Multi residue analysis of pesticides in wheat and khat collected from different regions of Ethiopia

The present study investigated the presence and level of pesticide residues in wheat and khat samples collected from various localities of Ethiopia. The khat samples from Galemso and Aseno had p,p'-DDT concentrations ranged from 141.2 to 973.0 μg/Kg and 194.3-999.0 μg/Kg, respectively. Diazinon was detected in all the khat samples from BadaBuna (173.9-686.9 μg/Kg) but not in any of the samples from Galemso and Aseno. Diazinon was detected in all the wheat samples obtained from both Arsi and Bale (125.8 and 125.6 μg/Kg, respectively) and aldrin levels in these samples were below the quantification limit. Khat may be a contributing factor in the pathological diseases found among khat users.

Elimination of persistent organic pollutants from fish oil with solid adsorbents

Fish oils are one of the main sources of ω-3 fatty acids in animal and human diet. However, they can contain high concentrations of persistent organic pollutants due to their lipophilic properties. The aim of this study is the reduction of persistent organic pollutants in fish oil using silicon-based and carbon-based solid adsorbents. A wide screening study with different commercially available adsorbents was carried out, in order to determine their capacity of pollutant removal from fish oil. Moreover, adsorption conditions were evaluated and optimized with using an experimental design and adjustment of the experimental results to response surfaces, obtaining removals rates of more than 99% of PCDD/Fs, 81% of dioxin-like PCBs, 70% of HCB, 41% of DDTs, 16% of marker PCBs and 10% of PBDEs. Finally, fish oil fatty acids were analyzed before and after the treatment with solid adsorbents, confirming that it did not affect its nutritive properties.

Role of non-ionic surfactants and plant oils on the solubilization of organochlorine pesticides by oil-in-water microemulsions

Screening low-cost, high efficacy and environmentally safe surface active agents is critical for achieving successful surfactant-enhanced remediation (SER) of soil contaminated with hydrophobic organic compounds. This study reports the solubilization of organochlorine pesticides (DDT or gamma-HCH) in oil-in-water (Winsor I) microemulsions (microE) composed of non-ionic surfactant (Tween 80 or Triton X-100), plant oil (linseed oil or soybean oil), and the cosurfactant (1-pentanol). Results show that the cosurfactant to surfactant ratio (C/S ratio, w/w) is the major factor influencing the microemulsion formation, and C/S ratios of 1:3 and 1:6 are superior to 1:1 for microemulsion formation. 66.9-95.6% and 51.9-80.9% of DDT solubilization enhancements were achieved by microemulsions based respectively on Tween 80 or Triton X-100 as compared to their respective surfactant solution alone, indicating the higher solubilizing capacities of microemulsion systems. The solubilization of gamma-HCH also increased by 40.6-57.5% in microemulsion formed with Tween 80 and 43.0-65.8% in microemulsion formed with Triton X-100, compared with that in corresponding surfactant solutions only. Further studies revealed that both cosurfactant content and oil content could influence the solubilizing capacity of microemulsions system, and higher solubilizing capacity could be obtained when more cosurfactant or oil were emulsified in microemulsion system. Between the two, oil content is more influential than cosurfactant content. The present results affirm the effective role of microemulsions formed with Tween 80 and Triton X-100 in enhancing the solubilization of DDT and gamma-HCH which would facilitate remediation of soils contaminated with these compounds.

Residues and chiral signatures of organochlorine pesticides in sediments from Xiangshan Bay, East China Sea

Residual levels and enantiomeric signatures of hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs) in surface sediments from Xiangshan Bay, East China Sea were investigated. The concentrations of ∑HCHs (sums of α-, β-, γ-, and δ-HCH) and ∑DDTs (sums of p, p'-DDT, p, p'-DDD,p, p'-DDE, o, p'-DDT, and o, p'-DDD) ranged from 0.14 to 0.67 ng g⁻¹ and 0.61 to 22.38 ng g⁻¹, respectively. A slight potential health risk to the organism was then indicated for the residual levels of DDTs according to the ERL/ERM guidelines. Moreover, the predominant β-HCH implied that the technical HCH contamination was mainly due to the historical usage. But the high ratio of DDT/∑DDTs depicted a cocktail input pattern of fresh and weathered DDTs. The enantiomeric fractions (EFs) of α-HCH, o, p'-DDT, and o, p'-DDD were also determined. The degradation of α-HCH was enantioselective in all sediments samples, resulting in an enrichment of (-)-enantiomers. However, the racemic residues of o, p'-DDT and o, p'-DDD were observed in all sediments samples.

A novel metabolic pathway for biodegradation of DDT by the white rot fungi, Phlebia lindtneri and Phlebia brevispora

1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane (DDT) was used as the substrate for a degradation experiment with the white rot fungi Phlebia lindtneri GB-1027 and Phlebia brevispora TMIC34596, which are capable of degrading polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated biphenyls (PCBs). Pure culture of P. lindtneri and P. brevispora with DDT (25 μmol l(-1)) showed that 70 and 30% of DDT, respectively, disappeared in a low-nitrogen medium after a 21-day incubation period. The metabolites were analyzed using gas chromatography/mass spectrometry (GC/MS). Both fungi metabolized DDT to 1,1-dichloro-2,2-bis(4-chlorophenyl)ethane (DDD), 2,2-bis(4-chlorophenyl)acetic acid (DDA) and 4,4-dichlorobenzophenone (DBP). Additionally, DDD was converted to DDA and DBP. DDA was converted to DBP and 4,4-dichlorobenzhydrol (DBH). While DBP was treated as substrate, DBH and three hydroxylated metabolites, including one dihydroxylated DBP and two different isomers of monohydroxylated DBH, were produced from fungal cultures, and these hydroxylated metabolites were efficiently inhibited by the addition of a cytochrome P-450 inhibitor, piperonyl butoxide. These results indicate that the white rot fungi P. lindtneri and P. brevispora can degrade DBP/DBH through hydroxylation of the aromatic ring. Moreover, the single-ring aromatic metabolites, such as 4-chlorobenzaldehyde, 4-chlorobenzyl alcohol and 4-chlorobenzoic acid, were found as metabolic products of all substrate, demonstrating that the cleavage reaction of the aliphatic-aryl carbon bond occurs in the biodegradation process of DDT by white rot fungi.

Effect of Aeromonas hydrophila on reductive dechlorination of DDTs by zero-valent iron

This study presents a reductive transformation method that combines zerovalent iron (ZVI) and Aeromonas hydrophila HS01 with iron oxide reduction property to degrade DDT (1,1-trichloro-2,2-bis(4-chlorophenyl)ethane) under anoxic conditions. The results suggest that HS01 has weak capability in terms of reducing DDT to DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane) and nearly failed to reduce DDD or its transformed intermediates. The coexistence of ZVI and HS01 results in a slight enhancement of DDT degradation compared with the ZVI system alone. The reduction of intermediates by ZVI, however, can be obviously accelerated in the presence of HS01, and the addition of anthraquinone-2,6-disulfonic disodium salt (AQDS) can accelerate the transformation rates further, especially for intermediate reduction. The analysis of the amount and electrochemical properties of Fe(III)/Fe(II) indicates that the presence of HS01 with or without AQDS is beneficial to the reduction of Fe(III) to Fe(II), resulting in the removal of passivating ferric precipitates on the ZVI surface. A mechanism and pathway that clarify the roles of ZVI, HS01, and AQDS in the ZVI + HS01 + AQDS system for DDT transformation are proposed. The quick removal of surface ferric precipitates is thought to be the reason for the enhancement of the transformation of DDT and its intermediates.

Using the o,p'-DDT/p,p'-DDT ratio to identify DDT sources in China

Dichlorodiphenyltrichloroethane (DDT) has been banned in China for decades, and yet high DDT concentrations are still being detected in the Chinese environment. This might be at least partly due to the current use of dicofol formulation, which contains DDT as an impurity. In this study, a method based on the ratios of two DDT isomers, o,p'-DDT and p,p'-DDT, was established and used to estimate the relative contributions of dicofol formulation and those of technical DDT to overall environmental DDT. Based on this method and field data from the literature, we calculated that dicofol formulation contributed >72% of atmospheric DDT in 2004 in the Taihu Lake region, China, and this value was >84% in summer when dicofol was applied for agricultural purposes. Sediment and soil, however, contained mostly residual DDT from the historical use of technical DDT. In most other regions of China, we found that dicofol contributed to a significant fraction of DDT in air samples.

Spatial variability and temporal trends of HCH and DDT in soils around Beijing Guanting Reservoir, China

Spatial variability and temporal trends in concentrations of the organochlorine pesticides (OCPs), hexachlorocyclohexane (HCH) and dichlorodiphenyltrichloroethane (DDT), in surface soils around Beijing Guanting Reservoir (GTR) were studied in 2003 and 2007. Concentrations of the two OCPs in soils around GTR were generally less than reference values set by the Chinese government for the protection of agricultural production and human health. Among the OCPs, β-HCH and p, p'-DDE were the two predominant compounds. This result indicates that the HCH and DDT residues in soils were primarily from historical use. Based on kriging, a spatial distribution of HCH and DDT around the GTR was observed. Spatial variability indicated how HCH and DDT had been applied and been distributed in the past. Between 2003 and 2007, concentrations of HCH and DDT decreased more rapidly in orchard soils than those in fallow soils.

Structural diversity of organochlorine compounds in groundwater affected by an industrial point source

Groundwater samples contaminated by an industrial point source were analysed in order to reveal the structural diversity of halogenated organic contaminants. Particular focus was laid on the metabolites and derivatives related to the pesticides DDT (2,2-bis(chlorophenyl)-1,1,1-trichlorethane) and lindane (γ-hexachlorocyclohexane). Additionally, a wide range of chlorinated and brominated xenobiotics were identified. These results represent a high degree of contamination with organochlorine compounds illustrating a considerable structural diversity in groundwater in the vicinity of the industrial plant. The polar DDT-metabolite DDA (2,2-bis(chlorophenyl)acetic acid), which has been neglected in water studies widely, represents the main DDT metabolite analysed in the water samples. Besides DDA, some unknown substances with structural relation to DDA and DDT were detected and identified, in detail 2,2-bis(4-chlorophenyl)acetic acid N-methyl amide (DDAMA) and 2,2-bis(4-chlorophenyl)acetic acid n-butyl ester (DDABE). As an overall implication of this study it has to be demanded that analysis of industrially affected ground waters have to be based on screening analysis for a comprehensive view on the state of pollution.

Assessing pesticide leaching and desorption in soils with different agricultural activities from Argentina (Pampa and Patagonia)

Pesticide distribution in the soil profile depends on soil and pesticide properties as well as on the composition of irrigation water. Water containing surfactants, acids or solvents, may alter pesticide desorption from soil. The distribution of organochlorine pesticides (OCPs) in two Argentinean agricultural areas, Pampa and Patagonia, was evaluated. Furthermore, pesticide desorption from aged and freshly spiked soils was performed by the batch technique, using solutions of sodium oxalate and citrate, dissolved organic carbon (DOC), wastewater and surfactants. Patagonian soil showed the highest OCP levels (46.5-38.1 μg g(-1) OC) from 0 to 30 cm depth and the predominance of p,p'-DDE residues reflected an extensive and past use of DDT. Pampean soil with lower levels (0.039-0.07 μg g(-1) OC) was mainly polluted by the currently used insecticide endosulfan. Sodium citrate and oxalate, at levels usually exuded by plant roots, effectively enhanced desorption of p,p'-DDT, p,p'-DDE and α-cypermethrin, while no effects were observed for α-endosulfan and endosulfan sulfate. The non-ionic surfactant Tween 80 behaved similarly to the acids, whereas the anionic sodium dodecyl sulfate enhanced desorption of all pesticides. Increased desorption of the hydrophobic pesticides also occurred when DOC from humic acids but not from sewage sludge or wastewater were used. Soil profile distribution of pesticides was in accordance with results from desorption studies. Data suggest pesticide leaching in Pampean and Patagonian soils, with risk of endosulfan to reach groundwater and that some organic components of wastewaters may enhance the solubilisation and leaching of recalcitrant compounds such as p,p'-DDT and p,p'-DDE.

Tenax TA extraction to assess the bioavailability of DDTs in cotton field soils

The rapid-desorbing fraction plays an important role in the bioavailability of organic pollutants in soil. In the present study, DDT's desorption from the cotton field soils was investigated by Tenax extraction. The results of the Tenax consecutive extraction (400 h) indicated that the rate constants were in the order of 10(-1), 10(-2), and 10(-4) h(-1) for the rapid, slow and very-slow desorption, respectively. The rapid-desorbing fraction was about 0.3 times the total soil DDTs, and about 2 times the Tenax 6 h-extracted fraction (single-point extraction). The rapid-desorbing fraction correlated well with the 6 h-extracted fraction (p<0.05), implying the feasibility of measuring the rapid-desorbing fraction with Tenax 6 h-extration. The strong correlation with the carrot accumulation suggested that Tenax 6 h-extrated fraction could serve as a good predictor of DDT's bioavailability to carrot roots. Risk assessment demonstrated that when based on the rapid-desorbing concentration and 6 h-extracted concentration, about 60.7% and 17.9% of the soil samples were moderately polluted, however, up to 78.5% were moderately polluted when based on the total soil DDTs concentration. The risk assessment might be more representative when based on Tenax extraction because of the strong correlation with the carrot accumulation. Our results provided implications for site risk assessment and cleanup strategies.

Residues and enantiomeric profiling of organochlorine pesticides in sediments from Yueqing Bay and Sanmen Bay, East China Sea

The spatial distributions and chiral signals of organochlorine pesticides (OCPs) in surface sediments from Yueqing Bay and Sanmen Bay, East China Sea were investigated. The total OCPs concentrations ranged from 2.11 to 18.15 ng g(-1) dry weight for all the sampling stations. HCHs and DDTs were dominant, although trans-chlorodane/cis-chlorodane were also observed in some sites. The predominant beta-HCH and the alpha-HCH/gamma-HCH ratios indicated that the residues of HCHs in these places mainly originated from the historical usage of technical HCH. However, the ratios of (DDE+DDD)/DDT reflected a cocktail input pattern of fresh and weathered DDTs. Based on ERL/ERM guidelines, p,p'-DDT and SigmaDDT posed a small risk to the bottom-dwelling consumers, while p,p'-DDD, p,p'-DDE and Sigmachlorodane were not found at concentrations expected to adversely affect sediment biota. Enantiomeric analysis showed that the degradation of chiral OCPs was enantioselective, resulting in enrichment of (-)-enantiomers for alpha-HCH, o,p'-DDT and o,p'-DDD in all samples. These results implied that the sediment quality guidelines of chiral OCPs should be reassessed using concentrations of their individual enantiomers.

Determination of organochlorine pesticides residues in human adipose tissue, data from Cukurova, Turkey

We determined and compared the levels of BHC and DDT isomers and metabolites and HCB in the adipose tissue of 82 cases of autopsies performed at the Morgue Department of Adana Branch of the Council of Forensic Medicine. The relationships between the age, gender, and body mass indexes of cases, and the accumulation of OCs residues were also investigated. Detectable concentrations of p,p'-DDE were found in 100% of adipose tissue samples. Concentrations of OCs in female adipose tissues were significantly higher than male adipose tissues (p < 0.05). The presented work is highly significant, being the first study pointing out the chronic exposure to organochlorine pesticides in Cukurova region.

Trace organic contaminants and their sources in surface sediments of Santa Monica Bay, California, USA

Spatial distribution of selected contaminants in the surface sediments of Santa Monica Bay (SMB), California was investigated. Sediments were analyzed for DDTs (DDT and metabolites), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), linear alkylbenzenes (LABs) and coprostanol. Effluent samples from the Hyperion Treatment Plant (HTP), which discharges treated municipal wastewater effluents into SMB, were also analyzed. The inter-correlation in the distribution trends of contaminants was examined. The concentrations of contaminants were interpolated in a geographic information system to visualize their spatial distribution in the Bay. Inventories of the contaminants were also estimated. The concentrations of coprostanol, LABs and PCBs are very high only in the vicinity of the sewage outfall whereas PAHs and DDTs occur widespread in the Bay. The poor correlation of DDTs with LABs, PAHs or coprostanol content confirms the historic origin of DDTs and their absence in the contemporary wastewaters. Moderate correlation of DDTs with PCBs implies historic deposits as a major origin of PCBs. There are hot spots of DDTs at water depths of 60 and 100m and the inventory of DDTs in Bay sediments is insignificant compared to that estimated in the Palos Verdes Shelf which extends from the southern edge of Redondo Canyon around Palos Verdes Peninsula. The concentration of toxic contaminants was examined according to published sediment quality guidelines. About 20 stations contain p, p'-DDE and/or total DDTs above ERM and, PCBs between ERL and ERM indicating potential for adverse biological effects.

Indirect determination of dichlorodiphenyltrichloroethane (DDT) after dechlorination with magnesium/palladium bimetallic particles and potentiometric measurements

Determination of chlorine ions of pesticides was performed after dechlorination reaction using a palladium/magnesium system. Chlorine ions were quantified by potentiometry with ion-specific electrode. Rates of dechlorination of 100 microg of DDT as a function of reaction time and percent (wt/wt) of palladium deposited on the magnesium particles were determined. The best reaction conditions to DDT dechlorination were achieved with an acetone/water (1:1) solution and DDT reaction with a 0.27% (wt/wt) palladium/magnesium bimetallic system at room temperature for 10 min. The detection limit was of 0.24 microg/mL. This low cost method showed an efficiency of 92% in determining chlorine ions derived from DDT, it is fast, requiring no specialized laboratory equipment.

Synergistic genotoxicity caused by low concentration of titanium dioxide nanoparticles and p,p'-DDT in human hepatocytes

The use of titanium dioxide nanoparticles (nano-TiO(2)) for the degradation of dichlorodiphenyltrichloroethane (p,p'-DDT) increases the risk of exposure to trace nano-TiO(2) and p,p'-DDT mixtures. The interaction of p,p'-DDT and nano-TiO(2) at low concentrations may alter toxic response relative to nano-TiO(2) or p,p'-DDT alone. In this work, the combined genotoxicity of trace nano-TiO(2) and p,p'-DDT on human embryo L-02 hepatocytes without photoactivation was studied. Nano-TiO(2) (0.1 g/L) was mixed with 0.01-1 mmol/L p,p'-DDT to determine adsorption isotherms. L-02 cells were exposed to different levels of p,p'-DDT (0, 0.001, 0.01, and 0.1 mumol/L) and nano-TiO(2) (0, 0.01, 0.1, and 1 microg/mL) respectively. The adsorption of p,p'-DDT by nano-TiO(2) was approximately 0.3 mmol/g. Cell viability, apoptosis, and DNA double strand breaks were similar among all test groups. Nano-TiO(2) alone (0.01-1 microg/mL) increased the levels of oxidative stress and oxidative DNA adducts (8-OHdG), but it did not induce DNA breaks or chromosome damage. Addition of trace nano-TiO(2) with trace p,p'-DDT synergistically enhanced genotoxicity via increasing oxidative stress, oxidative DNA adducts, DNA breaks, and chromosome damage in L-02 cells. Low concentrations of nano-TiO(2) and p,p'-DDT increased oxidativestress by reactive oxygen species (ROS) formation and lipid oxidation. Oxidative stress is a major pathway for DNA and chromosome damage. Dose-dependent synergistic genotoxicity induced by combined exposure of trace p,p'-DDT and nano-TiO(2) suggests a potential environmental risk of nano-TiO(2) assisted photocatalysis.

Analysis of DDT isomers with enzyme-linked immunosorbent assay and optical immunosensor based on rat monoclonal antibodies as biological recognition elements

New rat monoclonal antibodies (mAbs) for DDT [1,1,1 -trichloro-2,2-bis (4-chlorophenyl) ethane], namely DDT 7C12, DDT 1C1, and DDT 1B2, were developed, characterized, and applied in ELISA both in coating antigen and in enzyme-tracer format. The latter used horseradish peroxidase (HRP) or glucose oxidase as enzymes. The lowest concentration of p,p'-DDT was determined with mAb DDT 7C12 and DDT-hapten HRP, with a test midpoint (IC50) of 0.5 +/- 0.2 microg/L (n=10) in 40 mM PBS (phosphate-buffered saline). The mouse anti-rat immunoglobulin lambda-light chain mAb LA1B12 was used as capture mAb. The best IC50 for o,p'-DDT in 40 mM PBS was 1.0 +/- 0.3 microg/L (n=12) and was obtained with mAb DDT 1C1 and DDT-hapten HRP, whereas mAb DDT 1B2 was very selective for p,p'-DDT with an IC50 of 4.2 + 1.6 microg/L (in 40 mM PBS, n=9). An optical immunosensor was optimized and applied for the analysis of DDT (or DDT equivalents). This immunosensor consists of a bench-top optical readout device and disposable sensor chips, which include the fluidic system. Evanescent field excitation and emission of the fluorophore Oyster-645 was used. An IC50 for p,p'-DDT [in 5% (v/v) isopropanol in 40 mM PBS] of 4 microg/L was obtained using DDT 7C12-Oyster-645. ELISA and immunosensor were used for the analysis of p,p'-DDT in unspiked and spiked surface water samples. Within the working ranges of these immunotechniques, recoveries ranged from 80 to 120%.

Using shell-tunable mesoporous Fe3O4@HMS and magnetic separation to remove DDT from aqueous media

1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) is of concern in water treatment because of its persistence and health effects. A new concept is proposed to synthesize hexagonal mesoporous silica (HMS) with magnetic functionalization for DDT removal from aqueous media. Fe(3)O(4) nanocrystals were synthesized by a low-temperature solvothermal process, and then encapsulated in mesoporous silica through a packing approach, forming core-shell structured Fe(3)O(4)@HMS microspheres. The synthesized materials were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nitrogen adsorption-desorption techniques. The results indicate that the silica shell conserves mesoporous structure after the removal of surfactant templates. Different from previous studies, the thickness, pore volume and surface area of silica shell can be controlled by adjusting the reaction condition. These Fe(3)O(4)@HMS materials show high adsorption capacity and fast adsorption rate for DDT. Because of the useful magnetic property and unique mesoporous structure, the synthesized materials provide a fast, convenient and highly efficient means to remove DDT from aqueous media.

Application of high frequency ultrasound in the destruction of DDT in contaminated sand and water

High frequency ultrasound, as an alternative to high cost incineration, has been investigated to remediate DDT from sand and soil slurries. In this study, low power high frequency ultrasound (1.6 MHz; 150 W/L), with operating costs much lower than low frequency ultrasound, has been used to remediate DDT in liquid solution and in sand slurries. At 1.6 MHz, the wavelength, cycle time, bubble size and bubble life time are much smaller and the number of bubbles per litre is much larger than at frequencies below 50 kHz. These large differences affect the effective mass transfer to the bubbles and subsequent energy release, hydrolysis of water and degradation mechanism. Based on DDT measurement, using high frequency ultrasound, 90% of 8 mg/L of DDT from liquid solution was destroyed in 90 min. Removal efficiency from 32.6 mg/L of DDT in a 40 wt.% sand slurry was 22% in 90 min. Other slurry and DDT combinations are reported. Incremental chloride measurements indicated that combination of ultrasound and iron powder helps to increase the remediation rate of DDT from sand slurry, e.g. 46% cf. 32% for a 20 wt.% slurry. The results show that high frequency ultrasound is effective in degrading the non-polar pollutant DDT dispersed in water and in sand slurry. In practice, due to intensity limitations in currently available equipment and higher attenuation of energy, high frequency ultrasound has a low volume coverage and would require circulation of the slurry past the sonotrode, multiple sonotrodes, larger sonotrode area and lower slurry densities may still be required.

Sorption of dichlorodiphenyltrichloroethane (DDT) and its metabolites by activated carbon in clean water and sediment slurries

Polyethylene-water partitioning coefficients (K(PE)) and mass transfer coefficients (k(PE)) for the ortho and para isomers of the organochlorine pesticide dichlorodiphenyltrichloroethane (DDT) and its metabolites dichlorodiphenyldichloroethane (DDD), dichlorodiphenyldichloroethylene (DDE) and, dichlorodiphenylmonochloroethylene (DDMU) were measured. These data were used to derive activated carbon (AC) sorption isotherms in clean water in the sub-nanogram per litre free aqueous concentration range for a virgin and a regenerated AC. The sorption strength of AC for DDT and its metabolites was very high and logarithmic values of the AC-water partitioning coefficients, logK(AC), ranged from 8.47 to 9.26. A numerical mass transfer model was calibrated with this data to interpret previously reported reductions in DDT uptake by semipermeable membrane devices after AC amendment of sediment from Lauritzen Channel, California, USA. The activated carbon-water partitioning coefficient values (K(AC)) measured in clean water systems appear to overestimate the AC sorption capacity in sediment up to a factor 32 for DDT and its metabolites at long contact time with fine-sized AC. Modelling results show decreased attenuation of the AC sorption capacity with increased sediment-AC contact time. We infer that increased resistance in mass transfer of DDTs to sorption sites in the microporous region likely caused by deposits of dissolved organic matter in the macro- and mesopores of AC appears to be the most relevant fouling mechanism. These results suggest that DDTs may diffuse through possible deposits of dissolved organic matter over time, implying that the effects of sediment on the sorption of DDTs by AC may be more kinetic than competitive.

The effects of Mn2+, Ni2+, Cu2+, Co2+ and Zn2+ ions on pesticide adsorption and mobility in a tropical soil

The adsorption behaviour of DDT in a tropical silt-clay soil from Kenya showed that addition of increasing metal ion concentrations from 10 to 100 microg/g of soil, reduced the % DDT adsorption in the soil by factors ranging from 6.1% to 15.4% depending on the type of metal ion. The inhibition of adsorption by metal ions was most observed in the lower ranges of metal ion concentrations in soil, i.e. up to 100 microg/g, beyond which additional increase in metal ion concentration did not result in any further increase in % adsorption. In the standard adsorption test procedure with different soil samples saturated with 100 microg/g of metal ions, the rate of adsorption of the pesticide varied with the type of metal ion and the equilibrium maximum adsorption of DDT was lowered with addition of metal ions, with the % adsorption of 78.4% (control) being reduced to lower values in the range of 67%-77.4%, depending on the type of metal ion. The data for control and samples fitted well into the Freundlich adsorption model and showed that addition of metal ions lowered the K(f) values compared with control. The results suggested that in the natural soil-water environments where there was no disturbance, aggregation of humic substances-pesticide molecule complexes altered the observed lowering of adsorption caused by inhibition by metal ions, and enhanced binding to solid soil phase occurred instead. In the leaching experiments, DDT was found to leach more extensively in soils with lower %OC content and the extent of leaching in soil columns saturated with metal ions at 100 microg/g was found to be inhibited compared with control and depended on the type of metal ion.