MINIMIZE ARSENIC MOBILITY IN CONTAMINATED SOIL AS A NATURAL ATTENUATION APPROACH Franka Dankwarth, Joachim Gerth & Ulrich Förstner Technische Universität University Hamburg-Harburg, Arbeitsbereich Umweltschutztechnik Eißendorferstraße 40, D-21073 Hamburg, Germany (
[email protected],
[email protected],
[email protected] ) ABSTRACT Procedures are tested to estimate actual and potential arsenic release from soils on former infiltration fields and tannery sludge by batch extraction techniques and column leaching tests. Leachate concentrations obtained from both techniques are similar. Potentially mobile arsenic in iron oxide rich substrates can be assessed by time dependent dissolution in ascorbic acid. Arsenic mobility increases with pH and can be reduced by additions of calcium to an extent that leachate concentrations are limited to regulatory values. Arsenic is well retained in a calcite matrix under non-saturated conditions while in the water saturated zone permanent leaching of arsenic is observed. For arsenic, there is no natural attenuation per se. If no action is taken arsenic exceeds concentration limits in the soil leachate and groundwater. INTRODUCTION The concept of natural attenuation is based on processes such as biological degradation, dispersion, dilution, sorption, evaporation and/or chemical and biochemical stabilisation of pollutants. Typically for natural attenuation strategies is the indentation of biological, chemical and geotechnical approaches. Common objectives are: site characterization with regard to the efficiency of the expected retardation/degradation mechanisms, proof of applicability of the natural attenuation concept (i.e., time frame) and elucidation of questions relating to the persistence of critical pollution sources. Natural attenuation/intrinsic remediation has been applied to low-risk cases, such as contaminations by petroleum hydrocarbons. Its also considered for inorganic pollutants. In this project, we investigate the potential for arsenic mobility and retardation in former infiltration fields and sludge deposits of a tannery. The contaminated land of 15 ha is located in the periphery of a town and has been a nature reserve for 30 years. Plans exist to develop the area for housing and industry. So far, no information exists on arsenic leaching from the different soil substrates to the aquifer in a depth of about 5 m. The objective of this study is to establish procedures for characterizing potentially mobile and immobile arsenic fractions. This serves as a basis for assessing the natural attenuation potential and also for measures to minimize arsenic leaching. The dominant soil type is cambic podzol. In the topsoil (Ah horizon), 40-90 mg/kg arsenic are bound to organic matter consisting of humic material and sedimented fine leather particles. The subsoil (Cs horizon) contains iron oxide bound arsenic
with concentrations between 100 and 500 mg/kg. The iron oxide phase is poorly crystalline goethite. Both, iron and arsenic become mobilized to some extent by mildly reducing conditions after heavy rainfalls and form a co-precipitate in the lower soil profile. The soil seapage with Fe(II) and arsenic entering the groundwater is received by a nearby brook. A massive precipitate of ferrihydrite has formed at the bank side in contact with the contaminated land with 200 to 400 mg/kg As. The major source of arsenic is a 3 ha landfill of tannery waste consisting of calcite as the major phase with up to 2000 mg/kg As. This material is partly deposited below the water table.
METHODS Soil samples were taken from 4 profiles and mixed to give a representative sample of each Ah and Cs horizon (sample Ah and Cs). The ferrihydrite (sample Fh) was taken from a single location. Total contents were determined by aqua regia extraction in a microwave system. For mineral composition samples were investigated by XRD. Extraction agents (H2O, and oxalate/ascorbic acid (pH 3,25)) are used in combination with column leaching tests under non-saturated and saturated conditions. Moisture content is varied in column tests to account for intermittent wetting and drying under field conditions. The major proportion of arsenic is bound by iron oxides present in different modifications. This mineral fraction is characterized for stability against reductive dissolution using a modified version of the method of POSTMA (1993) with 40 ml of 100 mM ascorbic acid to extract 2g batches of sample for different shaking times. The results give an estimate of the arsenic mobilization potential during temporary water saturation in the vadose zone. Arsenic mobility as a function of pH was studied in batch experiments with 4 g of air-dried soil suspended in 40 ml of water and additions of HNO3 or NaOH to obtain pH values between 3 and 9 after 18 hours shaking. A separate series of acid/base additions was conducted in the presence of 0.01 M CaCl 2. This was to test if calcium can reduce arsenic mobility by the formation of a Ca/As precipitate. After filtering through a 0.45 µm membrane filter the solutions were analyzed by hydride generation graphite furnace AAS. RESULTS AND DISCUSSION Table 1 shows data on sample composition together with the results of batch extraction experiments. Arsenic in the water extracts of samples Ah and Cs clearly exceeds the permissable regulatory value of 10 µg/l. In a column experiment with sample Cs under non-saturated conditions (not shown) the leachate concentration is 35 µg/l and nearly identical to those obtained by the batch extraction test. The potentially mobile fraction is estimated by ascorbic acid extraction. Figure 1a shows the reductive dissolution of samples Cs and Fh. The potentially mobile arsenic fraction can be estimated from the change of high-rate to low-rate dissolution kinetics. About 20 and 60 % of total arsenic in sample Cs and Fh, respectively, are mobilized during the fast dissolution step. For sample Cs, the
time span for the fast step is also indicated by the As-to-Fe ratio showing a clear change in rate at about 160 hours (Figure 1b). Table 1: Selected data of soil substrates Sample
Depth
pHCaCl2
[cm]
Ct
Feasc/ox
Asasc/ox
Ast
AsH2O
[%]
[mg/kg]
[mg/kg]
[mg/kg]
[µg/l]
Ah
0-30
4,6
5,5
2563
32,5
37,3
26,2
Cs
120-150
4,6
0,13
12406
93,8
109,1
35,0
Fh
0-10
5,0
n.d.
60806
214,1
223,8
1,4
As/Feasc/ox: extractable in hot ascorbic acid/oxalate; As t: total arsenic in microwave extract with aqua regia; AsH2O: water extractable arsenic according to DIN 38414 (S4 test); Ct: total carbon
30
As/Fe [mol/mol x103]
dissolved As [%]
100 80
Fh 60
Cs 40 20 0 0
500
1000
1500
2000
2500
25
Fh
20
Cs
15 10 5 0 0
3000
500
time [min]
1000
1500
2000
2500
time [min]
Fig.1a: Ascorbic acid extractable As
Fig.1b: Fe/As ratio in ascorbic acid extract
As-concentration [µg/l]
The results suggest that easy-to-mobilize arsenic is bound to amorphous ferric hydroxide while the immobile fraction is associated with slowly dissolving goethite. In contrast, sample Fh as a co-precipitate shows a relatively constant As-to-Fe ratio and an even distribution of As in the ferric hydroxide matrix.
Cs Cs + calcium Ah Fh
300 250 200 150 100 50 0 2
4
6
8
pH
Fig. 2: Arsenic extraction as a function of pH
10
Arsenic extraction at different pH is summarized in Figure 2. Almost no change in arsenic mobility is observed with samples Ah and Fh when lowering the pH to 3. Mobility increases, however, in sample Cs. At higher pH, arsenic mobility increases in all samples, particularly in sample Cs the mobilizable fraction of which may consist of surface bound adsorbed arsenic. This is supported by the finding that mobilization also occurs much faster than in samples Ah and Fh (not shown). Arsenic mobility is reduced by a factor of at least 4 and higher when adding calcium as is demonstrated with sample Cs in Figure 2. Similar results were obtained with 0.1 and 0.01 M CaCl2 solutions. This suggests that arsenic mobility in the vadose zone can be controlled by precipitate formation using solution concentrations around 0.01 M calcium. In the presence of calcium, arsenic is probably bound as a calcium arsenate phase (SADIQ 1997). BOTHE and BROWN (1999) postulate the formation of arsenate apatite. In the field, arsenic contaminated waste with >220 mg/kg consisting of mainly calcite was deposited on an iron oxide rich soil horizon about 40 years ago. This horizon contains only background levels of arsenic and has therefore not received any contaminant from the matrix above. The contaminated material was exposed to reducing conditions in a column experiment and yielded up to 100 mg/L arsenic in the leachate. This agrees well with an arsenic concentration of 120 mg/L in the groundwater of the highly contaminated landfill with up to 2000 mg/kg As consisting of calcite as the major phase. According to DUTRÉ and VANDECASTEELE, dissolved arsenic is in equilibrium with calcium arsenite (CaHAsO3) under reducing conditions. It could be shown that leachates of moderately contaminated soil substrates with up to 100 mg/kg arsenic clearly exceed the limiting concentration of 10 µg/l by a factor of 2 to 3 under aerated and non-saturated conditions. Much higher values can be expected when soil pH increases to > 6 and during temporary water saturation. Potentially mobile arsenic can be estimated from time dependent ascorbic acid extraction. A significant reduction of arsenic mobility is achieved by additions of calcium causing the formation of a calcium arsenate. Arsenic is well retained in a calcite matrix under non-saturated conditions. In the water saturated zone, however, permanent leaching of arsenic from this material can be observed. For arsenic, there is no natural attenuation per se. If no action is taken arsenic exceeds regulatory concentration limits in the soil leachate and groundwater. The results suggest that contaminated soils should be treated with lime to maintain a sufficient calcium level in the soil solution. In the calcite matrix arsenic is only stable in its oxidized form. In the water saturated matrix immobilization can only be achieved by draining and lowering the water-table. REFERENCES BOTHE, J.V. and BROWN, P.L (1999): Environ. Sci. Technol. 33, 3806-3811. DUTRÉ, V. and VANDECASTEELE, C. (1998): Environ. Sci. Technol. 32, 27822787. POSTMA, D. (1993): Geochim. Cosmochim. Acta 57, 5027-5034. SADIQ, M. (1997): Water, Air and Soil Pollution 93, 117-136.
