Hydrocarbon-Contamination of Soils: The Potential of Sodium Clays to Decelerate Soil Toxicants

Hydrocarbon-contamination can change hydraulic conductivity (HC) in soils, and hence increase the spreading rate of aqueous toxicants in the ground. A constant head permeameter used in the laboratory to measure HC of soils taken from near the Pitch Lake in Trinidad determined that the HC exceeded that of a reference soil having “normal HC” for a loam. Although water moved rapidly through it, the Pitch Lake soil (PLS) remained dry due to water repellence. Treatment consisted of either of two red mud bauxite wastes mixed at 25 and 50% w/w with PLS at air dry. One of the bauxite wastes had undergone treatment with gypsum several years before and hence contained a greater proportion of calcium ions compared to the other red mud which contained more sodium ions. At 25% w/w the non-gypsum-treated red mud waste decreased HC of the PLS by 50%, and at 50% w/w caused a 10-fold decrease of HC on the PLS. The gypsum-treated red mud waste had no effect on the HC of the PLS. The drastic decrease in HC of the hydrocarbon-contaminated soil implies blocking of hydraulic channels by inorganic particles. The high levels of Na released in the Bayer beneficiation process dispersed and released fine < 5 mμ clay particles from the non-gypsum-treated red muds. This suggests that the rapid movement of aqueous pollutants in such hydrocarbon-polluted soils could be similarly curtailed under field conditions.


Introduction
Water repellent soils normally exhibit low infiltration rates (Blackwell, 2000).Hydrophobicity can reduce the affinity for soils to water such that infiltration or wetting may be delayed for periods ranging from as little as a few seconds to in excess of weeks (Hall, 2009).Soil hydrophobicity is thought to be caused primarily by a coating of long-chained hydrophobic organic molecules on individual soil particles, thereby influencing soil hydrological and ecological functions (Takawira et al., 2014).A study by Lourenco et al. (2015) confirmed the hypothesis that hydrocarbon contamination induces water repellence and reduces soil moisture retention at low suction (< 100 kPa) for laboratory contaminated soils.Water is not easily absorbed by such non-wetting soils.Yet, in a pilot study of the Trinidad Pitch Lake soils at La Brea: (1) Water moved rapidly through the soil fabric, and; (2) The soil remained completely dry, despite prodigious movement of water through it; (3) Soil crumbs (> 2 mm), though denser than water, literally floated on water; (4) Soil crumbs > 2 mm exhibited mutual repellence when floated in water.
Therefore, quite apart from the last three more esoteric above-mentioned reactions, it was concluded that aqueous contaminants released in such soils, could move just as quickly.There exists therefore, the potential (Figure 1) to rapidly contaminate large volumes of soil during toxic aqueous spills.o the sified water t (4).matter g the g the ld be raulic ess.
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Bauxite Waste Treatments
Two depths were selected as potential treatment additives because only the 0-15 cm layer was gypsum treated.It has been shown (Harris, 2008) that the gypsum applied to a depth of 15 cm had little or no physical effect on soil below 15 cm depth.Therefore the original chemistry of the RMW at 15-30 cm was not as altered as that of the 0-15 depth (GRMW).It was therefore postulated that the greater ESP and SAR of material from 15-30 cm may enhance displacement of inorganic ions in a hydraulic column.Removal of divalent ions from the exchange complex can contribute to reductions in aggregate stability and decreases in HC (Kopittke et al., 2006).

Red Mud Safety Concerns
Red mud can contain elevated levels of metals, and the pH of red mud characteristically ranges from 10 to 12 due to the use of caustic soda during the extraction process.Red mud is also known to contain technologically enhanced naturally occurring radioactive material (TENORM), including thorium and uranium.TENORM is naturally occurring radioactive material (NORM) that has been processed in such a manner that its concentration has increased (EPA, 2012).
However, concerns about the radioactivity in red mud waste were allayed because (Pinnock, 1991) showed that using 100% bauxite residue gave a dose equivalent to just over 2 m Sv y -1 and was judged to be acceptable.However, other work on Hungarian bauxites has recommended a maximum addition of 15% bauxite residue to avoid exceeding a level of 0.3 m Sv y -1 (Pinnock, 1991).

Statistics
Sample means compared using Tukey's Honestly Significant Difference (P ≤ 0.05) to examine differences in HC between treatments.

Water Repellence
Water repellence of the PLS was found to be at the highest value, i.e., extremely water repellent (4), compared with the reference soil, which had a value of highly wetting (0), i.e., showing very rapid water absorbance.Rapid water absorbance is unusual for hydrophobic soils.Further, the fact that soil aggregates after several days in water, and even aggregates as small as < 2 mm remained dry after several hours of water infiltration showed the extremely hydrophobic nature of the Pitch Lake soils (Figure 4).Note.*Water repellence, measured by counting the number of seconds required for water to be absorbed into the soil after the method of DeBano (1981), is classified as follows: lower than 3 seconds = highly wetting (0); 3-6 seconds = wetting (1); 6-60 seconds = slightly water repellent (2); 60-600 seconds moderately water repellent (3); > 600 seconds = extremely water repellent (4). jas.ccsenet.

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the Pitch Lake of the heavie nk to bottom a These provide extra negatively charged sites for adsorbing hydrated Na + cations.Similarly, the tendency of Na + ions to facilitate the formation of multiple layer hydrates in smectite clays leads to greater swelling and reduction of HC.On the contrary, Frenkel et al. (1992) showed that smectites did not release clay particles when added to a sandy soil unless anions were added, because the HC of smectite clay-sand mixtures decreased only following the addition of the various anions.Dispersed clay appeared in their effluent only upon addition of citrate or hexametaphosphate.Therefore, at 56% intrinsic organic matter (Table 1), such hydrocarbon contamination would have greatly increased the negative charges of the Pitch Lake soil.This could have increased repelling forces amongst the thoroughly mixed-in clay particles, thereby blocking hydraulic channels.
Further, this result conforms with the very high sensitivity of illite to even small amounts of exchangeable Na (Oster et al., 1980).They found that, for a given ESP, the critical flocculation concentrations (CFC) of illite was much larger than for smectite, and hypothesized that the explanation lies in the irregular nature of illite particles, which prevent good contact between edges and planar surfaces, thereby decreasing the potential for inter-particle attraction.Similarly, the red mud particles, having undergone beneficiation (extreme comminution), would have been altered to more irregular shapes.Compared with Sintering red mud, the Bayer Process (used in the Caribbean alumina processing) produces a relatively small particle diameter.The particle diameter of Bayer mud is between 0.8 μm and 50 μm with an average value of 14.8 μm (Wang & Liu, 2012).In the case of the smectite cited by Frenkel et al. (1992), HC is decreased through partial blocking of pores by short distance migration of dispersed particles, provided the particle size of the sand (or host soil) is sufficiently fine to retain these particles.Being a clay-loam, the particle size of the PLS is smaller than that of fine sand and therefore would be susceptible to pore blockage by the fine clay particles released from the dried red mud.

Conclusion
By repelling water from pore channel walls, hydrophobicity can markedly reduce resistance to movements of aqueous liquids through soils.Finely crushing RMW from < 2 mm to < 1 mm produced an at least 4-fold increase in the effective surface area of the RMW.Soil water repellence is a function of soil surface chemistry.More specifically, it is a function of the free energy of the solid/gas interface in soil (γSG).In contrast to the low Na + RMW, the high Na + RMW produced dislocated clay particles which blocked hydraulic channels.
With a high energy surface exhibited by rapidly decreasing the initial advancing contact angle (θ), the high Na + RMW either expanded, or released clay particles, either of which could have blocked hydraulic channels in the PLS.

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Table 2 .
Water repellence rating* of soil near a pitch lake, and that of bauxite wastes