Partitioning of hydrophobic organic compounds within soil-water-surfactant systems

Understanding the partitioning of hydrophobic organic compounds (HOCs) within soil-water-surfactant systems is key to improving the use of surfactants for remediation. The overall objective of this study was to investigate the soil properties that influence the effectiveness of surfactants used to remediate soil contaminated with hydrophobic pesticides, as an example of a more general application for removing strongly sorbing HOCs from contaminated soils via in-situ enhanced sorption, or ex-situ soil washing. In this study, the partitioning of two commonly used pesticides, atrazine and diuron, within soil-water-surfactant systems was investigated. Five natural soils, one nonionic surfactant (Triton-100 (TX)) and one cationic surfactant (benzalkonium chloride (BC)) were used. The results showed that the cation exchange capacity (CEC) is the soil property that controls surfactant sorption onto the soils. Diuron showed much higher solubility enhancement than atrazine with the micelles of either surfactant. Within an ex-situ soil washing system, TX is more effective for soils with lower CEC than those with higher CEC. Within an in-situ enhanced sorption zone, BC works significantly better with more hydrophobic HOCs. The HOC sorption capacity of the sorbed surfactant (K(ss)) was a non-linear function of the amount of surfactant sorbed. For the cationic surfactant (BC), the maximal K(ss) occurred when around 40% of the total CEC sites in the various soils were occupied by sorbed surfactant. Below a sub-saturation sorption range (~20 g/kg), under the same amount of BC sorbed, a soil with lower CEC tends to have higher K(ss) than the one with higher CEC.     

Surfactant enhanced electrokinetic remediation of DDT from soils

Electrokinetic remediation has been investigated extensively as one of the noble technologies in remediation of metal contaminated soils. However, its applications in remediation of organic contaminants have been limited due to low solubilities of organics in water. In addition, most organic contaminants are non-ionic and therefore, they are not mobile under electrical field. The use of surfactants may increase the remediation efficiency by increasing the solubility of organics. Significant fraction of organics associated with soil, can be transferred to micellar phase, which then can be transported toward either cathode or anode, depending on the ionic group of surfactants. In this study, the removal of hydrophobic organic contaminants from a soil using electrokinetic method was investigated in the presence of surfactants. A nonionic surfactant, Tween 80, and an anionic surfactant, SDBS, were used in the experiments. DDT was chosen as the model organic contaminant. Phase distribution studies and column experiments were conducted. It was found that both Tween 80 and SDBS had similar solubilization potentials for DDT. It was also shown that the aqueous DDT mass could reach from 0.01 to 13% of the total mass in the presence of 7500 mg/L of SDBS. No significant movement of DDT was observed when Tween 80 was used in the column experiments. This was attributed to low rates of electroosmotic flows and strong interaction of Tween 80 with the soil. The amount of surfactant was not enough to mobilize DDT significantly in the column studies. On the other hand, electrokinetic transport with SDBS yielded much better results. DDT transport toward the anode within the negatively charged micelles overcame the opposite electrosmotic flow. This was attributed to the lower degree of interaction between the soil and SDBS, and the electrokinetic transport of negatively charged micelles.     

A multi-component statistic analysis for the influence of sediment/soil composition on the sorption of a nonionic surfactant (Triton X-100) onto natural sediments/soils

The contents of soil/sediment organic carbon and clay minerals (i.e. montmorillonite, kaolinite, illite, gibbsite and 1.4 nm minerals) for 21 natural soil/sediment samples and the sorption of Triton X-100 on these samples were determined. A multi-component statistic analysis was employed to investigate the importance of soil/sediment organic matters and clay minerals on their sorption of Triton X-100. The sorption power of soil/sediment composition for Triton X-100 conforms to an order of montmorillonite>organic carbon>illite>1.4 nm minerals (vermiculite+chlorite+1.4 nm intergrade mineral)>kaolinite. The sorption of Triton X-100 on a montmorillonite, a kaolinite and a humic acid were also investigated and consistent with the result of multi-component statistic analysis. It is clear that the sorption of Triton X-100 on soils or sediments is the combined contribution of soil/sediment organic matters and clay minerals, which depended on both the contents of soil/sediment organic matters and the types and contents of clay minerals. The important influence of illite on the sorption of nonionic surfactants onto soils/sediments is suggested and demonstrated in this paper. Surfactants for aquifer remediation application may be more efficient for the contaminated soils/sediments that contain little clay minerals with 2:1 structure because of the less sorption of nonionic surfactants on these soils/sediments.     

Evaluation of surfactants/cosolvents for desorption/solubilization of Phenanthrene in clayey soils

Throughout the USA, numerous sites exist where the soils have been contaminated by polycyclic aromatic hydrocarbons (PAHs). These compounds may be toxic, mutagenic and/or carcinogenic, so these sites threaten human health and the environment and prompt remediation is warranted. In situ flushing with surfactants/cosolvents has shown promise for treating PAH‐contaminated soils that are uniform and possess a high permeability, but the efficiency of this process is severely limited when heterogeneous and/or low permeability soils are present. For these difficult situations, electrokinetically enhanced in situ flushing offers great potential, but this method is highly dependent on the type of purging agent that is used. Thus, in this laboratory investigation, batch desorption experiments were conducted to evaluate different surfactants/cosolvent solutions for use in electrokinetically enhanced in situ flushing. The surfactants/cosolvents were evaluated on their ability to desorb and solubilize phenanthrene, a representative PAH, from two widely varying clayey soil types. The soils were artificially contaminated at four PAH concentrations, and batch tests were conducted using six different surfactant/cosolvent solutions. The results indicated that phenanthrene was more strongly bound to the soil with the higher organic content, and the surfactants with a higher hydrophile – lipophile balance number (HLB) caused greater PAH desorption and solubilization. Furthermore, the surfactant solutions performed better when they were used at a higher concentration. Compared to the cosolvent solution or a combined mixture of the cosolvent and surfactant solutions, greater desorption and solubilization of the contaminant occurred when the surfactant solution was used by itself.     

Degradation of soil-sorbed trichloroethylene by stabilized zero valent iron nanoparticles: effects of sorption, surfactants, and natural organic matter

Zero valent iron (ZVI) nanoparticles have been studied extensively for degradation of chlorinated solvents in the aqueous phase, and have been tested for in-situ remediation of contaminated soil and groundwater. However, little is known about its effectiveness for degrading soil-sorbed contaminants. This work studied reductive dechlorination of trichloroethylene (TCE) sorbed in two model soils (a potting soil and Smith Farm soil) using carboxymethyl cellulose (CMC) stabilized Fe-Pd bimetallic nanoparticles. Effects of sorption, surfactants and dissolved organic matter (DOC) were determined through batch kinetic experiments. While the nanoparticles can effectively degrade soil-sorbed TCE, the TCE degradation rate was strongly limited by desorption kinetics, especially for the potting soil which has a higher organic matter content of 8.2%. Under otherwise identical conditions, ∼44% of TCE sorbed in the potting soil was degraded in 30 h, compared to ∼82% for Smith Farm soil (organic matter content = 0.7%). DOC from the potting soil was found to inhibit TCE degradation. The presence of the extracted SOM at 40 ppm and 350 ppm as TOC reduced the degradation rate by 34% and 67%, respectively. Four prototype surfactants were tested for their effects on TCE desorption and degradation rates, including two anionic surfactants known as SDS (sodium dodecyl sulfate) and SDBS (sodium dodecyl benzene sulfonate), a cationic surfactant hexadecyltrimethylammonium (HDTMA) bromide, and a non-ionic surfactant Tween 80. All four surfactants were observed to enhance TCE desorption at concentrations below or above the critical micelle concentration (cmc), with the anionic surfactant SDS being most effective. Based on the pseudo-first-order reaction rate law, the presence of 1×cmc SDS increased the reaction rate by a factor of 2.5 when the nanoparticles were used for degrading TCE in a water solution. SDS was effective for enhancing degradation of TCE sorbed in Smith Farm soil, the presence of SDS at sub-cmc increased TCE degraded by ∼10%. However, effect of SDS on degradation of TCE in the potting soil was more complex. The presence of SDS at sub-cmc decreased TCE degradation by 5%, but increased degradation by 5% when SDS dosage was raised to 5×cmc. The opposing effects were attributed to combined effects of SDS on TCE desorption and degradation, release of soil organic matter and nanoparticle aggregation. The findings strongly suggest that effect of soil sorption on the effectiveness of Fe-Pd nanoparticles must be taken into account in process design, and soil organic content plays an important role in the overall degradation rate and in the effectiveness of surfactant uses.     

Aqueous chemistry and interactive effects on non-ionic surfactant and pentachlorophenol sorption to soil

Non-ionic surfactant addition was investigated as a method to remediate pentachlorophenol (PCP) contaminated soil. The goal was to quantify surfactant (Tergitol NP-10 (TNP10)) and PCP sorption to soil and their interactive effects under varying pH, ionic strength, and soil conditions. Up to 16,700 mg/kg of TNP10 partitioned to soil, with increasing sorption far above the critical micelle concentration (CMC) and with greater amounts of PCP present. Approximately 40-45 times more TNP10 and 20-30 times more PCP sorbed to the finer soil with higher organic matter content. Aqueous TNP10 concentrations well above the CMC (>/=5500 mg/L) were required to enhance PCP desorption from the soil. As pH increased by 0.5-0.85 units, TNP10 sorption decreased by 14-25% and PCP sorption as measured by the log of the equilibrium partition coefficient decreased by 1-1.5. A lower ionic strength of 0.03 versus 0.112 M increased PCP desorption from contaminated soil by 5-17% in the presence of TNP10. This work is relevant to designing ex situ soil washing or surfactant-aided PCP remediation.     

Enhanced soil flushing of phenanthrene by anionic-nonionic mixed surfactant

Laboratory column experiments were conducted to investigate the performance of anionic-nonionic mixed surfactant, sodium dodecyl sulfate (SDS) with Triton X-100 (TX100), in enhancing phenanthrene flushing for contaminated soil in an aim to improve the efficiency of surfactant remediation technology. The experimental results showed that the sorption of TX100 onto soil was severely restricted in the presence of SDS in batch and column experiments and decreased with the increasing mass fraction of SDS in mixed surfactant solutions; meanwhile the enhanced solubilization of phenanthrene by SDS-TX100 mixed surfactant was greater than that by individual surfactant. These results can be attributed to the formation of mixed surfactant micelles in solution. The column flushing experiments showed that the flushing efficiencies for phenanthrene-contaminated soil by SDS-TX100 mixed surfactants were greater than that by individual surfactant and increased with the increasing mass fraction of SDS in mixed surfactant solutions.     

Enhanced Fenton degradation of hydrophobic organics by simultaneous iron and pollutant complexation with cyclodextrins

The effectiveness and selectivity of Fenton degradation of hydrophobic organic compounds (HOCs) can be improved by simultaneous complexation of Fe(2+) and the organic compound with a cyclodextrin or derivatized cyclodextrin. Such selective complexation of a target substrate and a catalytic metal is a crude mimic of enzyme systems. Both beta-cyclodextrin and carboxymethyl-beta-cyclodextrin (CMCD) were able to simultaneously complex Fe(2+) and an aromatic hydrocarbon, such as phenol, polycyclic aromatic hydrocarbons, and polychlorinated biphenyls (PCBs). Degradation of compounds included in cyclodextrins was unaffected by hydroxyl radical scavengers, indicating that the radical was formed at the ternary complex (HOC-cyclodextrin-iron) and in close proximity to the included molecule. Without cyclodextrins, humic acid (HA) decreased degradation efficiency. However, in the presence of CMCD, HA did not inhibit degradation of the target compound. CMCD is capable of removing HOCs from HA binding sites while at the same time complexing Fe(2+). PCBs sorbed to glass were resistant to Fenton degradation, but were significantly degraded using a cyclodextrin modified Fenton system. In all of these systems, the ternary HOC-cyclodextrin-iron complexes effectively direct hydroxyl radical reaction toward the HOC, increasing the efficiency of Fenton degradation. One potential application of such targeted degradation systems is the in situ remediation of hydrophobic organic pollutants in contaminated soil and groundwater or in industrial waste streams.     

Partitioning of hydrophobic pesticides within a soil-water-anionic surfactant system

Surfactants can be added to pesticide-contaminated soils to enhance the treatment efficiency of soil washing. Our results showed that pesticide (atrazine and diuron) partitioning and desorbability within a soil-water-anionic surfactant system is soil particle-size dependent and is significantly influenced by the presence of anionic surfactant. Anionic surfactant (linear alkylbenzene sulphonate, LAS) sorption was influenced by its complexation with both the soluble and exchangeable divalent cations in soils (e.g. Ca²⁺, Mg²⁺). In this study, we propose a new concept: soil system hardness which defines the total amount of soluble and exchangeable divalent cations associated with a soil. Our results showed that anionic surfactant works better with soils having lower soil system hardness. It was also found that the hydrophobic organic compounds (HOCs) sorbed onto the LAS-divalent cation precipitate, resulting in a significant decrease in the aqueous concentration of HOC. Our results showed that the effect of exchangeable cations and sorption of HOC onto the surfactant precipitates needs to be considered to accurately predict HOC behavior within soil-water-anionic surfactant systems.     

Partitioning of hydrophobic organic chemicals (HOC) into anionic and cationic surfactant-modified sorbents

Surfactant-modified sorbents have been proposed for the removal of organic compounds from aqueous solution. In the present study, one cationic (HDTMA) and three anionic (DOWFAX-8390, STEOL-CS330, and Aerosol-OT) surfactants were tested for their sorptive behavior onto different sorbents (alumina, zeolite, and Canadian River Alluvium). These surfactant-modified materials were then used to sorb a range of hydrophobic organic chemicals (HOCs) of varying properties (benzene, toluene, ethylbenzene, 1,2-dichlorobenzene, naphthalene, and phenanthrene), and their sorption capacity and affinity (organic-carbon-normalized sorption coefficient, K(oc)) were quantified. The HDTMA-zeolite system proved to be the most stable surfactant-modified sorbent studied because of the limited surfactant desorption. Both anionic and cationic surfactants resulted in modified sorbents with higher sorption capacity and affinity than the unmodified Canadian River Alluvium containing only natural organic matter. The affinities of the surfactant-modified sorbents (K(oc)) for most HOCs are lower than octanol/water partition coefficient (K(ow)) normalized to the organic carbon content (f(oc)) and the density of octanol (K(oc) octanol); naphthalene and phenanthrene are the exceptions to this rule.     

Soil particle-size dependent partitioning behavior of pesticides within water-soil-cationic surfactant systems

Cationic surfactants have been proposed for enhanced sorption zones to contain hydrophobic organic compound (HOC) contamination. Benzalkonium chloride (BC), a cationic surfactant, was selected to study the particle-size dependent sorption behavior of the surfactant and its role in the immobilization of two hydrophobic pesticides (atrazine and diuron) within soil-water-surfactant systems for this application. Five different soils were considered in this study. Our results showed significant particle-size dependent behavior for surfactant sorption and pesticide immobilization in the presence of the sorbed cationic surfactant. The cation exchange capacity (CEC) of the bulk soils and their size fractions (clay, silt, and sand fractions) determined BC sorption capacity. In the absence of BC the sand fractions were the least effective sorbent for the pesticides compared with silts and clays. However, at relatively low BC mass sorbed (<10,000mg/kg) to any of the soil fractions, well below sorption saturation, the sand fractions became more effective sorbents for either pesticide than the clay and silt fractions. The pesticide partitioning coefficient onto sorbed BC (K(ss)) was not constant. Particle CEC, availability of CEC sites for sorption of the cationic surfactant, and the amount of the BC sorbed determined the phase of K(ss). The maximum K(ss) occurred before BC saturation sorption capacity was reached and at different % CEC occupancy for the different size fractions. For the clay fractions, the maximum K(ss) occurred at lower % CEC occupancy ( approximately 30-40%) than for the silt and sand fractions. The maximal K(ss) for the sand fractions occurred at the highest % CEC occupancy among all fractions ( approximately 50-60%). These findings suggest that for an in situ surfactant-enhanced sorption zone it may be better to operate well below the saturation sorption of the cationic surfactant. This would enhance sorption of the HOCs onto the immobile fractions (silt and sand fractions) rather than the potentially mobile clay fractions.     

Surfactant enhanced remediation of subsurface chromium contamination

The objective of this research was identification of optimal surfactant systems for remediating chromate-contaminated subsurface environments. Batch and column studies were conducted utilizing chromium contaminated soil obtained from the U.S. Coast Guard Support Center, Elizabeth City, N.C. Results of the batch studies demonstrated that surfactants, when used alone, were able to enhance the extraction of chromate 2.0–2.5 times greater than water. When a complexing agent, diphenyl carbazide, was solubilized by aqueous micelles the system was able to enhance the chromate elution by 9.3 to 12.0 times greater than water (or 3.7–5.7 times greater than surfactant without the complexing agent). Column studies showed that when surfactants are used along with the complexing agent, 213% of Cr(VI) can be removed relative to D.I. water in less than 20 pore volumes, whereas D.I. water took 35 pore volumes to reach the baseline removal. The economics of surfactant enhanced subsurface remediation will be affected by surfactant losses (e.g. precipitation and sorption); batch and column studies were conducted to evaluate the losses of surfactants due to such phenomena. Results of these laboratory studies demonstrated that the surfactant system containing Dowfax 8390 and diphenyl carbazide was most effective in remediation of the chromium contaminated soil.     

有机物污染场地浅层异位固化稳定化试验研究

以某化工企业有机物污染浅层土为对象,应用4种固化药剂进行固化稳定化修复对比研究。通过现场异位固化稳定化结合室内无侧限抗压强度、毒性浸出等试验,讨论了固化剂组份、龄期对其物理力学性质和浸出特性影响,并对比分析了其固化稳定化效果及机理。结果表明:随着养护时间增长,修复土p H值、毒性浸出溶液的有机物浓度降低,其中pH值由0 d的12.76~13.11降至28 d时的12.11~12.69,而有机物浸出浓度14d降幅在65%~100%间;干密度及无侧限抗压强度qu则稳步提高,其中干密度28 d增幅达14.4%~23.2%,而强度最大增加到122 k Pa。固化剂3和4修复污染土干密度较大,28 d密度超过1.37 g/cm~3,密实作用明显;添加固化剂会立即显著增大污染土pH值,其中固化剂4(电石渣+凹凸棒土)各龄期pH值明显大于其他剂型;各固化剂对不同有机污染物的稳定效果有所差别,但均能有效固化稳定化苯胺、2-氯酚、萘、苯、甲苯、邻-二甲苯;就污染物总体固化稳定化效果而言,含活性炭组分的固化剂1效果最为突出,其总稳定率接近100%;含水泥组分的固化剂2,3对土体的增强作用较好,其28 d强度可达109 k Pa以上。相较其他药剂,固化剂4在成本、能耗及污染物排放方面表现最优,且能较好满足场地修复对有机物稳定率及强度的要求,综合效果最佳,为优选的最佳剂型。     

Transport of two naphthoic acids and salicylic acid in soil: experimental study and empirical modeling

In contrast to the parent compounds, the mechanisms responsible for the transport of natural metabolites of polycyclic aromatic hydrocarbons (PAH) in contaminated soils have been scarcely investigated. In this study, the sorption of three aromatic acids (1-naphthoic acid (NA), 1-hydroxy-2-naphthoic acid (HNA) and salicylic acid (SA)) was examined on soil, in a batch equilibrium single-system, with varying pH and acid concentrations. Continuous flow experiments were also carried out under steady-state water flow. The adsorption behavior of naphthoic and benzoic acids was affected by ligand functionality and molecular structure. All modeling options (equilibrium, chemical nonequilibrium, i.e. chemical kinetics, physical nonequilibrium, i.e. surface sites in the immobile water fraction, and both chemical and physical nonequilibrium) were tested in order to describe the breakthrough behavior of organic compounds in homogeneously packed soil columns. Tracer experiments showed a small fractionation of flow into mobile and immobile compartments, and the related hydrodynamic parameters were used for the modeling of reactive transport. In all cases, the isotherm parameters obtained from column tests differed from those derived from the batch experiments. The best accurate modeling was obtained considering nonequilibrium for the three organic compounds. Both chemical and physical nonequilibrium led to appropriate modeling for HNA and NA, while chemical nonequilibrium was the sole option for SA. SA sorption occurs mainly in mobile water and results from the concomitancy of instantaneous and kinetically limited sites. For all organic compounds, retention is contact condition dependent and differs between batch and column experiments. Such results show that preponderant mechanisms are solute dependent and kinetically limited, which has important implications for the fate and transport of carboxylated aromatic compounds in contaminated soils.     

Evaluation of an elevated non-ionic surfactant critical micelle concentration in a soil/aqueous system

In this study, an elevated non-ionic surfactant critical micelle concentration (CMC) in a soil/aqueous system was examined. Experimental measurements have been made of surfactant solubilization of polycyclic aromatic hydrocarbons (PAH) (i.e. fluoranthene and pyrene) in a 5-month aged PAH contaminated soil, as well as surfactant sorption onto soil. Fluoranthene and pyrene in the soil/aqueous system in the presence of three non-ionic surfactants (i.e. Tween 80, Triton X-100 and Brij 35) were extracted using dichloramethane and analyzed using GC-MS. Maximum sorption of non-ionic surfactant onto soil was evaluated using a surface tension technique. It was observed that PAH solubilization is proportional to surfactant dose after the elevated CMC, termed as the effective CMC (CMCeff), is achieved. The values of surfactant CMCeff assessed by the surface tension technique were found to be similar to those determined from surfactant PAH solublization, thereby proving the research hypothesis that surfactant sorption is the cause for the elevation of surfactant CMC in a soil/aqueous system.     

Effects of alcohols, anionic and nonionic surfactants on the reduction of PCE and TCE by zero-valent iron

The effects of surfactants, sodium dodecyl sulfate (SDS) and Triton X-a00 (TX), and alcohols (methanol, ethanol, and propanol) on the dehalogenation of TCE and PCE by zero-valent iron were examined. Surface concentrations of PCE and TCE on the iron were dependent on aqueous surfactant concentrations. At concentrations above the CMC, sorbed halocarbon concentrations declined and concentrations associated with solution phase micelles increased. The anionic surfactant SDS ([SDS] < CMC) did not affect reduction rates, until the CMC was exceeded after which reactivity decreased, possibly due to sequestering of the TCE and PCE in mobile micelles. The nonionic TX showed a mixed effect on reactivity, increasing the PCE reduction rate, but not affecting TCE removal. Production of TCE from PCE increased in the presence of TX. Similar experiments showed that methanol, ethanol, and propanol inhibited reduction of TCE and PCE by metallic iron. Zero-valent iron may be useful in recycling soil washing effluents contaminated with TCE and PCE.     

Simulated formation and flow of microemulsions during surfactant flushing of contaminated soil.

Contamination of groundwater resources by non-aqueous phase liquids (NAPLs) has become an issue of increasing environmental concern. This study investigated the formation and flow of microemulsions during surfactant flushing of NAPL-contaminated soil using the finite difference model UTCHEM, which was verified with our laboratory experimental data. Simulation results showed that surfactant flushing of NAPLs (i.e., trichloroethylene and tetrachloroethylene) from the contaminated soils was an emulsion-driven process. Formation of NAPL-in-water microemulsions facilitated the removal of NAPLs from contaminated soils. Changes in soil saturation pressure were used to monitor the mobilization and entrapment of NAPLs during surface flushing process. In general, more NAPLs were clogged in soil pores when the soil saturation pressure increased. Effects of aquifer salinity on the formation and flow of NAPL-in-water microemulsions were significant. This study suggests that the formation and flow of NAPL-in-water microemulsions through aquifer systems are complex physical-chemical phenomena that are critical to effective surfactant flushing of contaminated soils.     

Modeling the two stages of surfactant-aided soil washing

This paper provides new insights into modelling the distribution of hydrophobic compounds between soil and water phases in the presence of nonionic surfactant micelles. Experimental measurements were made of various systems comprising a non-ionic surfactant, five soils of different fractional organic carbon contents, and a hydrophobic (disperse) dye. Soil-washing performance was quantified using reciprocal surfactant-soil solubilization coefficients (1/Kd). Two stages of partitioning were identified. In stage 1, the dye concentration increased slightly with increasing surfactant dose until surfactant monomers saturated the bulk solution at the critical micelle concentration (cmc). The washing performance was 1:1 proportional to the surfactant monomer concentration. Most of the surfactant in this stage is sorbed. In stage 2, above the cmc, soil-washing performance increased linearly with increasing available surfactant micelles in the bulk solution. Reciprocal surfactant-soil solubilization coefficients (1/Kd), octanol-water partition coefficients (Kow), fractional organic carbon content of the soil (foc), and surfactant concentration were correlated for each stage in the soil-washing process using two simple equations.     

Contaminant interactions with geosorbent organic matter: insights drawn from polymer sciences

This is a state-of-science review of interrelationships between the sorption/desorption behaviors and chemical structures of natural organic matter (NOM) matrices associated with soils, sediments and aquifer materials. It identifies similarities between these behavior-property interrelationships for natural geosorbents and those for synthetic organic polymers. It then invokes, with appropriate restrictions and modifications, several structure-function relationships that have been developed for synthetic polymers to explain the behavior of NOM matrices with respect to the sorption and desorption of hydrophobic organic contaminants (HOCs). Previous research regarding HOC sorption and desorption by different types of NOM and by synthetic polymers is summarized, and research requirements for further refinement of the NOM-polymer analogy are examined. The discussion focuses on structural and compositional heterogeneities that exist at the particle and aggregate scale, a scale at which homogeneity is commonly, and often improperly, assumed in the development of contaminant fate and transport models.     

Influence of ionic surfactants on the flocculation and sorption of palladium and mercury in the aquatic environment

The influence of sub-micellar concentrations of an anionic surfactant (sodium dodecyl sulphate; SDS) and a cationic surfactant (hexadecyl trimethylammonium bromide; HDTMA) on the aquatic behaviour of the strongly complexing metals, Pd(II) and Hg(II), has been investigated. In river water, flocculation of organic complexes of metal was suppressed by SDS but accentuated by HDTMA, effects that are consistent with electrostatic and hydrophobic interactions between ionic surfactants and natural polyelectrolytes. In sea water, flocculation of metal complexes was enhanced by both surfactants because of the shielding and salting effects of inorganic ions on these interactions. Particle surface modification engendered by sorbed surfactant strongly influenced the sorption of Pd and Hg to estuarine particles. Thus, hydrophobically bound SDS enhances the negative charge at the particle surface and favours specific sorption of metal, while specifically sorbed HDTMA enhances the solvency of the particle surface, favouring non-specific sorption of metal complexes. Given the relatively short environmental half-life of SDS, its impacts on strongly complexing metals are predicted to be localised. However, greater stability of HDTMA suggests that its effects on such metals, including enhanced flocculation and sorption, are likely to be more pervasive.