Effect of SDBS–Tween 80 mixed surfactants on the distribution of polycyclic aromatic hydrocarbons in soil–water system

Purpose

Enhancing desorption of hydrophobic organic contaminants from soils is a promising approach for the effective remediation of soils contaminated with organic compounds. The desorption efficiency of chemical reagent, such as surfactant, should be evaluated. In this study, the effect of mixed anionic–nonionic surfactants sodium dodecylbenzene sulfonate (SDBS)–Tween 80 on the distribution of polycyclic aromatic hydrocarbons in soil–water system was evaluated.

Materials and methods

Batch desorption experiments were employed to evaluate the distribution of polycyclic aromatic hydrocarbons (PAHs) and surfactants in soil–water system. PAHs and SDBS were determined by high-performance liquid chromatography, Tween 80 by spectrophotometry, and total organic carbon with a carbon analyzer.

Results and discussion

Sorption of PAHs to soil was increased at low surfactant concentration due to the effective partition phase on soil formed by sorbed surfactants. The mixture of anionic and nonionic surfactants decreased the sorption of surfactants to soil, increasing the effective surfactant concentration in solution and thus decreasing the sorption of PAHs on soil. Anionic–nonionic mixed surfactant showed better performance on desorption of PAHs from soil than single surfactant. The greatest desorption efficiency was achieved with low proportions of SDBS (SDBS/Tween80?=?1:9).

Conclusions

SDBS–Tween 80 mixed surfactant showed the highest desorption rate with low proportion of SDBS, which indicated that the addition of relative low amount of anionic surfactant could significantly promote the desorption efficiency of PAHs by nonionic surfactants. Results obtained from this study did provide useful information in surfactant-enhanced remediation of soil and subsurface contaminated by hydrophobic organic compounds.     

Effect of Application of Surfactants on Hydraulic Properties of Soils

This study explores the effect of surfactants, commonly found in detergents, on the hydraulic properties of soils. The soil properties examined included hydraulic conductivity, infiltration characteristics, and effective suction at the wetting front, capillary rise and soil penetrability. Two agricultural soils—a loam and a sandy loam, and three surfactants—one anionic surfactant (Sulphonic) and two non-ionic surfactants (Rexol and Rexonic), were used in the study. Changes in hydraulic properties with the application of surfactants were compared with properties obtained with deionised water (control). The results showed that Sulphonic, the anionic surfactant, had a significant effect on hydraulic properties of both soils. Applications of Sulphonic caused decreases in the capillary rise and penetrability, and an increase in the solid–liquid contact angle, shape factor and sorptivity. Except for a slight decrease in hydraulic conductivity resulting from the application of Rexol, the non-ionic surfactants did not reveal significant impact on the hydraulic characteristics of test soils.     

The nature and kinetics of organic matter release from soil by salt solutions

Laboratory experiments in soil columns were performed to study the influence of dissolved salts on the amount and composition of organic matter (OM) released from soil. Samples of two surface soils from former wastewater infiltration sites were leached with solutions containing dissolved salts (NaH₂PO₄, NaNO₃, CaCl₂) and by deionized water. The NaH₂PO₄ solution induced strongest release with 0.6% of soil organic carbon (C_org) with 700 ml for 100 g of soil, while CaCl₂ released the least, summing to 0.1–0.2% of C_org. The OM released was characterized by UV absorbance (aromaticity), ultrafiltration (molecular size distribution) and solid-phase extraction (polarity). The results suggest that CaCl₂ preferentially released readily soluble OM. For the other solutions we assume solubilization by enhanced electrostatic repulsion (water), sodium exchange (NaNO₃), and sodium exchange and calcium decomplexation and displacement of sorbed organic anions (NaH₂PO₄) to be the major mechanisms of release. In all experiments a phase of spontaneous desorption was observed, followed by a phase of steady-state desorption. Activation energies for steady-state release were estimated from kinetic investigations and suggest that the release is controlled by diffusion towards the phase boundary. These investigations emphasize the influence of dissolved salts on the nature and quantity of organic matter released from soil. The method presented seems able to characterize soil organic matter with respect to its availability and its mode of association with the soil matrix.     

Soil physical properties of subsoils contaminated with light nonaqueous phase liquids (lNAPLs)

At 4 sub‐sites of a loess‐derived site, contaminated with light nonaqueous phase liquids (lNAPLs), in Thuringia (Central Germany) undisturbed core samples were obtained from depths down to almost 9 m for determining water retention characteristics. Due to bulk densities of 1.46—1.87 g cm^—3 and high water retention (˜ 19—24 % b.v.) at 15000 hPa the resulting water retention curves are relatively flat. This finding means that there exists only a small portion of pore volume participating in water transport. As known from literature, however, contaminants like lNAPLs decrease the surface tension of soil water or soil solution, resulting in that the soil solution is retained in pores of smaller size at the same matric potential as compared to pure or lNAPL‐free water. This fact must have consequences for the water regime of soils or soil material affected by organic contaminants like lNAPLs because of the changed water retention properties control deep percolation as well as capillary rise.     

Fractionated extraction of polycyclic aromatic hydrocarbons (PAHs) from polluted soils: estimation of the PAH fraction degradable through bioremediation

Chemical extractions were tested to estimate the fraction of 15 polycyclic aromatic hydrocarbons (PAHs) in polluted soils degradable through bioremediation. Of 30 soil samples, differing in PAH concentrations, from 10 long‐term polluted sites, four were used in screening tests conducted with 14 different extractants. PAH extractability increased in the order salt solutions < surfactant solutions < organic solvents and organic solvent–water mixtures. Closest correlations (r = 0.73–0.91) and ratios of c. 1:1 of the extracted to the degradable PAH concentrations were found with aqueous solutions of two surfactants (Genapol UDD 88, Synperonic LF/RA 30). The PAH extractability by surfactant solutions increased with surfactant concentration (0–50 mg ml^?1) and with decreasing degree of PAH condensation (6–2 rings). Kinetic behaviour of PAH solubilization by surfactants resembled the kinetics of PAH degradation (r = 0.84–0.99). With repeated extractions, a finite fraction of PAHs was extracted, which is thought to be the mobile and mobilizable PAH fraction, the latter being controlled by back‐diffusion from intra‐ and inter‐particle sites. Experiments with all 30 soil samples and a priori testing with an additional 10 soil samples confirmed that single extractions with surfactant solutions (5 mg ml^?1; soil:solution ratio 1:20) were suitable for estimating the degradable fraction of PAHs in soil. With separate linear regression equations for PAHs grouped according to their degree of condensation, the extracted concentrations described the degradable concentrations with correlation coefficients (r) ranging from 0.62 to 0.80 (P < 0.001). PAH extractions with the surfactant solutions can be used to assess the prospects of bioremediation of PAH‐polluted sites.     

Pentachlorophenol interactions with soil

Pentachlorophenol (PCP) adsorption and desorption equilibrium was studied with two Menfro silt loam soils — upper horizon and lower horizon. For the adsorption studies the variables were: temperature (10 and 30 °C) and the amount of organic matter. The variables for the desorption studies were: temperature (10 and 30 °C), pH and the presence of an anionic and a cationic surfactant. The results from these studies confirmed the importance of soil organic matter for adsorption of PCP on the soils. The adsorption data at different temperatures indicated the physical nature of the adsorption process. The desorption data produced non-singularity and some PCP was irreversibly adsorbed onto the soil despite repeated washings. Increased pH increased the desorption of PCP from the soil. The anionic surfactant, sodium dodecylbenzene sulfonate (SDS) was able to desorb significant amounts of PCP from the soil at doses equal to critical micelle concentration (CMC). But, the nonionic surfactant, surfactant, Triton X-405 required a much higher dose (twice the CMC) to cause a significant desorption of PCP from the soil.     

Adsorption of Acrylonitrile on Some Soils and Minerals from Aqueous Solutions

Equilibrium and kinetic studies have been made on the adsorption of acrylonitrile(CH2=CHCN) on three soils and four minerals from aqueous solutions.It was shown that the organic matter was the major factor affecting the adsorption process in the soils.The conformity of the equilibrium data to linear type(one soil) and Langmuir type(two soils) isotherms indicated that different mechanisms were involved in the adsorption.This behavior appears bo be related to the hydrophobicity of soil organic matter due to their composition and E4/E6 ratio of humic acids.The adsorption kinetics were also different among the soils,indicating the difference in porosity of organic matter among the soils,and the kinetics strongly affected the adsorption capacity of soils for acrylonitrile.Acrylonitrile was slightly adsorbed from aqueous solutions on pyrophyllite with electrically neutral and hydrophobic nature,and practically not on montmorillonite and kaolinite saturated with Ca.However,much higher adsorption occurred on the zeolitized coal ash,probably caused by high organic carbon content(107g/kg).     

Adsorptionsisothermen als Regelgrößen beim Transport von Schwermetallen in Böden

Adsorption isotherms as regulators controlling heavy metal transport in soils The adsorption and desorption of Pb²⁺ and Cd²⁺ from equilibrium solutions with heavy metal contents up to 5000 μg/1 were determined in bulk experiments for soil samples from an acid Braunerde developed in loess loam, taken from the humic surface layer of the mineral soil (0–10 cm) and from the subsoil fairly free from organic matter (30–40 cm). Pb and Cd in solutions were determined by flameless atomic absorption spectroscopy. Pb was more strongly retained in the solid phase than Cd, and higher amounts of heavy metals were retained in the humic surface soil than in the subsoil free of organic matter. In the case of Pb adsorption/desorption showed slight hysteresis in the subsoil. The quantity/intensity (Q/I) relationships found in the experiments could be described by the Freundlich equation. The Q/I-relationships were substituted in the general transport equation. With a simulation model the transport of Pb and Cd through the soil with vertical water flow was calculated by the use of the Continuous Simulation Programming Language (CSMP). Two different cases were considered: a small, continuous increase in the heavy metal input of the soil surface, and a high, instantaneous peak input. Simulation of the transport and distribution mechanisms induced by the inputs over a period of 10 years showed strong retention of lead in the surface layer and consequently a strong damping of the concentration peak in the soil solution. In contrast, cadmium is distributed more quickly over the whole profile, yet the concentration peak in the solution phase is, too, damped considerably in the surface layer by temporary retention in the solid phase. The results of the simulation runs are in accordance with the situation in real soils where often strong accumulation of Pb is found in the top soil, while Cd is accumulated only slightly.     

Acetochlor as a soil pollutant

The agricultural use of pesticides leads to diffuse pollution whereby the various contaminants of the soil infiltrate into the groundwater reaching lakes and drinking water aquifers. Due to the extensive application of these chemicals, their leaching presents a high environmental risk. Since the adsorption coefficient (K) characterizes the soil / water partitioning [1] and is also representative for leaching, the first step in understanding of the environmental fate of a pollutant is to study its adsorption properties. Weak binding to the soil constituents (low K) leads to groundwater pollution, while a strong binding (high K) results in surface water pollution through the erosion of the soil. Acetochlor is a widely used herbicide all over the world. Similar to other organic pollutants, the environmental fate of this chemical is strongly related to its adsorption properties. Static adsorption equilibrium measurements were carried out at 25°C on different types of Hungarian soils (chernozem, brown forest and sandy soil) characterized by varying amounts of organic matter and pH values. Acetochlor solutions were prepared in the presence of nitrate and phosphate ions (0.1 mol/L sodium nitrate and 0.1 mol/L phosphate uffer, pH=7) which are constituents of fertilizers occurring in high concentrations in the environment. In order to appreciate their effect, adsorption studies were also performed in pure aqueous medium. The equilibrated liquid was analyzed after centrifugation by two different methods (Total Organic Carbon measurement, High Performance Liquid Chromatography). Isotherms obtained under different conditions, as well as on various soils, exhibit a similar shape, thus indicating a two-step adsorption process. The plots cannot be interpreted according to the classes of isotherms suggested by Giles (H-, L- and C-type, [2]). The adsorption coefficients were estimated from the initial slope of the curves. These values were determined not only by the type of the soil, but also by the composition of the aqueous media. Due to the low value of the adsorption coefficients, the acetochlor is a rather mobile pollutant of the soil posing a potential danger to the aquatic environment. The organic matter adsorption coefficients (K_om) [3] were also calculated and they were approximately identical for soils of high organic matter. For the chernozem and brown forest soils, the values of the K and K_om parameters are increasing in the order from water < phosphate buffer < sodium nitrate. For soils of low organic content, the similarity of the K_om values cannot be expected (due to the hyperbolic nature of the equation) as the data really indicate it for the adsorption behavior of the sandy soil. Here, the organic matter plays a less important role and the adsorption is controlled by the solute / inorganic substance interactions. This conclusion is nicely proved by the adsorption of the acetochlor on quartz resulting thereby in a similar plot being obtained for the soils. According to the hypothesis presented here, the first step of the isotherms is controlled by the solute / surface interactions while the solute / adsorbed solute interactions are operating in the second step of the isotherm. The role of the organic matter in this region of the isotherm is probably negligible. The comparison of the adsorption coefficients leads to the conclusion that the presence of nitrate and phosphate ions enhances the adsorption of acetochlor on the soils containing a high percentage of organic matter. As these ions are present in the environment due to the extensive use of fertilizers, they may increase the acetochlor pollution of water by erosion. This conclusion corroborates those earlier observations that reported frequent acetochlor contamination of the surface waters [4–5]. As the organic matter content of the soils plays an important role in the acetochlor adsorption, humic substances must have a strong influence on the transport of this compound. Experiments to obtain adsorption isotherms of further pesticides and the development of a quantitative model are in progress.     

Surface-active properties of particle size fractions in two humic acids

The relationships between surface active properties and humic acid (HA) particle sizes were investigated. Two HAs from an Ando soil and a Brown forest soil were separated into 6 particle size fractions by gel permeation chromatography. Surface-active properties characterized by surface excess value (\gT mol cm^-2), cross-sectional surface area per molecule (A nm²), critical micelle concentration (CMC g L^-1), efficiency and effectiveness of water surface tension reduction were obtained by the measurement of the surface tension of HA solutions from different particle size fractions. For the HA from the Ando soil, except for the smallest particle size fraction, increasing particle size enhanced the efficiency of reduction of the water surface tension and decreased the CMC, while the effectiveness of reduction of the water surface tension was about the same. The surface activity of the HA from the Ando soil increased with increasing particle size. This phenomenon was similar to the surface activity of a homologous series of surfactants, which increased with increasing alkyl chain length. For the HA from the Brown forest soil, the smallest particle size fraction and three large fractions showed a high efficiency, namely a high surface activity. The smallest fraction from the Brown forest soil showed the highest efficiency and the lowest CMC value. In both HAs, the smallest particle size fraction showed exceptional surface-active properties compared with the other fractions and three fractions with large particle size showed a higher surface activity than other smaller fractions.     

Remediation of a Diesel Contaminated,Sandy-Loam Soil Using Low Concentrated Surfactant Solutions (5 pp)

Background, Aims and Scope   Surfactant enhanced ex-situ soil washing can be used to remediate diesel contaminated soils. Surfactants enhance the diesel removal from soils by two processes: mobilization and solubilization. Mobilization occurs at surfactant concentrations below the critical micelle concentration (CMC), while solubilization occurs at surfactant concentrations beyond the CMC of the surfactant. In this paper, the leaching of diesel from a self contaminated (10 000 mg diesel/kg) sandy-loam soil was studied using low concentrated non-ionic surfactant solutions of Tergitol NP-10 (10-6 to 10-3 mol/L). Methods   The surface tension of the supernatant solutions of soil suspensions in non-ionic surfactant (Tergitol NP-10) solutions (10-6 to 10-3 mol/L) was measured to determine the total surfactant concentration (surfactant sorbed onto the soil + surfactant in solution) at which micelles are present in the water phase of the soil suspension, or 'effective critical micelle concentration' (ECMC), and to calculate the sub-CMC sorption isotherm of Tergitol NP-10 onto the soil. The diesel removal was measured by soil leaching experiments with Tergitol solutions ranging from 10-6 to 10-3 mol/L. Results and Discussion   The sub-CMC sorption isotherm of Tergitol onto the soil could be described by the Freundlich equation. Even at very low surfactant concentrations (10-6 mol/L) the surfactant enhanced the diesel removal from the soil. Up to the surfactants' ECMC the diesel removal increased in a linear way with an increasing surfactant concentration in the wash water. A significant part (20%) of the diesel oil was removed in the presence of surfactant, but in absence of micelles. Beyond the surfactants' ECMC, the increase in diesel removal efficiency with increasing surfactant concentration was less pronounced. When the added surfactant concentration was increased to 10-3 mol/L, which corresponds to an equilibrium surfactant concentration in the supernatant solution (6.77.10-4 mol/L) above the CMC, the diesel oil removal was increased up to 50%. At this surfactant concentration emulsion formation was observed. Conclusion   Surfactant aided leaching of diesel from a self-contaminated, sandy-loam soil with surfactant solutions below their ECMC was able to remove in one step 20% of diesel from the soil by the mobilization mechanism. Succesive treatments or continuous leaching with surfactant solutions below their ECMC was not studied but is likely to reduce the diesel contamination further and at the same time avoid problems with emulsion formation.     

Carbon storage in loess derived surface soils from Central Germany: Influence of mineral phase variables

The influence of the soil mineral phase on organic matter storage was studied in loess derived surface soils of Central Germany. The seven soils were developed to different genetic stages. The carbon content of the bulk soils ranged from 8.7 to 19.7 g kg^—1. Clay mineralogy was confirmed to be constant, with illite contents > 80 %. Both, specific surface area (SSA, BET‐N₂‐method) and cation exchange capacity (CEC) of bulk soils after carbon removal were better predictors of carbon content than clay content or dithionite‐extractable iron. SSA explained 55 % and CEC 54 % of the variation in carbon content. The carbon loadings of the soils were between 0.57 and 1.06 mg C m^—2, and therefore in the ”︁monolayer equivalent” (ME) level. The increase in SSA after carbon removal (ΔSSA) was significantly and positively related to carbon content (r² = 0.77). Together with CEC of carbon‐free samples, ΔSSA explained 90 % of the variation in carbon content. Clay (< 2 μm) and fine silt fractions (2—6.3 μm) contained 68—82 % of the bulk soil organic carbon. A significantly positive relationship between carbon content in the clay fraction and in the bulk soil was observed (r² = 0.95). The carbon pools of the clay and fine silt fractions were characterized by differences in C/N ratio, δ¹³C ratio, and enrichment factors for carbon and nitrogen. Organic matter in clay fractions seems to be more altered by microbes than organic matter in fine silt fractions. The results imply that organic matter accumulates in the fractions of smallest size and highest surface area, apparently intimately associated with the mineral phase. The amount of cations adhering to the mineral surface and the size of a certain and specific part of the surface area (ΔSSA) are the mineral phase properties which affect the content of the organic carbon in loess derived arable surface soils in Central Germany most. There is no monolayer of organic matter on the soil surfaces even if carbon loadings are in the ME level.     

Adsorption of mercury compounds by tropical soils II. Effect of soil: Solution ratio,ionic strength,pH, and organic matter

Mercury adsorbed from HgCl₂ and 2-methoxy-ethylmercury chloride (Aretan) solutions by three contrasting soils showed a dependence on soil: solution ratio and initial Hg concentration in soil solution. Changing the soil solution ratio from 1: 10 to 1 : 100 but keeping the initial concentration constant resulted in an increase in Hg adsorption from both Hg compounds. A similar change in soil: solution ratio accompanied by a decrease in initial concentration, on the other hand, resulted in decrease in Hg adsorption. Upon manipulating of the pH of the surface soils, adsorption of HgCl₂ at 100 mg Hg L^?1 concentration increased from about 701 :o over 95 mg Hg kg^?1 when pH was raised from 5.0 to 8.0. Precipitation of Hg may also have contributed to this trend. Aretan adsorption by these soils, on the other hand, changed little with change in pH. Removal of organic matter from soil resulted in large reductions of Hg adsorbed, as much as 95 % from the HgCl₂ solutions, but only up to 31 % from Aretan solutions. This suggests that organic matter in soil played a mayor role in the adsorption of inorganic Hg whereas the soils' mineral fractions were involved more in the adsorption of the organic Hg compound.     

再生水灌溉对农田土壤水流运动影响的研究进展

再生水灌溉农田既可节约宝贵的水资源、缓解农业用水紧缺,同时再生水中的多种营养元素和微量元素可促进作物生长、提高粮食产量。但再生水中的物质进入农田后将引起土壤孔隙结构、团聚体结构、黏粒分散特征和水土作用关系等一系列的变化,进而引起土壤入渗性能和导水性能的改变,增大环境污染风险。该文综述了再生水中的悬浮无机固体、大分子有机质、油脂、表面活性剂和盐分对农田土壤水流运动的影响及其作用机理,指出受灌农田土壤结构性质演化过程与驱动机制、受灌农田土壤与灌溉入渗水流之间的相互作用关系为该领域亟需开展的2个研究方向。文章对再生水农田灌溉制度制定、污染风险控制和生态环境保护均有参考价值。     

Volatilisation of aromatic hydrocarbons from soil: Part I,fluxes from coal tar contaminated surface soils

The non-steady-state fluxes of aromatic hydrocarbons were measured in the laboratory from the surface of soils contaminated with coal tar. Four soil samples from a former gasworks site were used for the experiments. The fluxes were quantified for 11 selected compounds, 4 mono- and 7 polycyclic aromatic hydrocarbons, for a period of up to 8 or 16 days. The concentrations of the selected compounds in the soils were between 0.2 and 3,100 µg/g. The study included the experimental determination of the distribution coefficient of the aromatic hydrocarbons between the sorbed phase and the water under saturated conditions. The determined distribution coefficients showed that the aromatic hydrocarbons were more strongly sorbed to the total organic carbon including the coal tar pitch — by a factor of 8 to 25 — than expected for natural organic matter. The fluxes were also estimated using an analytical solution of the Fick's diffusion equation and assuming that the compounds diffused independently of each other and that instant equilibrium existed between the air, water and sorbed phases. A relatively good agreement was found between the predicted and the measured flux. The predicted fluxes were between 0.11 and 7.5 time the measured fluxes. The fluxes were overestimated for the monocyclic aromatic hydrocarbons and underestimated for 3-rings-PAHs.     

Ultrasonic Enhanced Desorption of DDT from Contaminated Soils

In this study, using high-power low-frequency ultrasound, heated slurries with anionic surfactant sodium dodecyl sulfate (SDS) were treated to enhance desorption of DDT from soils with high clay, silt, and organic matter content and different pH (5.6?C8.4). The results were compared with DDT extracted using a strong solvent combination as reference. Slurry ranges from 5 to 20 wt.% were studied. For a soil slurry (10 wt.%) at pH 6.9 with 0.1% v/v SDS surfactant heated to 40°C for 30 min, desorption was above 80% in 30 s using 20 kHz, 932 W/L ultrasonic intensity without solvent extraction. Other soils gave lower desorption efficiency in the range 40?C60% after 30 s ultrasonic treatment. The percentage of organic matter, dissolved organic carbon, soil surface area, clay and silt percentage, and soil pH level were the key parameters influencing variations in desorption of DDT in the three soils in similar experimental conditions. DDT dissolution in SDS and soil organic matter removal employing the ultrasonic-enhanced organic matter roll-up mechanism emerged as the two best possible methods of DDT desorption. The method offers a practical, potentially low-cost alternative to high volume, costly, hazardous solvent extraction of DDT.     

THE SURFACE TENSION OF SOIL WATER

Because of their organic matter content, the surface tension of water of soils is about 8 to 9 erg cm ^-2 (8 to 9 × 10 ^-7 J cm ^-2) less than that of pure water. It is estimated that the surface tension of soil solution is 63–64 erg cm ^-2 in the surface soils studied.     

Influence of organic matter content, moisture status and time after reworking on soil shear strength

Recovery of soil shear strength was measured in disturbed samples of a calcareous clay loam soils, with four different organic matter contents. Repacked soil cores were put on tension tables at 25 or 50 cm water tension. Shear strength and water content were measured at one of eight time intervals, from 48 h to 7 weeks after the tension was applied. Soil strength increased with increasing organic matter content or increasing the applied tension. The major factor affecting strength, however, was the length of time that tension was applied. Because water contents showed little or no change with time on the tension table these results demonstrate thixotropic recovery of shear strength. This property is discussed with relevance to the soil's potential for mole drainage.     

Decomposition of Organic Matter with Previous Cadmium Adsorption in Soils

Decomposition of organic matter with previous Cd adsorption (thereafter referred to as OMACd) in soils and in water was studied in order to clarify the mechanism of Cd-induced inhibition of organic matter decomposition in soil. Two types of organic materials (sludge, rice straw) with or without previous Cd adsorption were mixed with a Gley soil or a Light-colored Andosol in a proportion of 1%. In the soils amended with the Cd-free organic materials, a CdCl₂ solution was added to the soils. The decomposition of the organic matter was examined by measuring the CO₂ evolution for 4 weeks at 28°C. Although the same amount of Cd was added to the soils, the decomposition of OMACd was inhibited to a greater extent than that in the soils to which a CdCl₂ solution had been added.

Furthermore the decomposition of sludge with previous Cd adsorption (thereafter referred to as SACd) in water after inoculation of soil microorganisms was investigated. Although the control sludge without Cd was markedly decomposed at 30°C during 4 weeks, SACd was not appreciably decomposed. These results suggest that OMACd cannot be readily decomposed by microorganisms.     

Incidence of soil surface crust types in semi-arid Niger

Soil water availability is most essential in the Sahelian agriculture but is hampered by several factors. Surface crusts or crust-like surfaces, which are characteristic of most Sahelian soils, have been shown to decrease water infiltrability and increase runoff. Their type and structure are influenced by soil texture, vegetation cover, erosion and deposition effects of wind and water. A soil and terrain survey in semi-arid SW-Niger was carried out to explain the patterns of soil surface crusts and the deterioration of the land. The soil surface crusts were shown to depend also on specific terrain factors including land use type and intensity, and terrain type and position. Chemical and physical soil factors such as organic carbon, soil colour and texture occurring with specific crusts indicate soil degradation, especially in sloping terrain, which increases runoff and soil erosion. For sandy soils, surface tillage is required to break up the crusts. Higher surface organic matter is recommended to enhance water infiltration in soils.