties and Pb-speciation on the feasibility of EDR. In a departure from the practice of spiking samples, the bench-scale EDR experiments described in this work are performed using industrially contaminated soils such that a range of soil physical and chemical properties are investigated for a variety of realistically occurring Pb species.
Effect of Soil Type TABLE II
Contamination sources and likely Pb speciation as revealed by (Jensen et al., 2006).
Dominant species are in bold font.
Soil Activity and probable Pb-source Activity
period Pb-Associations 1 Ancient church with Pb-roof burned down 1627 Solder
Phosphates (pyromorphite + plumbogummite)
Al/Fe-minerals Oxide-bound Organically bound 2 Extraction of metals from scrap and ore. Smelter
waste products deposited on site. Battery recycling.
1938-1985 Phosphates (pyromorphite) Alloys (smelter waste products)
Sulfate (jarosite) Oxide bound Organically bound 3 Car painting. Use of lead based paints. 1900’s Chromate
Organically bound 4 Harbor area, filled up with waste from porcelain
production and a gasworks. Braking up and scrapping of locomotives.
1880-1988 Phosphates (pyromorphite + plumbogummite)
Fe/Al-minerals Oxide-bound Organically bound 5 Former waste-dump with mixed industrial waste
and household waste. Cowered with sewage sludge, ash and mould.
1913-1937 Al/Fe-minerals Alloy
Organically bound Oxide bound 6 Former waste-dump with mixed industrial waste
and household waste. Cowered with sewage sludge, ash and mould.
1913-1920 Alloy
Solder Phosphates (plumbogummite) Organically bound Oxide bound 7 Gravel pit used as waste dump. 1900’s Carbonates/oxides
(hydrocerussite, cerussite, litharge)
Oxide bound Organically bound
8 Metal foundry
1921-1976 Sulphates (anglesite) Solder
Organically bound 9 Harbor area filled up with harbor sludge and
surface soil from central Copenhagen between 1780 and 1820. Site laid out as army ammunition site with chemical storage.
After
1780 Metallic Oxide bound Organically bound 10 Soil collected by contractor north of Copenhagen. Unknown Fe/Al -minerals
Metallic Alloys Solder Chloride
Organically bound
Effect of Soil Type
Conductivity was measured by electrode (MeterLab CDM210) in a solution prepared by constantly mixing of 10g soil and 25ml distilled water for 30min, followed by settling for 20min. Phosphate was measured after digestion of 0.2-0.5g sample at 550ºC followed by boiling with HCl, reaction with ammonium molybdate to form yellow phosphor-molybden acid, and reduction by ascorbic acid in the presence of antimony. The strong blue color was measured by spectrophotometer (Shimadzu UV-1601).pH-dependent desorption of Pb from the soil was investigated by extraction of 5.00g dry, crushed soil with 25.00ml reagent at 200rpm for 7 days. The reagents were as follows: 1.0M NaOH, 0.5M NaOH, 0.1M NaOH, 0.05M NaOH, 0.01M NaOH, distilled water, 0.01M HNO3, 0.05M HNO3, 0.1M HNO3, 0.5M HNO3, 1.0M HNO3. pH was measured after 10min settling, after which the liquid was filtered through a 0.45 m filter for subsequent measurement on AAS. Non acidic samples were preserved with one part of conc. HNO3 to four parts of liquid in autoclave at 200 kPa and 120ºC for 30 minutes prior to AAS measurement. Metals were analyzed according to the Danish standard method DS259 (Dansk Standardiseringsråd, 1991), which entails a 30min acid digestion of 1g soil with 20.00mL of half concentrated HNO3 in an autoclave at 200kPa and 120ºC. The metal-content in solution was measured by atomic adsorption spectrophotometry (AAS, Perkin Elmer 5000 or GBC 932AA) following filtration through a 0.45µm filter. AAS analyses were for all metals validated through analysis of reference samples.
TABLE III
Measured physical and chemical characteristics of the soils from Table II.
Soil number
Parameter 1 2 3 4 5 6 7 8 9 10
Clay (%) (< 2 m) 5 35 8 8 6 11 20 13 14 10
pH 6.9 6.1 7.2 6.9 6.3 7.2 6.8 7.6 7.6 7.8
CaCO3 (%) 3.7 0.5 3.7 6.7 0.9 4.9 1.0 9.1 7.7 9.2 Organic matter (%) 2.6 4.1 3.5 7.4 21.3 11.7 3.6 3.4 7.0 2.8 CEC (meq/100g) 6.6 15.3 3.7 6.3 26.0 10.3 5.3 4.9 8.2 4.5 Fe (g/kg) 5.2 18.8 9.5 24.4 37.6 30.4 21.5 12.0 13.6 13.6 Conductivity
( S/cm) 244 446 194 455 1820 351 281 1848 1659 637 Phosphate [mg/kg] 1200 745 2224 1257 2455 539 741 1036 1123 1547 2.2 EDR FEASIBILITY EXPERIMENTS
Electrodialysis experiments were carried out in cylindrical Plexiglas cells with three compartments (Figure 2.1). The center compartment contained the soil specimen which was 10 cm long and 8 cm in diameter. The anolyte and catholyte were separated from the soil specimen by anion- and cation-exchange membranes, respectively (Ionics, AR204SZRA and CR67 HVY HMR427, respectively).
Electrolytes were circulated between electrolyte chambers and glass reservoirs by a mechanical pump (Masterflex® model 7553-76). Platinum-coated electrodes (Permascand) were used as working electrodes. The electrolytes initially consisted of 500mL 0.01 M NaNO3 adjusted to pH 2 with HNO3. Prior to the beginning of each experiment, soil specimens were mixed with deionized water to a moist but unsaturated consistency.
Effect of Soil Type A constant current of 0.2mA/cm2 was maintained in all experiments except where noted in table IV. pH in the electrolytes, current and voltage were observed approximately once every 24 hours. Each experiment was terminated after approximately 67500coulomb/kg had passed through the soil. The quantification of charge with respect to mass and not volume of soil was necessary because the soil mass and water content varied considerably from one sample to another due to variations in soil organic matter and soil structural differences. During the electrodialysis experiments H+, and OH- were produced at the anode and cathode, respectively. The ion-exchange membranes hindered intrusion of these ions into the soil specimen, and pH-changes occurred in the electrolytes. An electrode (MeterLab CDM220) was used to measure pH in electrolyte compartments, which was maintained between 1 and 2 by manual addition of HNO3 and NaOH.
OH-
Figure 2.1: Schematic drawing of a cell used for experimental electrodialytic remediation of contaminated soil. AN = anion-exchange-membrane, CAT =
cation-exchange-membrane TABLE VI
Summary of bench-scale EDR feasibility experiments (see Tables 2 and 3 for soil origins and characteristics). The charge passage was kept constant at 67.5 C/g DW.
*Due to imperfect contact between soil and membranes current decreased during a period of the remediation. This was compensated for by longer remediation, to reach the wished passage of current.
**During part of the period current was increased to 0.4mA/cm2
*** The high voltages were observed only during short periods, when catholytes needed pH adjustment in all experiments but 7, 9 and 10.
Exp./Soil
No Current
density [mA/cm2]
Voltage
(range)*** Soil [g dry weight]
Time
[days] Initial water content [%]
1 0.2 1.8-116.8 750 59 22
2 0.2** 6.1-104.7 1131 70 27
3 0.2 2.8-44.0 764 60 20
4 0.2 3.2-29.6 700 55 24
5 0.2 4.1-33.4 508 40 49
6 0.2 4.8-137.2 566 44 30
7 0.2* 3.0-139.3 848 78 19
8 0.2 2.4-137.1 854 67 18
9 0.2* 4.3-135.8 696 86 27
10 0.2* 3.1-138.3 889 71 17
Effect of Soil Type
After each experiment, the soil specimen was divided into five sections perpendicular to the current-direction. Pb, pH and water content were measured in each slice.
Membranes were cleaned overnight in 1M HNO3 and electrodes were cleaned overnight in 5M HNO3. Volumes of the cleaning acids as well as the electrolytes were measured followed by analysis of the Pb-concentration by AAS.
METAL-MOBILIZATION LENGTH
Results from electrokinetic soil remediation feasibility experiments presented in terms of the percentage of metal removed clearly dependent on experimental geometry all else being equal, shorter systems will exhibit greater removal. Coletta et al., (1997) suggested evaluation of the moments of synoptic concentration distributions for comparison. We modified this approach in order to facilitate comparisons between distributions observed over various soil specimen lengths, and suggest the metal-mobilization-length (Mm) parameter defined as:
Mm = 1
0
M
M
(1)Where M1 = 1st moment of contaminant (position of the center of mass of contaminant being mobilized) and M0 = 0th moment (total mass of contaminant in the sample). For a discretely sampled experiment, M0 and M1 can be approximated as:
M0 =
i i i
0
c l
c L
M1 =i i i i
i i i
( c l d ) -L
c l 2 . (2a, b)
For our experiments, c0 is the initial Pb concentration in the soil sample, ci is the final concentration in the soil slice i, L is the length of the soil specimen and li is the length of soil slice i, while di is the distance of slice i from the anode-end of the soil-specimen. With Mm it is possible to express the extent of contaminant transport obtained in a single number, which simplifies evaluation of experiments significantly:
A positive Mm is obtained if the contaminant is transported towards the cathode; a negative Mm is obtained if the contaminant is transported towards the anode; greater
|Mm|, the better remediation.