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Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils
Journal of Hydrology (2008) 356, 261– 270
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Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils T.M.H. Le a, R.N. Collins b, T.D. Waite
a,*
a
School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Centre for Water and Waste Technology, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
b
Received 23 September 2007; received in revised form 28 March 2008; accepted 11 April 2008
KEYWORDS Sorptivity; Hydraulic conductivity; Consolidation coefficient; Filtration cell; Material coordinates theory
Summary Acid sulfate soils (ASS) cover extensive areas of east Australian coastal floodplains. Upon oxidation, these hydromorphic pyritic sediments produce large quantities of sulfuric acid. In addition, due to their geographic location, these soils may also come in contact with high ionic strength estuarine tidal waters. As a result, there is typically a large variation in acidity (pH) and cation concentrations in soil porewaters and adjacent aquatic systems (e.g., agricultural field drains, rivers, estuaries, etc.). Acid sulfate soils, especially from the unoxidized gelatinous deeper layers, contain a relatively high proportion of montmorillonite, which is wellknown for its shrink-swell properties. Variations in cation concentrations, including H3O+, can influence montmorillonite platelet interactions and may, thus, also significantly affect the hydraulic conductivity of materials containing this clay. In this paper we report on the effect of four common cations, at reasonable environmental concentrations, on the hydraulic properties of potential (unoxidized) acid sulfate soil materials. The natural system was simplified by examining individually the effects of each cation (H+, Ca2+, Fe2+ and Na+) on a soil–water suspension in a filtration cell unit. Moisture ratio, hydraulic conductivity and the consolidation coefficient of the deposited filter cakes were calculated using material coordinates theory. The results indicate that the hydraulic conductivity of potential acid sulfate soils increases at low pH and with cation concentration. Although an increase in the charge of amphoteric edge groups on montmorillonite clays may result in some aggregation between individual clay platelets, we conclude that the extent of these changes are unlikely to cause significant increases in the transportation of acidity (and contaminants) through potential acid sulfate soils as the hydraulic conductivity of these materials remain low (<109 m/s) at pH and ionic conditions normally experienced in the field. ª 2008 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +61 2 9385 5059; fax: +61 2 9385 6139. E-mail address: [email protected] (T.D. Waite). 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.04.014
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Introduction Rapid development in Australian coastal regions has initiated growing demand for improved understanding of factors controlling the generation of acid and high dissolved concentrations of elements such as iron and aluminium from so-called ‘‘acid sulfate soils’’ (ASS) and the transport of these potentially harmful species through subsurface environments. Commonly known as Holocene coastal sediments or marine muds, these originally hydromorphic soils contain (or contained) sulfidic minerals, commonly in the form of pyrite (FeS2). Potential (or unoxidized) acid sulfate soils (PASS) often contain a high proportion of swelling clays and are difficult to dewater. However, extreme weather conditions or anthropogenic disturbances, such as prolonged drought, drainage, dredging or excavation, enable oxygen to oxidize sulfides in PASS to form actual ASS (van Oploo, 2000; Smith et al., 2003). The oxidation process ultimately results in the formation of acidic porewaters and, in many instances, high concentrations of dissolved cations (particularly of Fe(II) and Al(III) (Glamore, 2003), which can be exported into nearby streams and waterbodies destroying aquatic life and corroding civil infrastructure. Potential acid sulfate soils are defined as sulfidic soil materials containing pyrite which have not oxidised to the extent that soil pH has decreased below 3.5 (Tulau, 1999). Given this definition, the ‘partial’ oxidation of pyrite in PASS can result in significant soil porewater concentrations of ferrous iron and sulfuric acid: FeS2 þ 7=2O2 þ H2 O ! Fe2þ þ 2Hþ þ 2SO2 4 While the production and transformation of these initial oxidation products in the groundwaters of acid sulfate soil affected regions have been investigated in a number of studies (White et al., 1993, 1997; Blunden and Indraratna, 2000; van Oploo, 2000; Smith et al., 2003; Johnston et al., 2004; Green et al., 2006), there have been few reports of the impacts of these constituents on ASS physical properties (Le et al., 2008). Almost 50% w/w of PASS consists of clay minerals and more than 60 % of the clay-sized fraction is montmorillonite (Smiles, 2000; White et al., 2003). It is well-known that cations can compress the diffuse double layer of clay particles leading to changes in the modes of particle association (i.e., face-to-face, edge-to-face or edge-to-edge) (van Olphen, 1963; Stein, 1986; Wakeman, 1986; Mohan and Fogler, 1997; Tombacz and Szekeres, 2004) and, as a result, considerably alter the hydraulic conductivity and consolidation properties of consolidated assemblages of these materials (Mohan and Fogler, 1997; Mata and Ledesma, 2003; Santiwong et al., 2008). Nevertheless, in a previous study (Le et al., 2008) we observed that dissolved milli-molar concentrations of H+ and Ca2+ had little, if any, measureable impact on the hydraulic conductivity and consolidation of PASS in experiments employing a hydraulic consolidometer, despite theoretical calculations suggesting that changing pore water electrolyte concentrations should dramatically change the hydraulic properties of these soils (White et al., 2003, 2005). However, it may reasonably be argued that the use of a bulk soil sample
T.M.H. Le et al. in the consolidometer studies as well as the short timescale (7 days) of the experiments may not be representative of either the material’s physical state in the natural environment or the long timescales (months to years) required to induce significant physical changes. This may have important ramifications to ASS management as tidal flushing of east Australian coastal ASS drains with high ionic strength estuarine waters is increasingly becoming the preferred acid neutralisation technique to minimise the environmental problems caused by ASS discharge (Le et al., 2008). The principal issues of concern then are (i) whether any changes in hydraulic properties will occur in unconsolidated unoxidized PASS on change in ionic composition, and (ii) the timescale over which any changes might occur. Both aspects can be investigated simultaneously by varying the pH and cation concentrations of diluted PASS suspensions which are filtered through a dead-end filtration device. In this fashion, chemical interactions between the solution and the solid phase of the soil are not limited by the initial hydraulic properties of the soil (1010 m/s) which inherently limit the transport of solution through bulk soil samples (Le et al., 2008). Dead-end filtration cells have been employed for many industrial applications including investigation of membrane fouling in water and wastewater treatment, assessment of the permeability of landfill containers, examination of the permeability of substrates to drilling fluid and ease of sediment dewatering in civil constructions (Siyag et al., 1983; Petrov and Rowe, 1997; Shackelford et al., 2000; Graham et al., 2001; Vasco et al., 2001). The method has also previously been used for the examination of the hydraulic conductivity and consolidation properties of PASS (White et al., 2003).
Materials and methods Study site Potential acid sulfate soil samples were collected at 1.5 m below the ground surface of sugar cane fields bordered by McLeod’s Creek, a tributary of the Tweed River, Northeastern New South Wales (28 18 0 S, 153 31 0 E), Australia. The site is located in a typical backswamp environment surrounded by small streams feeding into the creek. Similar to most of the Tweed floodplain area, the soil profile of the study site can be divided into four strata. The organic topsoil, with a dark-brown to black appearance, varies in thickness from 0.1 to 0.4 m. A layer of oxidized ASS materials around 0.1 to 0.3 m in thickness, commonly known as the acidic horizon, lies below the organic topsoil and is followed by the transition zone (0.3–0.8 m in thickness) which also contains oxidized materials, but in a much smaller proportion. The PASS commences at approximately 0.9 m below the ground surface. The solution concentration of dominant cations (Na+, Ca2+, Fe2+ and Mg2+) in both the drainwater from McLeod’s Creek and PASS porewater can vary significantly depending on weather condition, time and sample location. The range of chemical characteristics that are frequently obtained for waters obtained at the study site are given in Table 1.
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils Table 1 Typical range of major cation concentrations (mg/L), pH and electrical conductivity (EC) values of PASS porewaters and McLeod’s Creek drainwaters obtained at the study site Species +
Na K+ Ca2+ Fe2+ Mg2+ pH EC (mS) a b
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pressure filtration cell combined with the materials coordinate approach to derive the material hydraulic conductivity and consolidation coefficient of PASS materials and found the results obtained to be consistent with those reported by other investigators.
Concentration (mg/L) Pore water
Drain water
Experimental procedure
253–570 13–23 72–260 0–350a 63–292 3–6a 0.7–2.8a
6200 170 180 Very low 520 3.5–6.5b 0.1–11.7
Experiments were conducted with PASS suspensions in deionised, membrane filtered ‘‘Milli-Q’’ water (Millipore Corporation)and with suspensions that contained one of three different salts (NaCl, FeCl2 or CaCl2) at ‘low’ (i.e., 60.02 mol/L) or ‘high’ (i.e., P0.05 mol/L) concentrations. A number of experiments were also conducted at pH 4 (a pH at which these soils are still defined as PASS) in order to examine the combined effects of pH and cation concentrations on the hydraulic conductivity of the PASS that accumulated on the filter. The effect of using an NaCl concentration of 0.5 mol/L was also examined in order to recreate conditions typical of estuarine tidal waters since such waters are increasingly being used to ‘flush’ acidic drains in coastal eastern Australia. Soil suspensions were prepared daily before each set of experiments in order to avoid oxidation of the soil materials. A clay:water ratio of 1:20 by weight (i.e., 50 g of clay in 1 L of distilled water) was used in all instances. Prior to creating the suspension, all visible coarse contaminants such as remnant plant roots and small pebbles were removed by hand. After dilution, small sand particles and remaining coarse contaminants tended to either float or quickly settle. These components were removed from the suspensions in order to improve sample homogeneity. The diluted PASS suspensions were mixed thoroughly by an electromagnetic stirrer and sonicated to ensure suspension homogeneity. Concentrated acid (H2SO4) and/or monovalent or divalent chloride salts (NaCl, FeCl2 or CaCl2) were subsequently added to the suspensions. During the experiments the pH of the suspensions tended to increase due to the buffering capacity of the soil. This problem was overcome by continual monitoring and, when needed, addition of small amounts of concentrated acid in order to maintain the desired pH. Subsamples of the suspensions were taken prior to filtration in order to determine the electrophoretic mobility (le) of particles present using a Brookhaven ZetaPLUS (Brookhaven Instruments Corporation) instrument. The Smoluchowski equation (BIC, 2002) was used to convert (le) to zeta potential (f). Dead-end batch filtration experiments were conducted with 50 mL volume of feed suspension poured directly into a Perspex cylindrical filtration cell with a membrane of 0.044 m diameter at its base. The membrane selected for use in this study was a microfiltration (MF) membrane with an average pore size of 0.22 lm (0.22 lm GV, DURAPORE Membrane Filters, Millipore). A fresh piece of membrane was used in each experiment. Pressure controllers (P602C-FAC-002R and P-602C-FAC-004R, Bronkhorst Hi-Tech S.V.) were employed to maintain precise supply of constant pressure in the 3–60 kPa and 5–300 kPa (±0.1%) range. Instrument Grade Air (BOC Group) was used for the application of pressure. Instantaneous mass of cumulative permeate was measured by electronic balances (PB3002-SDR, ¨ TTINGEN) Mettler Toledo Ltd., and LC1201S, Sartorius AG GO
van Oploo (2000). White et al. (1993).
Soil samples Approximately 1 kg of PASS was collected from the study site using a handheld auger. The sample was wrapped tightly in plastic, and then covered with aluminium foil and placed in a plastic bag. Air was carefully excluded from the bag and the sample was then constantly kept refrigerated (below 4 C) to avoid oxidation and drying of the soil. For PASS at this site, clay and fine silt account for as much as 99% of the soil material (White et al., 2003). Expandable lattice clays (i.e., smectites) dominate, comprising 60% of the clay fraction, while kaolinite and illite account for, respectively, 30% and 10% (White et al., 2003). As such, the clay fraction, especially montmorillonite, is expected to control the hydraulic conductivity of these PASS. Measured moisture ratios (the volume of water to soil) of PASS are typically 3.5. Diluted fresh PASS suspensions generally have pH ranging from 7.0 to 8.0, but quickly decreases to 6.0 or less if exposed to air for a few hours. With an abundant supply of oxygen, the suspension can be continuously oxidised and acidified to pH as low as 1.5–2.0.
Filtration cell methodology Filtration cell methods are widely used in determination of the dewaterability of sludges and mineral assemblages and in the examination of membrane fouling in water and wastewater treatment (Wakeman and Tarleton, 1999; Iritani, 2003; White et al., 2003). Filtration studies are often used to examine the performance of filter media and/or to quantify the membrane fouling behaviour of suspended solids. In this study however, the filtration cell method is used to characterise the hydraulic properties and consolidation behaviour of predispersed then reconsolidated PASS under various solution conditions. Estimating the hydraulic conductivity and consolidation coefficients of filter cakes using a filtration cell is, however, less straightforward than use of an odeometer or hydraulic consolidometer. Despite the complexity, the material coordinates approach typically used in data analysis is well developed with the approach used successfully in many instances (Smiles, 1986). For example, White et al. (2003) have used a constant applied
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and the data were electronically transferred to a connected PC at predetermined time intervals. After completion of the filtration run, the wet and dry weight of the filtration cakes was determined for triplicate samples with the dry weight determined after drying at 105 C for at least 48 h.
Data analysis Outflow and cake moisture ratio data were analysed using the material coordinate framework which has been developed and detailed by Smiles (1986, 2000) and White et al. (2003). This framework converts the conventional constant pressure filtration theory based on spatial coordinates to a set of equations that are particularly applicable to swelling materials, such as montmorillonite, by employing material coordinates (Smiles, 1986). The theory is summarised here as it is considered to be particularly appropriate to gelatinous PASS. Space coordinates have previously been used to model unsteady flow through swelling materials by Phillip (1957) and Wakeman (1978). Lagrangian analyses, employing material coordinates, were then introduced based on the distribution of the solid in order to simplify analysis of the unsteady problem (Smiles, 1986). The theory is based on two inter-connected equations of continuity for the liquid and solid phases for one-dimensional flow of water in a swelling system (Smiles, 1986, 2000): ohw oF w ohs oF s ¼ and ¼ ð1Þ ot z oz t ot z oz t
i ¼ S0 t1=2
ð6Þ
The sorptivity (S0) is then obtained from the slope of the linear regression of cumulative outflow plotted against the square root of time at the early stages of flow. This procedure reveals that S0 contains similar information as the slope t/V against V that Wakeman and Tarleton (1999) uses in the conventional cake filtration model and the inverse slope of t vs. V2 used in compressive rheology theory by de Krester et al., 2001. Integral manipulations are then used to solve the flow equation with step changes in imposed pressure, which provides the relationship between S0 and material hydraulic conductivity km. The consequential equations that allow calculation of the material hydraulic conductivity km and material moisture diffusivity (or, in geomechanics, the material consolidation coefficient) Dm from sorptivity S0 and sample moisture ratio # data are S0 oS0 S0 o#0 km ðw0 Þ ¼ ð7Þ D# ow 4D# ow 0 S0 oS0 S0 ð8Þ Dm ð#0 Þ ¼ D# o#0 4D# In the data analysis process, cumulative outflow i is first plotted against the square root of time and fitted with a regression line to obtain sorptivity. This parameter as well as cake moisture ratio are then plotted against matric potential in order to calculate the separate terms in Eqs. (7) and (8).
where Fw and Fs are the volume flux densities and hw and hs are, respectively, the liquid and solid volume fractions (Smiles, 1986, 2000). The m(z, t) materials coordinate is defined by
Results
om om ¼ hs and ¼ F s ð2Þ oz ot and the continuity equation for water in materials space becomes o# ou ¼ ð3Þ ot m om t
Fig. 1a shows that higher moisture content cakes were produced at pH 4 than at pH 8 with the effect of increasing pressure in producing drier cake less pronounced at pH 8. The experimental results were highly reproducible, especially for the pH 8 suspension (which is similar to the natural pH of the PASS). The cake moisture ratio appears to be slightly higher in the presence of ‘low’ concentrations (0.02 mol/L) of Na+ compared to 0.5 mol/L (Fig. 1b), however this difference is unlikely to be statistically significant. Similarly, very little difference in filter cake properties was observed for suspensions containing 0.01 and 0.05 mol/L Ca2+ and Fe2+ at pH 4 (Fig. 1c and d). Comparison of the moisture ratios of cakes formed from suspensions containing the salts with values obtained for the clay suspended in distilled water (Fig. 1a–d) indicate that the salts (at least at the concentrations used in these experiments) had very little effect on cake moisture ratios obtained at pH 4. Interestingly, when the pH of the PASS suspension was varied between 4 and 8 in the presence of 0.02 mol/L NaCl (Fig. 2), the apparent difference in cake moisture ratio at the two pHs is less significant than obtained in distilled water (Fig. 1a).
where # = hw/hs is the moisture ratio of the suspension and u is the flux of water relative to the solid particles (Smiles, 1986). Darcy’s Law for swelling systems using materials coordinates can then be written as oU ow u ¼ km ð#Þ ð4Þ ¼ km ð#Þ þ ð1 qs Þ oz om where km is material hydraulic conductivity and U is the total potential energy in a swelling system and involves three components: the gravitational potential z, the unloaded matric potential w and the overburden potential X (Smiles, 2000): U¼zþwþX
ð5Þ
To formulate equations that can be applied to data obtained from filtration experiments and to obtain hydraulic conductivity and moisture diffusivity values, a parameter termed sorptivity (S0) is introduced which embodies the change in the properties of the pore system between the initial and final moisture ratio of the sample (Phillip, 1957; White et al., 2003). The formula relating sorptivity with cumulative outflow at early stages, i is
Moisture ratio, #
Material hydraulic conductivity (km) and material consolidation coefficient (Dm) In Fig. 3a it can be seen that the material hydraulic conductivity (km), in the absence of added salts, is higher at pH 4 than at pH 8. As the moisture ratio increased with decreas-
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils 5
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Distilled water - pH8
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FeCl l/L - pH4 2 0.05 Fe Cl2 0.05mo mol/LpH4
0.05 mol/L mo l/L -- pH4 pH4 CaCl 2 0.05 CaCl2
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2 1
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Imposed gas pressure, P (kPa) Figure 1 Dependence of filter cake moisture ratio on: (a) pH, (b) Na+, (c) Ca2+ and (d) Fe2+ concentrations as a function of imposed gas pressure.
Moisture ratio
5
4
NaCl 0.02 mol/L- pH4
3
NaCl 0.02 mol/L - pH5 NaCl 0.02 mol/L - pH6 NaCl 0.02 mol/L - pH8
2 1
10
100
Imposed gas pressure, P (kPa)
Figure 2 Dependence of filter cake moisture ratio on pH, in the presence of 0.02 mol/L NaCl, as a function of imposed gas pressure.
ing pH, this result may be due to the filter cake having a more porous structure. However, the km values of the filter cakes are generally higher when the concentrations of the cations are increased from ‘low’ to ‘high’ (Fig. 3b–d) even though the moisture ratios are essentially identical (Fig. 1). This trend departs from the observed pattern in the absence of added cations and suggests that particle arrangement on deposition may possibly be different. The corresponding results of material consolidation coefficients (Dm) shown in Fig. 4, indicate, however, that differences in this parameter are only observed in response to increasing cation concentrations from ‘low’ to ‘high’ (i.e., increasing to pH 4 in the absence of cations had little if any effect on the material consolidation coefficient). As shown in Figs. 5 and 6, the material hydraulic conductivity and consolidation coefficient of the cakes deposited in
the presence of ‘high’ concentrations of the cations are markedly higher than values measured in distilled water at pH 4. Indeed, there is less than 30% difference in the km and Dm values for cakes formed from suspensions containing ‘high’ cation concentrations. Results shown in Figs. 7 and 8 for suspensions at pH 4, 5, 6 and 8 containing both no added salt and low (0.02 mol/L) NaCl concentrations reveal that almost no change in material hydraulic conductivity or consolidation coefficient occurs between pH 5 and pH 8, but increases dramatically for km, at pH 4. This trend, which is not reflected in the cake moisture ratios (cf. Fig. 2), indicates that a combination of factors, other than void volume, are impacting on the permeability and consolidation properties of the filter cake as the pH of the suspension is varied.
Electrophoretic mobility and zeta potential At a H+ concentration of 1011 mol/L (pH 11), the zeta potential of the PASS suspension is approximately 40 mV. The values decrease to between 25 and 20 mV at 108–104 mol/L (pH 8–4) and peak at 0.68 mV at the highest H+ concentration examined (101.5 mol/L, pH 1.5) (Fig. 9). This trend is similar to that reported by Aydin et al. (2004), who conducted zeta potential measurements of a mixed clay suspension from pH 4 to 12 and observed that the zeta potential of kaolinite tends to be more susceptible to pH changes than that of smectitic clays. Therefore, the changes in zeta potential observed here are likely to be a combined result of both the kaolinite and montmorillonite fractions that exist naturally in these PASS materials.
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Material hydraulic conductivity, 1010k m (m/s)
100
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a
10
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NaCl 0.02 mol/L - pH4
distilled water - pH8
NaCl 0.5 mol/L - pH4
distilled water - pH4
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0.1 1 100
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FeCl2 FeCl2 0.01 mol/L -pH4 - pH4
CaCl2 0.01 mol/L - pH4 CaCl2 mol/L-pH4
FeCl2 mol/L - pH4 FeCl2 0.05 mol/L-pH4
CaCl2 CaCl2 0.05 mol/L - pH4
0.1
0.1 1
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100
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100
Imposed gas pressure, P (kPa) Figure 3 Dependence of material hydraulic conductivity km on: (a) pH, (b) Na+, (c) Ca2+ and (d) Fe2+ concentrations as a function of imposed gas pressure.
Material consolidation coefficient, 1010Dm (m/s)
100
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NaCl 0.02 mol/L - pH4
distilled water - pH4
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CaCl2 CaCl2 0.01 0.01 mol/L-pH4 mol/L - pH4
FeCl2 FeCl 0.01 mol/L mol/L --pH4 pH4 2 0.01
CaCl2 CaCl2 0.05 0.05 mol/L mol/L -- pH4 pH4
FeCl2 0.05 mol/L mol/L-pH4 FeCl2 0.05 - pH4
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Imposed gas pressure, P (kPa) Figure 4 Dependence of the material consolidation coefficient Dm on: (a) pH, (b) Na+, (c) Ca2+ and (d) Fe2+ concentrations as a function of imposed gas pressure.
Material consolidation coefficient, 2 10 10Dm (m /s)
100
1010k m (m/s)
10
1
NaCl 0.5mol/L mol/L- pH4 - pH4 NaCl 0.5 CaCl2 0.05 CaCl2 0.05 mol/L mol/L -- pH4 pH4 - pH4 FeCl FeCl2 0.05 mol/L mol/L-pH4 2 0.05 - pH4 distilled water - pH4
0.1 1
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10 0 NaCl 0.02 mol/L - pH8 NaCl 0.02 mol/L - pH6 NaCl 0.02 mol/L - pH5 NaCl 0.02 mol/L - pH4 distilled water - pH8 distilled water - pH4
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1 1
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Imposed gas pressure, P (kPa)
Imposed gas pressure, P (kPa) Figure 5 Material hydraulic conductivity km of filter cakes deposited from pH 4 PASS suspensions containing ‘high’ concentrations of Na+, Ca2+, Fe2+ and distilled water.
Figure 8 Material consolidation coefficient Dm of filter cakes deposited from PASS suspensions containing ‘low’ concentrations of Na+, or distilled water, at varying pH values.
100
Zeta potential (mV)
2
10 Dm (m /s)
NaCl - pH4 NaCl 0.5 0.5mol/L mol/L - pH4 CaCl2 CaCl2 0.05 0.05 mol/L - pH4 FeCl2 0.05 mol/L-pH4 mol/L - pH4 FeCl2 0.05 distilled distilled water water - pH4 - pH4
10
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Material consolidation coefficient,
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15.00 5.00 -5.00 -15.00 -25.00 -35.00 -45.00 0
4
6
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pH 1 1
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Imposed gas pressure, P (kPa)
Figure 9 of pH.
Figure 6 Material consolidation coefficient Dm of filter cakes deposited from pH 4 PASS suspensions containing ‘high’ concentrations of Na+, Ca2+, Fe2+ and distilled water.
1
Zeta potential of the PASS suspensions as a function
10.00
Zeta potential (mV)
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Material hydraulic conductivity, 10 10k m (m/s)
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1.00 0.50 0.00 -0.50 -1.00 -1.50 -2.00 -2.50 -3.00 12
Mobility (108 m2/V.s)
Material hydraulic conductivity,
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils
0.00 -10.00
2+ Fe Fe 2+ Ca Ca + Na Na
-20.00 -30.00 -40.00 -50.00 0.1
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NaCl 0.02 mol/L - pH8 NaCl 0.02 mol/L - pH6 NaCl 0.02 mol/L - pH5 NaCl 0.02 mol/L - pH4 distilled water - pH8 distilled water - pH4
Concentration (10 mol/L)
Figure 10 Zeta potential of the PASS suspensions as a function of cation concentration.
0.1 1
10
100
Imposed gas pressure, P (kPa)
Figure 7 Material hydraulic conductivity km of filter cakes deposited from PASS suspensions containing ‘low’ concentrations of Na+, or distilled water, at varying pH values.
The PASS suspensions to which NaCl had been added were observed to have zeta potentials that fluctuated slightly around 29 mV until a Na+ concentration of 0.1 mol/L was exceeded. Above this concentration, the zeta potential increased to a maximum of 12 mV at 0.5 mol/L (Fig. 10). These results are in agreement with the trend observed by de Kretser et al. (1998) for a montmorillonite-
dominated clay suspension. However, the suspensions become highly unstable at Na+ concentrations >0.1 mol/L, as a result of particle flocculation and settlement, which resulted in the large standard deviations observed for these measurements. The divalent cations, Ca2+ and Fe2+, exhibited very similar impacts on the zeta potential of the PASS suspensions (Fig. 10). Increasing the concentration of either of these cations from 0.001 to 0.005 mol/L resulted in a sharp increase in the zeta potential of the PASS suspensions. Above 0.005 mol/L, however, the zeta potential of the suspensions remained rather constant fluctuating around 10 mV with a gradual increase in measurement error due to particle flocculation and settlement.
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Discussion Clay platelet arrangement The PASS used in these experiments is dominated by montmorillonite, which is well-known for its shrink-swell characteristics as well as its reaction to solution electrolytes (White et al., 2003). Thus, current knowledge on the properties of montmorillonite will be used as a basis for interpreting the obtained experimental results. A rather simplified, but useful model to describe the role of cations on the structure of the electrical double layer surrounding clay particles has been proposed by Tombacz and Szekeres (2004). In this model, the clay lamellae are envisaged to possess positive edges when the solution pH is smaller than the pH of point of zero charge (i.e., the pH at which the pH dependent surface charge is zero; pHPZC).These edges ‘emerge’ (as opposed to being ‘hidden’) if cation concentrations are higher than a threshold value (Tombacz and Szekeres, 2004). The magnitude, sign and distribution of charge on individual clay lamellae impact upon the association of these particles. The association between individual clay particles, in turn, controls the porosity, hydraulic conductivity and compressibility of the cake formed during filtration. It has been reported that the pHPZC of montmorillonite varies between pH 6 (Goodwin, 1971) and pH 7.7 (van Olphen, 1963) though more recent work by Tombacz and Szekeres (2004), Bourg et al. (2007) and others highlights the complex effects of pH both on the negative facial charge and on the amphoteric aluminol edge groups. The PZC of 1.5–2 of the PASS clay-sized materials examined in this study are much lower than reported for montmorillonite, possibly due to the presence of different clay minerals (such as kaolinite). Despite the predominant negative charge observed, the edge groupings on the montmorillonite particles present are likely to protonate at low pH and, provided the salt concentration is low, exhibit a distinct zone of positive charge which may result in edge-face attraction and formation of aggregates at the lower pH. These aggregates may subsequently deposit on the membrane in more conducting
assemblages than is the case for non-aggregated clay particles which form more resistive cakes as a result of their nonaggregated state (Waite et al., 1999). The change in km and Dm when pH is decreased to 4 (Figs. 7 and 8), compared with pH values P5, suggests that structural change in the filter cake may indeed have occurred. These results are consistent with an increase in zeta potential observed in the PASS suspensions at pH values <4 (Fig. 9). As a result, it is certainly possible that clay platelets may have existed predominantly in a dispersed state at pH values >4 resulting in relatively lower hydraulic conductivity and consolidation coefficient values once deposited on the membrane. Increasing the salt concentration at pH 4 is, in all cases, seen to result in an increase, albeit relatively small, in material hydraulic conductivity (Fig. 3) and consolidation coefficient (Fig. 5). Increased aggregation of clay particulates may well have occurred at the higher salt concentrations with the result that the assemblages exhibit higher hydraulic conductivity (and higher compressibility) once deposited on the membrane. The effect of increasing ionic strength is less evident for ferrous and calcium salts, possibly because the critical coagulation concentration of the clay particulates present has been exceeded even at the lower salt concentrations used (Heimenz and Rajagopalan, 1997).
Fitting empirical equations In previous studies attempts have been made to establish an empirical correlation between consolidation pressure (in mH2O) and hydraulic conductivity of marine-origin clay soils (Kim et al., 1992; White et al., 2003). The strong linear trends observed in Figs. 3–8 provoke examination of this correlation with the results obtained in this study. Material hydraulic conductivity values were graphed against compressive pressure on a log–log scale and fitted with a trendline. The fitting equation is in the power form km ðwÞ ¼ AwB
ð9Þ
Table 2 Comparison of the empirical constants M, N their standard errors (StE) and coefficients of determination (R2) obtained for the PASS suspensions as a function of solution composition and pH Solution
pH
Concentration (mol/L)
M
StE (M)a
N
StE (N)a
R2
Distilled water
8 4
0 0
23.29 22.03
0.04 0.04
0.34 0.59
0.03 0.03
0.98 0.99
Na+
8 6 5 4 4
0.02 0.02 0.02 0.02 0.5
23.26 23.32 23.08 22.20 21.27
0.07 0.03 0.02 0.06 0.04
0.36 0.26 0.41 0.50 0.68
0.06 0.03 0.02 0.05 0.03
0.93 0.97 0.99 0.94 0.99
Ca2+
4 4
0.01 0.05
21.71 21.14
0.02 0.02
0.66 0.61
0.02 0.02
1.00 1.00
Fe2+
4 4
0.01 0.05
21.47 21.06
0.03 0.05
0.63 0.59
0.02 0.04
1.00 0.99
a
Standard error.
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils where log (A) and B are, respectively, the intercept and slope of the linear regression on these log–log graphs. Eq. (9) can be converted to the more popular form suggested by White et al. (2003): lnðkm ðwÞÞ ¼ M N lnðwÞ
ð10Þ
where M and N are empirical constants which can be obtained easily by using the LINEST function in Excel. The value of N for the natural PASS diluted in distilled water is 0.34 (Table 2) with 95% confidence interval from 0.28 to 0.40 which encompasses the value of 0.39 reported by White et al. (2003) for the slope of the linear regression for similar PASS materials. However, the most remarkable feature of the data listed in Table 2 is the similarity between the constants M and N for all of the conditions examined in this study – considering that the cation concentrations varied by as much as four orders of magnitude. These results reiterate the main findings of this work that, while porewater electrolyte concentrations may impart a measureable effect on the hydraulic properties of this soil material, environmentally relevant cation concentrations found in nearby ground and drain water systems are unlikely to dramatically change the hydraulic conductivity of these soils.
Implications From the results obtained in this study, it would appear that the export of groundwater from PASS into local drains or, alternatively, the intrusion of high ionic strength estuarine tidal waters into PASS is likely to be rather minor. Similarly, waters highly charged with cations (e.g., brackish estuarine waters) currently used to flush or neutralise acidic drainwaters in ASS regions are not expected to induce significant changes in the hydraulic conductivity of PASS materials. These data support our earlier observations made on bulk PASS samples. In fact, examination of the hydraulic properties of resuspended PASS also suggests that this situation is unlikely to change in the future if these materials remain saturated. Although the hydraulic conductivity of the PASS examined in this study could be increased by up to 10- to 15-fold, in real terms this only results in a hydraulic conductivity of approximately 109 m/s – a value that may reasonably be considered to be far from the ‘conductive’ range.
References Aydin, M., Yano, T., Kilic, S., 2004. Dependence of zeta potential and hydraulic conductivity on adsorbed cations and aqueous phase properties. Soil Sci. Soc. Am. J. 68, 450–459. Bourg, I.C., Sposito, G., Bourg, A.C.M., 2007. Modeling the acid– base chemistry of montmorillonite. J. Colloid Interf. Sci. 312, 297–310. Blunden, B.G., Indraratna, B., 2000. Evaluation of surface and groundwater management strategies for drained sulfidic soil using numerical simulation models. Aust. J. Soil Res. 38, 569– 590. Brookhaven Instruments Corporation (BIC), 2002. Zeta Potential Analyzer – Instruction Manual. Brookhaven Corporate Park, NewYork. Glamore, W.C., 2003. Evaluation and analysis of acid sulfate soils impact via tidal restoration. Ph.D. Thesis, University of Wollongong, Wollongong, NSW, Australia, p. 469.
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Goodwin, J.W., 1971. Rheological studies on the dispersion of kaolinite suspensions. Trans. Brit. Ceram. Soc. 70, 65–70. Graham, J., Yuen, K., Goh, T.B., Janzen, P., Sivakumar, V., 2001. Hydraulic conductivity and pore fluid chemistry in artificially weathered plastic clay. Eng. Geol. 60, 69–81. Green, R., Macdonald, B.C.T., Melville, M.D., Waite, T.D., 2006. Hydrochemistry of episodic drainage waters discharged an acid sulfate soil affected catchment. J. Hydrol. 325, 356–375. Heimenz, P.C., Rajagopalan, R., 1997. Principles of Surface and Colloid Chemistry. Marcel Dekker Inc., New York. Iritani, E., 2003. Properties of filter cake in cake filtration and membrane filtration. KONA 21, 19–40. Johnston, S.G., Slavich, P.G., Hirst, P., 2004. The acid flux dynamics of two artificial drain in acid sulfate soil backswamps on the Clarences River floodplain, Australia. Aust. J. Soil Res. 42, 623–637. Kim, D.J., Vereecken, H., Feyen, J., Boels, D., Bronswijk, J.J.B., 1992. On the characterisation of properties of an unripe marine clay soil, II. A method on the determination of hydraulic properties. Soil Sci. 154, 59–71. de Kretser, R.G., Usher, S.P., Scales, P.J., Boger, D.V., Landman, K.A., 2001. Rapid filtration measurement of dewatering design and optimization parameters. AIChE J. 47 (8), 1758–1769. Le, T.M.H., Pham, A.N., Collins, R.N., Waite, T.D., 2008. Impacts of compressive pressure and solution composition on hydraulic conductivity and consolidation coefficients of acid sulfate soils. Aust. J. Soil Res. 46, 112–121. Mata, C., Ledesma, A., 2003. Permeability of a bentonite–crushed granite rock mixture using different experimental techniques. Geotechnique 53 (8), 747–758. Mohan, K.K., Fogler, H.S., 1997. Effect of pH and layer charge on formation damage porous media containing swelling clays. Langmuir 13, 2863–2872. Petrov, R.J., Rowe, R.K., 1997. Geosynthetic clay liner chemical compatibility by hydraulic conductivity testing and factors impacting its performances. Can. Geotech. J. 30 (6), 863. Phillip, J.R., 1957. The theory of infiltration: sorptivity and algebraic infiltration equations. Soil Sci. Soc. Am. J. 84, 257– 264. Santiwong, S.R., Guan, J., Waite, T.D., 2008. Effect of ionic strength and pH on hydraulic properties and structure of accumulating solid assemblages during microfiltration of montmorillonite suspension. J. Colloid Interf. Sci. 317, 214–227. Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B., Lin, L., 2000. Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids. Geotext. Geomem. 18, 133–161. Siyag, R.S., Pal, R., Poonia, S.R., Baruah, T.C., 1983. Effect of mixed cation solutions on hydraulic soil properties. Agric. Water Manage. 6, 15–25. Smiles, D.E., 1986. Principles of constant pressure filtration. In: Cheremisinoff, N.P. (Ed.), Encyclopedia of Fluid Mechanics: Slurry Flow Technology. Gulf Publishing, Houston, pp. 792–822. Smiles, D.E., 2000. Hydrology of swelling soils: a review. Aust. J. Soil Res. 38, 501–521. Smith, J., van Oploo, P., Marston, H., Melville, M.D., Macdonald, B.C.T., 2003. Spatial distribution and management of total actual acidity in an acid sulfate soil environment, McLeods Creek, northeastern NSW, Australia. Catena 51 (1), 61–79. Stein, H.N., 1986. Rheological behavior of suspensions. In: Cheremisinoff, N.P. (Ed.), Encyclopedia of Fluid Mechanics. Gulf Publishing, Houston, pp. 3–48. Tombacz, E., Szekeres, M., 2004. Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75–94. Tulau, M.J., 1999. Acid sulfate soil management priority areas in the lower tweed floodplains. Department of Land and Water Conservation, Sydney.
270 van Olphen, H., 1963. An Introduction to Clay Colloid Chemistry. Interscience, New York. van Oploo, P., 2000. Soil and pore-water relations of a coastal acid sulfate soil in northern New South Wales, Australia. Ph.D. Thesis, The University of New South Wales, Sydney, Australia, p. 386. Vasco, S.M., Jo, H.Y., Benson, C.H., Edil, T.B., Katsumi, T., 2001. Hydraulic conductivity of partially prehydrated geosynthetic clay liners permeated with aqueous calcium chloride solutions. In: Proceedings of the 7th International Conference on Geosynthetics, Nice, France, pp. 685–699. Waite, T.D., Schaefer, A., Fane, A.J., Heuer, A., 1999. Colloidal fouling of ultrafiltration membranes: impact of aggregate structure and size. J. Colloid Interf. Sci. 212, 264–274. Wakeman, R.J., 1978. The influence of entrapped gas and resaturation on diffusional extraction in porous media. Chem. Eng. J. 16, 73–78. Wakeman, R.J., 1986. Theoretical approaches to thickening and filtration. In: Cheremisinoff, N.P. (Ed.), Encyclopedia of Fluid Mechanics: Slurry Flow Technology. Gulf Publishing, Houston, pp. 649–681.
T.M.H. Le et al. Wakeman, R.J., Tarleton, E.S., 1999. Filtration: Equipment Selection, Modelling and Process Simulation. Elsevier Advanced Technology, The Boulevard, Langford lane, Kidlington, Oxford, UK. White, I., Melville, M.D., Wilson, B.P., Price, C.B., Willett, I.R., 1993. Understanding acid sulfate soils in Canelands. In: National Conference on Acid Sulfate Soils, Coolangatta, Australia, pp. 130–147. White, I., Melville, M.D., Wilson, B.P., Sammut, J., 1997. Reducing acidic discharges from coastal wetlands in eastern Australia. Wetl. Ecol. Manag. 5, 55–72. White, I., Smiles, D.E., Santomartino, S., van Oploo, P., Macdonald, B.C.T., Waite, T.D., 2003. Dewatering and hydraulic properties of soft, sulfidic, coastal clay soils. Water Resour. Res. 39 (10), 1295–1308. White, I., Macdonald, B.C.T, Riley, R., Melville, M.D., Waite, T.D, 2005. Unconfined groundwater hydrology of soft sulfidic soils. In: Engineers Australia, 29th Hydrology and Water Resources Symposium, 21–23 February 2005, Canberra, Australia.
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Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils T.M.H. Le a, R.N. Collins b, T.D. Waite
a,*
a
School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Centre for Water and Waste Technology, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
b
Received 23 September 2007; received in revised form 28 March 2008; accepted 11 April 2008
KEYWORDS Sorptivity; Hydraulic conductivity; Consolidation coefficient; Filtration cell; Material coordinates theory
Summary Acid sulfate soils (ASS) cover extensive areas of east Australian coastal floodplains. Upon oxidation, these hydromorphic pyritic sediments produce large quantities of sulfuric acid. In addition, due to their geographic location, these soils may also come in contact with high ionic strength estuarine tidal waters. As a result, there is typically a large variation in acidity (pH) and cation concentrations in soil porewaters and adjacent aquatic systems (e.g., agricultural field drains, rivers, estuaries, etc.). Acid sulfate soils, especially from the unoxidized gelatinous deeper layers, contain a relatively high proportion of montmorillonite, which is wellknown for its shrink-swell properties. Variations in cation concentrations, including H3O+, can influence montmorillonite platelet interactions and may, thus, also significantly affect the hydraulic conductivity of materials containing this clay. In this paper we report on the effect of four common cations, at reasonable environmental concentrations, on the hydraulic properties of potential (unoxidized) acid sulfate soil materials. The natural system was simplified by examining individually the effects of each cation (H+, Ca2+, Fe2+ and Na+) on a soil–water suspension in a filtration cell unit. Moisture ratio, hydraulic conductivity and the consolidation coefficient of the deposited filter cakes were calculated using material coordinates theory. The results indicate that the hydraulic conductivity of potential acid sulfate soils increases at low pH and with cation concentration. Although an increase in the charge of amphoteric edge groups on montmorillonite clays may result in some aggregation between individual clay platelets, we conclude that the extent of these changes are unlikely to cause significant increases in the transportation of acidity (and contaminants) through potential acid sulfate soils as the hydraulic conductivity of these materials remain low (<109 m/s) at pH and ionic conditions normally experienced in the field. ª 2008 Elsevier B.V. All rights reserved.
* Corresponding author. Tel.: +61 2 9385 5059; fax: +61 2 9385 6139. E-mail address: [email protected] (T.D. Waite). 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.04.014
262
Introduction Rapid development in Australian coastal regions has initiated growing demand for improved understanding of factors controlling the generation of acid and high dissolved concentrations of elements such as iron and aluminium from so-called ‘‘acid sulfate soils’’ (ASS) and the transport of these potentially harmful species through subsurface environments. Commonly known as Holocene coastal sediments or marine muds, these originally hydromorphic soils contain (or contained) sulfidic minerals, commonly in the form of pyrite (FeS2). Potential (or unoxidized) acid sulfate soils (PASS) often contain a high proportion of swelling clays and are difficult to dewater. However, extreme weather conditions or anthropogenic disturbances, such as prolonged drought, drainage, dredging or excavation, enable oxygen to oxidize sulfides in PASS to form actual ASS (van Oploo, 2000; Smith et al., 2003). The oxidation process ultimately results in the formation of acidic porewaters and, in many instances, high concentrations of dissolved cations (particularly of Fe(II) and Al(III) (Glamore, 2003), which can be exported into nearby streams and waterbodies destroying aquatic life and corroding civil infrastructure. Potential acid sulfate soils are defined as sulfidic soil materials containing pyrite which have not oxidised to the extent that soil pH has decreased below 3.5 (Tulau, 1999). Given this definition, the ‘partial’ oxidation of pyrite in PASS can result in significant soil porewater concentrations of ferrous iron and sulfuric acid: FeS2 þ 7=2O2 þ H2 O ! Fe2þ þ 2Hþ þ 2SO2 4 While the production and transformation of these initial oxidation products in the groundwaters of acid sulfate soil affected regions have been investigated in a number of studies (White et al., 1993, 1997; Blunden and Indraratna, 2000; van Oploo, 2000; Smith et al., 2003; Johnston et al., 2004; Green et al., 2006), there have been few reports of the impacts of these constituents on ASS physical properties (Le et al., 2008). Almost 50% w/w of PASS consists of clay minerals and more than 60 % of the clay-sized fraction is montmorillonite (Smiles, 2000; White et al., 2003). It is well-known that cations can compress the diffuse double layer of clay particles leading to changes in the modes of particle association (i.e., face-to-face, edge-to-face or edge-to-edge) (van Olphen, 1963; Stein, 1986; Wakeman, 1986; Mohan and Fogler, 1997; Tombacz and Szekeres, 2004) and, as a result, considerably alter the hydraulic conductivity and consolidation properties of consolidated assemblages of these materials (Mohan and Fogler, 1997; Mata and Ledesma, 2003; Santiwong et al., 2008). Nevertheless, in a previous study (Le et al., 2008) we observed that dissolved milli-molar concentrations of H+ and Ca2+ had little, if any, measureable impact on the hydraulic conductivity and consolidation of PASS in experiments employing a hydraulic consolidometer, despite theoretical calculations suggesting that changing pore water electrolyte concentrations should dramatically change the hydraulic properties of these soils (White et al., 2003, 2005). However, it may reasonably be argued that the use of a bulk soil sample
T.M.H. Le et al. in the consolidometer studies as well as the short timescale (7 days) of the experiments may not be representative of either the material’s physical state in the natural environment or the long timescales (months to years) required to induce significant physical changes. This may have important ramifications to ASS management as tidal flushing of east Australian coastal ASS drains with high ionic strength estuarine waters is increasingly becoming the preferred acid neutralisation technique to minimise the environmental problems caused by ASS discharge (Le et al., 2008). The principal issues of concern then are (i) whether any changes in hydraulic properties will occur in unconsolidated unoxidized PASS on change in ionic composition, and (ii) the timescale over which any changes might occur. Both aspects can be investigated simultaneously by varying the pH and cation concentrations of diluted PASS suspensions which are filtered through a dead-end filtration device. In this fashion, chemical interactions between the solution and the solid phase of the soil are not limited by the initial hydraulic properties of the soil (1010 m/s) which inherently limit the transport of solution through bulk soil samples (Le et al., 2008). Dead-end filtration cells have been employed for many industrial applications including investigation of membrane fouling in water and wastewater treatment, assessment of the permeability of landfill containers, examination of the permeability of substrates to drilling fluid and ease of sediment dewatering in civil constructions (Siyag et al., 1983; Petrov and Rowe, 1997; Shackelford et al., 2000; Graham et al., 2001; Vasco et al., 2001). The method has also previously been used for the examination of the hydraulic conductivity and consolidation properties of PASS (White et al., 2003).
Materials and methods Study site Potential acid sulfate soil samples were collected at 1.5 m below the ground surface of sugar cane fields bordered by McLeod’s Creek, a tributary of the Tweed River, Northeastern New South Wales (28 18 0 S, 153 31 0 E), Australia. The site is located in a typical backswamp environment surrounded by small streams feeding into the creek. Similar to most of the Tweed floodplain area, the soil profile of the study site can be divided into four strata. The organic topsoil, with a dark-brown to black appearance, varies in thickness from 0.1 to 0.4 m. A layer of oxidized ASS materials around 0.1 to 0.3 m in thickness, commonly known as the acidic horizon, lies below the organic topsoil and is followed by the transition zone (0.3–0.8 m in thickness) which also contains oxidized materials, but in a much smaller proportion. The PASS commences at approximately 0.9 m below the ground surface. The solution concentration of dominant cations (Na+, Ca2+, Fe2+ and Mg2+) in both the drainwater from McLeod’s Creek and PASS porewater can vary significantly depending on weather condition, time and sample location. The range of chemical characteristics that are frequently obtained for waters obtained at the study site are given in Table 1.
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils Table 1 Typical range of major cation concentrations (mg/L), pH and electrical conductivity (EC) values of PASS porewaters and McLeod’s Creek drainwaters obtained at the study site Species +
Na K+ Ca2+ Fe2+ Mg2+ pH EC (mS) a b
263
pressure filtration cell combined with the materials coordinate approach to derive the material hydraulic conductivity and consolidation coefficient of PASS materials and found the results obtained to be consistent with those reported by other investigators.
Concentration (mg/L) Pore water
Drain water
Experimental procedure
253–570 13–23 72–260 0–350a 63–292 3–6a 0.7–2.8a
6200 170 180 Very low 520 3.5–6.5b 0.1–11.7
Experiments were conducted with PASS suspensions in deionised, membrane filtered ‘‘Milli-Q’’ water (Millipore Corporation)and with suspensions that contained one of three different salts (NaCl, FeCl2 or CaCl2) at ‘low’ (i.e., 60.02 mol/L) or ‘high’ (i.e., P0.05 mol/L) concentrations. A number of experiments were also conducted at pH 4 (a pH at which these soils are still defined as PASS) in order to examine the combined effects of pH and cation concentrations on the hydraulic conductivity of the PASS that accumulated on the filter. The effect of using an NaCl concentration of 0.5 mol/L was also examined in order to recreate conditions typical of estuarine tidal waters since such waters are increasingly being used to ‘flush’ acidic drains in coastal eastern Australia. Soil suspensions were prepared daily before each set of experiments in order to avoid oxidation of the soil materials. A clay:water ratio of 1:20 by weight (i.e., 50 g of clay in 1 L of distilled water) was used in all instances. Prior to creating the suspension, all visible coarse contaminants such as remnant plant roots and small pebbles were removed by hand. After dilution, small sand particles and remaining coarse contaminants tended to either float or quickly settle. These components were removed from the suspensions in order to improve sample homogeneity. The diluted PASS suspensions were mixed thoroughly by an electromagnetic stirrer and sonicated to ensure suspension homogeneity. Concentrated acid (H2SO4) and/or monovalent or divalent chloride salts (NaCl, FeCl2 or CaCl2) were subsequently added to the suspensions. During the experiments the pH of the suspensions tended to increase due to the buffering capacity of the soil. This problem was overcome by continual monitoring and, when needed, addition of small amounts of concentrated acid in order to maintain the desired pH. Subsamples of the suspensions were taken prior to filtration in order to determine the electrophoretic mobility (le) of particles present using a Brookhaven ZetaPLUS (Brookhaven Instruments Corporation) instrument. The Smoluchowski equation (BIC, 2002) was used to convert (le) to zeta potential (f). Dead-end batch filtration experiments were conducted with 50 mL volume of feed suspension poured directly into a Perspex cylindrical filtration cell with a membrane of 0.044 m diameter at its base. The membrane selected for use in this study was a microfiltration (MF) membrane with an average pore size of 0.22 lm (0.22 lm GV, DURAPORE Membrane Filters, Millipore). A fresh piece of membrane was used in each experiment. Pressure controllers (P602C-FAC-002R and P-602C-FAC-004R, Bronkhorst Hi-Tech S.V.) were employed to maintain precise supply of constant pressure in the 3–60 kPa and 5–300 kPa (±0.1%) range. Instrument Grade Air (BOC Group) was used for the application of pressure. Instantaneous mass of cumulative permeate was measured by electronic balances (PB3002-SDR, ¨ TTINGEN) Mettler Toledo Ltd., and LC1201S, Sartorius AG GO
van Oploo (2000). White et al. (1993).
Soil samples Approximately 1 kg of PASS was collected from the study site using a handheld auger. The sample was wrapped tightly in plastic, and then covered with aluminium foil and placed in a plastic bag. Air was carefully excluded from the bag and the sample was then constantly kept refrigerated (below 4 C) to avoid oxidation and drying of the soil. For PASS at this site, clay and fine silt account for as much as 99% of the soil material (White et al., 2003). Expandable lattice clays (i.e., smectites) dominate, comprising 60% of the clay fraction, while kaolinite and illite account for, respectively, 30% and 10% (White et al., 2003). As such, the clay fraction, especially montmorillonite, is expected to control the hydraulic conductivity of these PASS. Measured moisture ratios (the volume of water to soil) of PASS are typically 3.5. Diluted fresh PASS suspensions generally have pH ranging from 7.0 to 8.0, but quickly decreases to 6.0 or less if exposed to air for a few hours. With an abundant supply of oxygen, the suspension can be continuously oxidised and acidified to pH as low as 1.5–2.0.
Filtration cell methodology Filtration cell methods are widely used in determination of the dewaterability of sludges and mineral assemblages and in the examination of membrane fouling in water and wastewater treatment (Wakeman and Tarleton, 1999; Iritani, 2003; White et al., 2003). Filtration studies are often used to examine the performance of filter media and/or to quantify the membrane fouling behaviour of suspended solids. In this study however, the filtration cell method is used to characterise the hydraulic properties and consolidation behaviour of predispersed then reconsolidated PASS under various solution conditions. Estimating the hydraulic conductivity and consolidation coefficients of filter cakes using a filtration cell is, however, less straightforward than use of an odeometer or hydraulic consolidometer. Despite the complexity, the material coordinates approach typically used in data analysis is well developed with the approach used successfully in many instances (Smiles, 1986). For example, White et al. (2003) have used a constant applied
264
T.M.H. Le et al.
and the data were electronically transferred to a connected PC at predetermined time intervals. After completion of the filtration run, the wet and dry weight of the filtration cakes was determined for triplicate samples with the dry weight determined after drying at 105 C for at least 48 h.
Data analysis Outflow and cake moisture ratio data were analysed using the material coordinate framework which has been developed and detailed by Smiles (1986, 2000) and White et al. (2003). This framework converts the conventional constant pressure filtration theory based on spatial coordinates to a set of equations that are particularly applicable to swelling materials, such as montmorillonite, by employing material coordinates (Smiles, 1986). The theory is summarised here as it is considered to be particularly appropriate to gelatinous PASS. Space coordinates have previously been used to model unsteady flow through swelling materials by Phillip (1957) and Wakeman (1978). Lagrangian analyses, employing material coordinates, were then introduced based on the distribution of the solid in order to simplify analysis of the unsteady problem (Smiles, 1986). The theory is based on two inter-connected equations of continuity for the liquid and solid phases for one-dimensional flow of water in a swelling system (Smiles, 1986, 2000): ohw oF w ohs oF s ¼ and ¼ ð1Þ ot z oz t ot z oz t
i ¼ S0 t1=2
ð6Þ
The sorptivity (S0) is then obtained from the slope of the linear regression of cumulative outflow plotted against the square root of time at the early stages of flow. This procedure reveals that S0 contains similar information as the slope t/V against V that Wakeman and Tarleton (1999) uses in the conventional cake filtration model and the inverse slope of t vs. V2 used in compressive rheology theory by de Krester et al., 2001. Integral manipulations are then used to solve the flow equation with step changes in imposed pressure, which provides the relationship between S0 and material hydraulic conductivity km. The consequential equations that allow calculation of the material hydraulic conductivity km and material moisture diffusivity (or, in geomechanics, the material consolidation coefficient) Dm from sorptivity S0 and sample moisture ratio # data are S0 oS0 S0 o#0 km ðw0 Þ ¼ ð7Þ D# ow 4D# ow 0 S0 oS0 S0 ð8Þ Dm ð#0 Þ ¼ D# o#0 4D# In the data analysis process, cumulative outflow i is first plotted against the square root of time and fitted with a regression line to obtain sorptivity. This parameter as well as cake moisture ratio are then plotted against matric potential in order to calculate the separate terms in Eqs. (7) and (8).
where Fw and Fs are the volume flux densities and hw and hs are, respectively, the liquid and solid volume fractions (Smiles, 1986, 2000). The m(z, t) materials coordinate is defined by
Results
om om ¼ hs and ¼ F s ð2Þ oz ot and the continuity equation for water in materials space becomes o# ou ¼ ð3Þ ot m om t
Fig. 1a shows that higher moisture content cakes were produced at pH 4 than at pH 8 with the effect of increasing pressure in producing drier cake less pronounced at pH 8. The experimental results were highly reproducible, especially for the pH 8 suspension (which is similar to the natural pH of the PASS). The cake moisture ratio appears to be slightly higher in the presence of ‘low’ concentrations (0.02 mol/L) of Na+ compared to 0.5 mol/L (Fig. 1b), however this difference is unlikely to be statistically significant. Similarly, very little difference in filter cake properties was observed for suspensions containing 0.01 and 0.05 mol/L Ca2+ and Fe2+ at pH 4 (Fig. 1c and d). Comparison of the moisture ratios of cakes formed from suspensions containing the salts with values obtained for the clay suspended in distilled water (Fig. 1a–d) indicate that the salts (at least at the concentrations used in these experiments) had very little effect on cake moisture ratios obtained at pH 4. Interestingly, when the pH of the PASS suspension was varied between 4 and 8 in the presence of 0.02 mol/L NaCl (Fig. 2), the apparent difference in cake moisture ratio at the two pHs is less significant than obtained in distilled water (Fig. 1a).
where # = hw/hs is the moisture ratio of the suspension and u is the flux of water relative to the solid particles (Smiles, 1986). Darcy’s Law for swelling systems using materials coordinates can then be written as oU ow u ¼ km ð#Þ ð4Þ ¼ km ð#Þ þ ð1 qs Þ oz om where km is material hydraulic conductivity and U is the total potential energy in a swelling system and involves three components: the gravitational potential z, the unloaded matric potential w and the overburden potential X (Smiles, 2000): U¼zþwþX
ð5Þ
To formulate equations that can be applied to data obtained from filtration experiments and to obtain hydraulic conductivity and moisture diffusivity values, a parameter termed sorptivity (S0) is introduced which embodies the change in the properties of the pore system between the initial and final moisture ratio of the sample (Phillip, 1957; White et al., 2003). The formula relating sorptivity with cumulative outflow at early stages, i is
Moisture ratio, #
Material hydraulic conductivity (km) and material consolidation coefficient (Dm) In Fig. 3a it can be seen that the material hydraulic conductivity (km), in the absence of added salts, is higher at pH 4 than at pH 8. As the moisture ratio increased with decreas-
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils 5
5
a
4
4
3
3
b
NaCl 0.02 mo l/L - pH4
Distilled water - pH8
Moisture ratio
265
NaCl 0.5 mol/L - pH4
Distilled water - pH4
2
2 1
5
10
1
100
5
c
10
100
d
4
4
3
3 FeCl l/L - pH4 2 0.01 Fe Cl2 0.01mo mol/L -p H4
CaCl 2 0.01 0.01 mol/Lmo l/L pH4 - pH4 CaCl2
FeCl l/L - pH4 2 0.05 Fe Cl2 0.05mo mol/LpH4
0.05 mol/L mo l/L -- pH4 pH4 CaCl 2 0.05 CaCl2
2
2 1
10
100
1
10
100
Imposed gas pressure, P (kPa) Figure 1 Dependence of filter cake moisture ratio on: (a) pH, (b) Na+, (c) Ca2+ and (d) Fe2+ concentrations as a function of imposed gas pressure.
Moisture ratio
5
4
NaCl 0.02 mol/L- pH4
3
NaCl 0.02 mol/L - pH5 NaCl 0.02 mol/L - pH6 NaCl 0.02 mol/L - pH8
2 1
10
100
Imposed gas pressure, P (kPa)
Figure 2 Dependence of filter cake moisture ratio on pH, in the presence of 0.02 mol/L NaCl, as a function of imposed gas pressure.
ing pH, this result may be due to the filter cake having a more porous structure. However, the km values of the filter cakes are generally higher when the concentrations of the cations are increased from ‘low’ to ‘high’ (Fig. 3b–d) even though the moisture ratios are essentially identical (Fig. 1). This trend departs from the observed pattern in the absence of added cations and suggests that particle arrangement on deposition may possibly be different. The corresponding results of material consolidation coefficients (Dm) shown in Fig. 4, indicate, however, that differences in this parameter are only observed in response to increasing cation concentrations from ‘low’ to ‘high’ (i.e., increasing to pH 4 in the absence of cations had little if any effect on the material consolidation coefficient). As shown in Figs. 5 and 6, the material hydraulic conductivity and consolidation coefficient of the cakes deposited in
the presence of ‘high’ concentrations of the cations are markedly higher than values measured in distilled water at pH 4. Indeed, there is less than 30% difference in the km and Dm values for cakes formed from suspensions containing ‘high’ cation concentrations. Results shown in Figs. 7 and 8 for suspensions at pH 4, 5, 6 and 8 containing both no added salt and low (0.02 mol/L) NaCl concentrations reveal that almost no change in material hydraulic conductivity or consolidation coefficient occurs between pH 5 and pH 8, but increases dramatically for km, at pH 4. This trend, which is not reflected in the cake moisture ratios (cf. Fig. 2), indicates that a combination of factors, other than void volume, are impacting on the permeability and consolidation properties of the filter cake as the pH of the suspension is varied.
Electrophoretic mobility and zeta potential At a H+ concentration of 1011 mol/L (pH 11), the zeta potential of the PASS suspension is approximately 40 mV. The values decrease to between 25 and 20 mV at 108–104 mol/L (pH 8–4) and peak at 0.68 mV at the highest H+ concentration examined (101.5 mol/L, pH 1.5) (Fig. 9). This trend is similar to that reported by Aydin et al. (2004), who conducted zeta potential measurements of a mixed clay suspension from pH 4 to 12 and observed that the zeta potential of kaolinite tends to be more susceptible to pH changes than that of smectitic clays. Therefore, the changes in zeta potential observed here are likely to be a combined result of both the kaolinite and montmorillonite fractions that exist naturally in these PASS materials.
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Material hydraulic conductivity, 1010k m (m/s)
100
100
a
10
10
1
1
b
NaCl 0.02 mol/L - pH4
distilled water - pH8
NaCl 0.5 mol/L - pH4
distilled water - pH4
0.1
0.1 1 100
10
1
100
10
100
c
10
10
1
1
100
d
FeCl2 FeCl2 0.01 mol/L -pH4 - pH4
CaCl2 0.01 mol/L - pH4 CaCl2 mol/L-pH4
FeCl2 mol/L - pH4 FeCl2 0.05 mol/L-pH4
CaCl2 CaCl2 0.05 mol/L - pH4
0.1
0.1 1
10
1
100
10
100
Imposed gas pressure, P (kPa) Figure 3 Dependence of material hydraulic conductivity km on: (a) pH, (b) Na+, (c) Ca2+ and (d) Fe2+ concentrations as a function of imposed gas pressure.
Material consolidation coefficient, 1010Dm (m/s)
100
100
a
NaCl 0.02 mol/L - pH4
distilled water - pH4
NaCl 0.5 mol/L - pH4
10
10
1
1 1 100
b
distilled water - pH8
10
1
100 100
c
10
100
d
CaCl2 CaCl2 0.01 0.01 mol/L-pH4 mol/L - pH4
FeCl2 FeCl 0.01 mol/L mol/L --pH4 pH4 2 0.01
CaCl2 CaCl2 0.05 0.05 mol/L mol/L -- pH4 pH4
FeCl2 0.05 mol/L mol/L-pH4 FeCl2 0.05 - pH4
10
10
1
1
1
10
100
1
10
100
Imposed gas pressure, P (kPa) Figure 4 Dependence of the material consolidation coefficient Dm on: (a) pH, (b) Na+, (c) Ca2+ and (d) Fe2+ concentrations as a function of imposed gas pressure.
Material consolidation coefficient, 2 10 10Dm (m /s)
100
1010k m (m/s)
10
1
NaCl 0.5mol/L mol/L- pH4 - pH4 NaCl 0.5 CaCl2 0.05 CaCl2 0.05 mol/L mol/L -- pH4 pH4 - pH4 FeCl FeCl2 0.05 mol/L mol/L-pH4 2 0.05 - pH4 distilled water - pH4
0.1 1
10
10 0 NaCl 0.02 mol/L - pH8 NaCl 0.02 mol/L - pH6 NaCl 0.02 mol/L - pH5 NaCl 0.02 mol/L - pH4 distilled water - pH8 distilled water - pH4
10
1 1
100
10
100
Imposed gas pressure, P (kPa)
Imposed gas pressure, P (kPa) Figure 5 Material hydraulic conductivity km of filter cakes deposited from pH 4 PASS suspensions containing ‘high’ concentrations of Na+, Ca2+, Fe2+ and distilled water.
Figure 8 Material consolidation coefficient Dm of filter cakes deposited from PASS suspensions containing ‘low’ concentrations of Na+, or distilled water, at varying pH values.
100
Zeta potential (mV)
2
10 Dm (m /s)
NaCl - pH4 NaCl 0.5 0.5mol/L mol/L - pH4 CaCl2 CaCl2 0.05 0.05 mol/L - pH4 FeCl2 0.05 mol/L-pH4 mol/L - pH4 FeCl2 0.05 distilled distilled water water - pH4 - pH4
10
10
Material consolidation coefficient,
267
15.00 5.00 -5.00 -15.00 -25.00 -35.00 -45.00 0
4
6
8
10
pH 1 1
10
100
Imposed gas pressure, P (kPa)
Figure 9 of pH.
Figure 6 Material consolidation coefficient Dm of filter cakes deposited from pH 4 PASS suspensions containing ‘high’ concentrations of Na+, Ca2+, Fe2+ and distilled water.
1
Zeta potential of the PASS suspensions as a function
10.00
Zeta potential (mV)
10
Material hydraulic conductivity, 10 10k m (m/s)
2
1.00 0.50 0.00 -0.50 -1.00 -1.50 -2.00 -2.50 -3.00 12
Mobility (108 m2/V.s)
Material hydraulic conductivity,
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils
0.00 -10.00
2+ Fe Fe 2+ Ca Ca + Na Na
-20.00 -30.00 -40.00 -50.00 0.1
1
10
100
1000
10000
-3
NaCl 0.02 mol/L - pH8 NaCl 0.02 mol/L - pH6 NaCl 0.02 mol/L - pH5 NaCl 0.02 mol/L - pH4 distilled water - pH8 distilled water - pH4
Concentration (10 mol/L)
Figure 10 Zeta potential of the PASS suspensions as a function of cation concentration.
0.1 1
10
100
Imposed gas pressure, P (kPa)
Figure 7 Material hydraulic conductivity km of filter cakes deposited from PASS suspensions containing ‘low’ concentrations of Na+, or distilled water, at varying pH values.
The PASS suspensions to which NaCl had been added were observed to have zeta potentials that fluctuated slightly around 29 mV until a Na+ concentration of 0.1 mol/L was exceeded. Above this concentration, the zeta potential increased to a maximum of 12 mV at 0.5 mol/L (Fig. 10). These results are in agreement with the trend observed by de Kretser et al. (1998) for a montmorillonite-
dominated clay suspension. However, the suspensions become highly unstable at Na+ concentrations >0.1 mol/L, as a result of particle flocculation and settlement, which resulted in the large standard deviations observed for these measurements. The divalent cations, Ca2+ and Fe2+, exhibited very similar impacts on the zeta potential of the PASS suspensions (Fig. 10). Increasing the concentration of either of these cations from 0.001 to 0.005 mol/L resulted in a sharp increase in the zeta potential of the PASS suspensions. Above 0.005 mol/L, however, the zeta potential of the suspensions remained rather constant fluctuating around 10 mV with a gradual increase in measurement error due to particle flocculation and settlement.
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Discussion Clay platelet arrangement The PASS used in these experiments is dominated by montmorillonite, which is well-known for its shrink-swell characteristics as well as its reaction to solution electrolytes (White et al., 2003). Thus, current knowledge on the properties of montmorillonite will be used as a basis for interpreting the obtained experimental results. A rather simplified, but useful model to describe the role of cations on the structure of the electrical double layer surrounding clay particles has been proposed by Tombacz and Szekeres (2004). In this model, the clay lamellae are envisaged to possess positive edges when the solution pH is smaller than the pH of point of zero charge (i.e., the pH at which the pH dependent surface charge is zero; pHPZC).These edges ‘emerge’ (as opposed to being ‘hidden’) if cation concentrations are higher than a threshold value (Tombacz and Szekeres, 2004). The magnitude, sign and distribution of charge on individual clay lamellae impact upon the association of these particles. The association between individual clay particles, in turn, controls the porosity, hydraulic conductivity and compressibility of the cake formed during filtration. It has been reported that the pHPZC of montmorillonite varies between pH 6 (Goodwin, 1971) and pH 7.7 (van Olphen, 1963) though more recent work by Tombacz and Szekeres (2004), Bourg et al. (2007) and others highlights the complex effects of pH both on the negative facial charge and on the amphoteric aluminol edge groups. The PZC of 1.5–2 of the PASS clay-sized materials examined in this study are much lower than reported for montmorillonite, possibly due to the presence of different clay minerals (such as kaolinite). Despite the predominant negative charge observed, the edge groupings on the montmorillonite particles present are likely to protonate at low pH and, provided the salt concentration is low, exhibit a distinct zone of positive charge which may result in edge-face attraction and formation of aggregates at the lower pH. These aggregates may subsequently deposit on the membrane in more conducting
assemblages than is the case for non-aggregated clay particles which form more resistive cakes as a result of their nonaggregated state (Waite et al., 1999). The change in km and Dm when pH is decreased to 4 (Figs. 7 and 8), compared with pH values P5, suggests that structural change in the filter cake may indeed have occurred. These results are consistent with an increase in zeta potential observed in the PASS suspensions at pH values <4 (Fig. 9). As a result, it is certainly possible that clay platelets may have existed predominantly in a dispersed state at pH values >4 resulting in relatively lower hydraulic conductivity and consolidation coefficient values once deposited on the membrane. Increasing the salt concentration at pH 4 is, in all cases, seen to result in an increase, albeit relatively small, in material hydraulic conductivity (Fig. 3) and consolidation coefficient (Fig. 5). Increased aggregation of clay particulates may well have occurred at the higher salt concentrations with the result that the assemblages exhibit higher hydraulic conductivity (and higher compressibility) once deposited on the membrane. The effect of increasing ionic strength is less evident for ferrous and calcium salts, possibly because the critical coagulation concentration of the clay particulates present has been exceeded even at the lower salt concentrations used (Heimenz and Rajagopalan, 1997).
Fitting empirical equations In previous studies attempts have been made to establish an empirical correlation between consolidation pressure (in mH2O) and hydraulic conductivity of marine-origin clay soils (Kim et al., 1992; White et al., 2003). The strong linear trends observed in Figs. 3–8 provoke examination of this correlation with the results obtained in this study. Material hydraulic conductivity values were graphed against compressive pressure on a log–log scale and fitted with a trendline. The fitting equation is in the power form km ðwÞ ¼ AwB
ð9Þ
Table 2 Comparison of the empirical constants M, N their standard errors (StE) and coefficients of determination (R2) obtained for the PASS suspensions as a function of solution composition and pH Solution
pH
Concentration (mol/L)
M
StE (M)a
N
StE (N)a
R2
Distilled water
8 4
0 0
23.29 22.03
0.04 0.04
0.34 0.59
0.03 0.03
0.98 0.99
Na+
8 6 5 4 4
0.02 0.02 0.02 0.02 0.5
23.26 23.32 23.08 22.20 21.27
0.07 0.03 0.02 0.06 0.04
0.36 0.26 0.41 0.50 0.68
0.06 0.03 0.02 0.05 0.03
0.93 0.97 0.99 0.94 0.99
Ca2+
4 4
0.01 0.05
21.71 21.14
0.02 0.02
0.66 0.61
0.02 0.02
1.00 1.00
Fe2+
4 4
0.01 0.05
21.47 21.06
0.03 0.05
0.63 0.59
0.02 0.04
1.00 0.99
a
Standard error.
Influence of metal ions and pH on the hydraulic properties of potential acid sulfate soils where log (A) and B are, respectively, the intercept and slope of the linear regression on these log–log graphs. Eq. (9) can be converted to the more popular form suggested by White et al. (2003): lnðkm ðwÞÞ ¼ M N lnðwÞ
ð10Þ
where M and N are empirical constants which can be obtained easily by using the LINEST function in Excel. The value of N for the natural PASS diluted in distilled water is 0.34 (Table 2) with 95% confidence interval from 0.28 to 0.40 which encompasses the value of 0.39 reported by White et al. (2003) for the slope of the linear regression for similar PASS materials. However, the most remarkable feature of the data listed in Table 2 is the similarity between the constants M and N for all of the conditions examined in this study – considering that the cation concentrations varied by as much as four orders of magnitude. These results reiterate the main findings of this work that, while porewater electrolyte concentrations may impart a measureable effect on the hydraulic properties of this soil material, environmentally relevant cation concentrations found in nearby ground and drain water systems are unlikely to dramatically change the hydraulic conductivity of these soils.
Implications From the results obtained in this study, it would appear that the export of groundwater from PASS into local drains or, alternatively, the intrusion of high ionic strength estuarine tidal waters into PASS is likely to be rather minor. Similarly, waters highly charged with cations (e.g., brackish estuarine waters) currently used to flush or neutralise acidic drainwaters in ASS regions are not expected to induce significant changes in the hydraulic conductivity of PASS materials. These data support our earlier observations made on bulk PASS samples. In fact, examination of the hydraulic properties of resuspended PASS also suggests that this situation is unlikely to change in the future if these materials remain saturated. Although the hydraulic conductivity of the PASS examined in this study could be increased by up to 10- to 15-fold, in real terms this only results in a hydraulic conductivity of approximately 109 m/s – a value that may reasonably be considered to be far from the ‘conductive’ range.
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Goodwin, J.W., 1971. Rheological studies on the dispersion of kaolinite suspensions. Trans. Brit. Ceram. Soc. 70, 65–70. Graham, J., Yuen, K., Goh, T.B., Janzen, P., Sivakumar, V., 2001. Hydraulic conductivity and pore fluid chemistry in artificially weathered plastic clay. Eng. Geol. 60, 69–81. Green, R., Macdonald, B.C.T., Melville, M.D., Waite, T.D., 2006. Hydrochemistry of episodic drainage waters discharged an acid sulfate soil affected catchment. J. Hydrol. 325, 356–375. Heimenz, P.C., Rajagopalan, R., 1997. Principles of Surface and Colloid Chemistry. Marcel Dekker Inc., New York. Iritani, E., 2003. Properties of filter cake in cake filtration and membrane filtration. KONA 21, 19–40. Johnston, S.G., Slavich, P.G., Hirst, P., 2004. The acid flux dynamics of two artificial drain in acid sulfate soil backswamps on the Clarences River floodplain, Australia. Aust. J. Soil Res. 42, 623–637. Kim, D.J., Vereecken, H., Feyen, J., Boels, D., Bronswijk, J.J.B., 1992. On the characterisation of properties of an unripe marine clay soil, II. A method on the determination of hydraulic properties. Soil Sci. 154, 59–71. de Kretser, R.G., Usher, S.P., Scales, P.J., Boger, D.V., Landman, K.A., 2001. Rapid filtration measurement of dewatering design and optimization parameters. AIChE J. 47 (8), 1758–1769. Le, T.M.H., Pham, A.N., Collins, R.N., Waite, T.D., 2008. Impacts of compressive pressure and solution composition on hydraulic conductivity and consolidation coefficients of acid sulfate soils. Aust. J. Soil Res. 46, 112–121. Mata, C., Ledesma, A., 2003. Permeability of a bentonite–crushed granite rock mixture using different experimental techniques. Geotechnique 53 (8), 747–758. Mohan, K.K., Fogler, H.S., 1997. Effect of pH and layer charge on formation damage porous media containing swelling clays. Langmuir 13, 2863–2872. Petrov, R.J., Rowe, R.K., 1997. Geosynthetic clay liner chemical compatibility by hydraulic conductivity testing and factors impacting its performances. Can. Geotech. J. 30 (6), 863. Phillip, J.R., 1957. The theory of infiltration: sorptivity and algebraic infiltration equations. Soil Sci. Soc. Am. J. 84, 257– 264. Santiwong, S.R., Guan, J., Waite, T.D., 2008. Effect of ionic strength and pH on hydraulic properties and structure of accumulating solid assemblages during microfiltration of montmorillonite suspension. J. Colloid Interf. Sci. 317, 214–227. Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B., Lin, L., 2000. Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids. Geotext. Geomem. 18, 133–161. Siyag, R.S., Pal, R., Poonia, S.R., Baruah, T.C., 1983. Effect of mixed cation solutions on hydraulic soil properties. Agric. Water Manage. 6, 15–25. Smiles, D.E., 1986. Principles of constant pressure filtration. In: Cheremisinoff, N.P. (Ed.), Encyclopedia of Fluid Mechanics: Slurry Flow Technology. Gulf Publishing, Houston, pp. 792–822. Smiles, D.E., 2000. Hydrology of swelling soils: a review. Aust. J. Soil Res. 38, 501–521. Smith, J., van Oploo, P., Marston, H., Melville, M.D., Macdonald, B.C.T., 2003. Spatial distribution and management of total actual acidity in an acid sulfate soil environment, McLeods Creek, northeastern NSW, Australia. Catena 51 (1), 61–79. Stein, H.N., 1986. Rheological behavior of suspensions. In: Cheremisinoff, N.P. (Ed.), Encyclopedia of Fluid Mechanics. Gulf Publishing, Houston, pp. 3–48. Tombacz, E., Szekeres, M., 2004. Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75–94. Tulau, M.J., 1999. Acid sulfate soil management priority areas in the lower tweed floodplains. Department of Land and Water Conservation, Sydney.
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