The addition of organic carbon and nitrate affects reactive transport of heavy metals in sandy aquifers

Organic carbon introduction in the soil to initiate remedial measures, nitrate infiltration due to agricultural practices or sulphate intrusion owing to industrial usage can influence the redox conditions and pH, thus affecting the mobility of heavy metals in soil and groundwater. This study reports the fate of Zn and Cd in sandy aquifers under a variety of plausible in-situ redox conditions that were induced by introduction of carbon and various electron acceptors in column experiments. Up to 100% Zn and Cd removal (from the liquid phase) was observed in all the four columns, however the mechanisms were different. Metal removal in column K1 (containing sulphate), was attributed to biological sulphate reduction and subsequent metal precipitation (as sulphides). In the presence of both nitrate and sulphate (K2), the former dominated the process, precipitating the heavy metals as hydroxides and/or carbonates. In the presence of sulphate, nitrate and supplemental iron (Fe(OH)(3)) (K3), metal removal was also due to precipitation as hydroxides and/or carbonates. In abiotic column, K4, (with supplemental iron (Fe(OH)(3)), but no nitrate), cation exchange with soil led to metal removal. The results obtained were modeled using the reactive transport model PHREEQC-2 to elucidate governing processes and to evaluate scenarios of organic carbon, sulphate and nitrate inputs.     

Sorption and mobility of metronidazole, olaquindox, oxytetracycline and tylosin in soil

Laboratory studies were conducted to characterise four different antibiotic compounds with regard to sorption and mobility in various soil types. Distribution coefficients (Kd values) determined by a batch equilibrium method varied between 0.5 and 0.7 for metronidazole, 0.7 and 1.7 for olaquindox and 8 and 128 for tylosin. Tylosin sorption seems to correlate positively with the soil clay content. No other significant interactions between soil characteristics and sorption were observed. Oxytetracycline was particularly strongly sorbed in all soils investigated, with Kd values between 417 in sand soil and 1026 in sandy loam, and no significant desorption was observed. Soil column leaching experiments indicated large differences in the mobility of the four antibiotic substances, corresponding to their respective sorption capabilities. For the weakly adsorbed substances metronidazole and olaquindox the total amounts added were recovered in the leachate of both sandy loam and sand soils. For the strongly adsorbed oxytetracyline and tylosin nothing was detected in the leachate of any of the soil types, indicating a much lower mobility. Results from defractionation and extraction of the columns (30 cm length) showed that 60-80% of the tylosin added had been leached to a depth of 5 cm in the sandy loam soil and 25 cm in the sand soil.     

Dynamic chemical characteristics of soil solution after pig manure application: A column study

When manures from intensive livestock operations are applied to agricultural or vegetable fields at a high rate, large amounts of salts and metals will be introduced into soils. Using a column leaching experiment, this study assessed the leaching potential of the downward movement of Cu and Zn as well as some salt ions after an intensive farm pig manure at rates of 0%, 5% and 10% (w/w) were applied to the top 20 cm of two different textured soils (G soil -sandy loam soil; H soil-silty clay loam soil), and investigated the growth of amaranth and Cu and Zn transfer from soil to amaranth (Amaranthus tricolor). Soil solutions were obtained at 20, 40 and 60 cm depth of the packed column and analyzed for pH, electrical conductivity (EC), dissolved organic matter (DOC) and Cu and Zn concentrations. The results indicated that application of pig manure containing Cu and Zn to sandy loam soil might cause higher leaching and uptake risk than silty clay loam soil, especially at high application rates. And manure amendment at 5% and 10% significantly decreased the biomass of amaranth, in which the salt impact rather than Cu and Zn toxicity from manures played more important role in amaranth growth. Thus the farmer should avoid application the high rate of pig manure containing metal and salt to soil at a time, especially in sandy soil.     

Effect of soil on microbial responses to metal contamination

An experiment was conducted to investigate microbial responses to metal inputs in five soils with varying clay and organic contents; one soil had also a higher pH. These soils were treated with a low metal, sewage sludge control or with this sludge contaminated to achieve Cu=112, Ni=58 and Zn=220 mg kg(-1) in medium and Cu=182, Ni=98 and Zn=325 mg kg(-1) in high metal soils. CO(2) evolution rates were measured at 1 week and at 4-5-day intervals thereafter until the end of the incubation (7 weeks). Extractable metals (CaCl(2) and water), biomass C, metabolic quotient, ergosterol, bacterial-fungal phospholipid fatty acid (PLFA-3 weeks only) ratio and mineral N were measured at 3 and 7 weeks. Metal inputs caused a marked increase in metal availability in the slightly acidic sandy loams, a smaller increase in slightly acidic clays and had little effect in the alkaline loam. After an initial increase in CO(2) evolution with metal inputs in all soils, the high metal treatment alone caused a significant decrease at later stages, mainly in sandy loams. Although biomass C and metabolic quotient decreased in all soils with higher metal inputs, the effect was more pronounced in the sandy loams. Metal inputs increased ergosterol and decreased bacterial-fungal PLFA ratios in most soils. Larger mineral N contents were found in all high metal soils at 3 weeks but, after 7 weeks, metals caused a significant decrease in sandy loams. CaCl(2) and water-extractable Cu, Ni and Zn contents were closely correlated with microbial indices in sandy loam but not in clay soils. Overall, the effect of treatments on microbial and extractable metal indices was greater in loams. Within a single series, higher organic soils showed less pronounced responses to metal inputs, although this trend was not always consistent.     

Dynamic chemical characteristics of soil solution after pig manure application: a column study

When manures from intensive livestock operations are applied to agricultural or vegetable fields at a high rate, large amounts of salts and metals will be introduced into soils. Using a column leaching experiment, this study assessed the leaching potential of the downward movement of Cu and Zn as well as some salt ions after an intensive farm pig manure at rates of 0%, 5% and 10% (w/w) were applied to the top 20 cm of two different textured soils (G soil -sandy loam soil; H soil-silty clay loam soil), and investigated the growth of amaranth and Cu and Zn transfer from soil to amaranth (Amaranthus tricolor). Soil solutions were obtained at 20, 40 and 60 cm depth of the packed column and analyzed for pH, electrical conductivity (EC), dissolved organic matter (DOC) and Cu and Zn concentrations. The results indicated that application of pig manure containing Cu and Zn to sandy loam soil might cause higher leaching and uptake risk than silty clay loam soil, especially at high application rates. And manure amendment at 5% and 10% significantly decreased the biomass of amaranth, in which the salt impact rather than Cu and Zn toxicity from manures played more important role in amaranth growth. Thus the farmer should avoid application the high rate of pig manure containing metal and salt to soil at a time, especially in sandy soil.     

Concurrent nitrate and Fe(III) reduction during anaerobic biodegradation of phenols in a sandstone aquifer

The biodegradation of phenols (5, 60, 600 mg l⁻¹) under anaerobic conditions (nitrate enriched and unamended) was studied in laboratory microcosms with sandstone material and groundwater from within an anaerobic ammonium plume in an aquifer. The aqueous phase was sampled and analyzed for phenols and selected redox sensitive parameters on a regular basis. An experiment with sandstone material from specific depth intervals from a vertical profile across the ammonium plume was also conducted. The miniature microcosms used in this experiment were sacrificed for sampling for phenols and selected redox sensitive parameters at the end of the experiment. The sandstone material was characterized with respect to oxidation and reduction potential and Fe(II) and Fe(III) speciation prior to use for all microcosms and at the end of the experiments for selected microcosms.The redox conditions in the anaerobic microcosms were mixed nitrate and Fe(III) reducing. Nitrate and Fe(III) were apparently the dominant electron acceptors at high and low nitrate concentrations, respectively. When biomass growth is taken into account, nitrate and Fe(III) reduction constituted sufficient electron acceptor capacity for the mineralization of the phenols observed to be degraded even at an initial phenols concentration of 60 mg l⁻¹ (high) in an unamended microcosm, whereas nitrate reduction alone is unlikely to have provided sufficient electron acceptor capacity for the observed degradation of the phenols in the unamended microcosm.For microcosm systems, with solid aquifer materials, dissolution of organic substances from the solid material may occur. A quantitative determination of the speciation (mineral types and quantity) of electron acceptors associated with the solids, at levels relevant for degradation of specific organic compounds in aquifers, cannot always be obtained. Hence, complete mass balances of electron acceptor consumption for specific organic compounds degradation are difficult to confine. For aquifer materials with low initial Fe(II) content, Fe(II) determinations on solids and in aqueous phase samples may provide valuable information on Fe(III) reduction. However, in microcosms with natural sediments and where electron acceptors are associated with the sediments, complete mass-balances for substrates and electron acceptors are not likely to be obtained.     

Effect of nitrogen amendment on respiration and respiratory quotient (RQ) in three hydrocarbon contaminated soils of different type

Three soil types (sandy gravel, silty clay and sandy loam) from sites historically contaminated with total petroleum hydrocarbon (TPH) were amended with NH(4)NO(3) at concentrations ranging from 16 to 2133 mg/kg soil(dry weight). Microbial activity was measured as O(2) consumption and CO(2) production in order to assess nitrogen limitation. Although activity was stimulated in all three soils under NH(4)NO(3) amendment (after 72 h), the level of nitrogen required was soil specific. For the sandy gravel and silty clay soils, O(2) consumption and CO(2) production both showed enhanced microbial activity when amended with 16 mg/kg soil(dry weight) NH(4)NO(3), whereas, these two parameters gave differing results for the sandy loam soil. Specifically, CO(2) production and O(2) consumption were stimulated with 66 mg/kg and 133 mg/kg soil(dry weight) of NH(4)NO(3) respectively. In addition, respiratory quotient kinetic analysis suggested different decomposition processes occurring in this soil under different NH(4)NO(3) amendment concentrations.     

Herbicide movement and dissipation at four Midwestern sites

This study was conducted to evaluate atrazine (2-chloro-4-ethylamino-6-isopropyl-1,3,5-triazine) and alachlor (2-chloro-N-(methoxymethyl)acetamide) dissipation and movement to shallow aquifers across the Northern Sand Plains region of the United States. Sites were located at Minnesota on a Zimmerman fine sand, North Dakota on Hecla sandy loam, South Dakota on a Brandt silty clay loam, and Wisconsin on a Sparta sand. Herbicide concentrations were determined in soil samples taken to 90 cm four times during the growing season and water samples taken from the top one m of aquifer at least once every three months. Herbicides were detected to a depth of 30 cm in Sparta sand and 90 cm in all other soils. Some aquifer samples from each site contained atrazine with the highest concentration in the aquifer beneath the Sparta sand (1.28 microg L(-1)). Alachlor was detected only once in the aquifer at the SD site. The time to 50% atrazine dissipation (DT50) in the top 15 cm of soil averaged about 21 d in Sparta and Zimmerman sands and more than 45 d for Brandt and Hecla soils. Atrazine DT50 was correlated positively with % clay and organic carbon (OC), and negatively with % fine sand. Alachlor DT50 ranged from 12 to 32 d for Zimmerman and Brandt soils, respectively, and was correlated negatively with % clay and OC and positively with % sand.     

Degradation of metribuzin in two soil types of Lebanon

The degradation of metribuzin [4-amino-6-tert-butyl-3-methylthio-1,2,4-triazin-5(4H)-one] as influenced by soil type, temperature, humidity, organic fertilizers, soil sterilization, and ultra-violet radiation was studied in two soil types of Lebanon under laboratory conditions. The two soil types were sandy loam and clay. Deamination of metribuzin in the sandy loam soil to its deaminometribuzin (DA) derivative was basically a result of biological activity. In the clay soil the first metabolite diketometribuzin (DK) was a result of oxidative desulfuration, while diketo-deaminometribuzin (DADK) was the product of reductive deamination. The two soils represented major differences in the pesticide transformation processes. Photodecomposition on the soil surface and in aqueous media was also an important process in the degradation of metribuzin. Furthermore, the increase in soil organic matter enhanced degradation.     

The effects of pH and soil type on concentrations of zinc, copper and nickel extracted by calcium chloride from sewage sludge-treated soils

The effects of pH on concentrations of zinc, copper and nickel extracted by calcium chloride from a clay loam and two sandy loam soils that had been treated with sewage sludge were studied. Concentrations of all the metals increased rapidly as pH decreased below a threshold value ranging from 6.2 to 7.0 for Zn, 6.2 to 7.2 for Ni and 4.7 to 5.7 for Cu. Both the total concentrations and the threshold pH values were influenced by differences in soil texture and (between the two light-textured soils) by differences in soil cation exchange capacity. The amount of zinc and copper extracted from the mixtures was considerably less than that extracted from the same quantity of sludge alone.     

Sorption of 3,4-dichloroaniline on four contrasting Greek agricultural soils and the effect of liming

Sorption of 3,4-dichloroaniline (3,4-DCA) on four typical Greek agricultural soils, with distinct texture, organic matter content and cation exchange capacities, was compared by using sorption isotherms and the parameters calculated from the fitted Freundlich equations. The sorption process of 3,4-DCA to the soil was completed within 48–72 h. The 3,4-DCA sorption on all soils was well described by the Freundlich equation and all sorption isotherms were of the L-type. The sandy clay loam soil with the highest organic matter content and a slightly acidic pH was the most sorptive, whereas the two other soil types, a high organic matter and neutral pH clay and a low organic matter and acidic loam, had an intermediate sorption capacity. A typical calcareous soil with low organic matter had the lowest sorption capacity which was only slightly higher than that of river sand. The 3,4-DCA sorption correlated best to soil organic matter content and not to clay content or cation exchange capacity, indicating the primary role of organic matter. The distribution coefficient (K _d) decreased with increasing initial 3,4-DCA concentration and the reduction was most pronounced with the highly sorptive sandy clay loam soil, suggesting that the available sorption sites of the soils are not unlimited. Liming of the two acidic soils (the sandy clay loam and the loam) raised their pH (from 6.2 and 5.3, respectively) to 7.8 and reduced their sorption capacity by about 50 %, indicating that soil pH may be the second in importance factor (after organic matter) determining 3,4-DCA sorption.     

Sorptive interactions and binding of δ‐endotoxin protein from bacillus thuringiensis subsp. kurstaki inforest soils

Abstract

The adsorption, desorption and binding of the insecticidal protein from Bacillus thuringiensis subsp. kurstaki (Btk toxin) onto autoclaved sandy and clay loam forest soils were studied at 23°C in a buffer medium (pH 10.2) using the precipitated protein mixture (active + inactive) obtained from a commercial Btk formulation. The active protein in the buffer solution was quantified by ELISA technique. Maximum adsorption of the toxin onto the sandy (301 μg/g) and clay (474 μg/g) loam soils was found to occur after 3 and 4 hours of agitation, respectively. Adsorption of the toxin was higher in the clay loam soil than in sandy loam. Adsorption parameters were calculated using the Freundlich and linear isotherm equations. The K_F and 1/n values for the soils were 1.12 and 1.48 (sandy), and 20.42 and 0.874 (clay), respectively, indicating stronger affinity of the toxin for the clay compared to the sandy loam soil. The linear model showed deviations at higher concentrations, nevertheless using the best fit, K_D and K_OC values were computed for the two soils. For sandy loam, the K_D and K_OC values were 9.38 and 391, respectively; the corresponding values for clay loam were 13.19 and 425, confirming the higher sorption affinity of the toxin for clay loam. The adsorption data did not fit the Langmuir equation because of heterogeneity of the soil surface. Desorption studies showed that more than half of the adsorbed toxic protein remained firmly attached to sandy (162.6 μg/g or 54.5%) and clay (314.0 μg/g or 67.4%) loam soils after six 0.5‐h washes (total 3.0 h wash time). Although the toxin appears to be a non‐leacher, its lateral mobility, soil persistence and biological consequences, including bioavailability of the bound residues, are poorly understood and require further investigation.     

Sorption of 3,4-dichloroaniline on four contrasting Greek agricultural soils and the effect of liming

Sorption of 3,4-dichloroaniline (3,4-DCA) on four typical Greek agricultural soils, with distinct texture, organic matter content and cation exchange capacities, was compared by using sorption isotherms and the parameters calculated from the fitted Freundlich equations. The sorption process of 3,4-DCA to the soil was completed within 48-72 h. The 3,4-DCA sorption on all soils was well described by the Freundlich equation and all sorption isotherms were of the L-type. The sandy clay loam soil with the highest organic matter content and a slightly acidic pH was the most sorptive, whereas the two other soil types, a high organic matter and neutral pH clay and a low organic matter and acidic loam, had an intermediate sorption capacity. A typical calcareous soil with low organic matter had the lowest sorption capacity which was only slightly higher than that of river sand. The 3,4-DCA sorption correlated best to soil organic matter content and not to clay content or cation exchange capacity, indicating the primary role of organic matter. The distribution coefficient (K(d)) decreased with increasing initial 3,4-DCA concentration and the reduction was most pronounced with the highly sorptive sandy clay loam soil, suggesting that the available sorption sites of the soils are not unlimited. Liming of the two acidic soils (the sandy clay loam and the loam) raised their pH (from 6.2 and 5.3, respectively) to 7.8 and reduced their sorption capacity by about 50 %, indicating that soil pH may be the second in importance factor (after organic matter) determining 3,4-DCA sorption.     

Degradation of Metribuzin in Two Soil Types of Lebanon

The degradation of metribuzin [4-amino-6-tert-butyl-3-methylthio-1,2,4-triazin-5(4H)-one] as influenced by soil type, temperature, humidity, organic fertilizers, soil sterilization, and ultra-violet radiation was studied in two soil types of Lebanon under laboratory conditions. The two soil types were sandy loam and clay. Deamination of metribuzin in the sandy loam soil to its deaminometribuzin (DA) derivative was basically a result of biological activity. In the clay soil the first metabolite diketometribuzin (DK) was a result of oxidative desulfuration, while diketo-deaminometribuzin (DADK) was the product of reductive deamination. The two soils represented major differences in the pesticide transformation processes. Photodecomposition on the soil surface and in aqueous media was also an important process in the degradation of metribuzin. Furthermore, the increase in soil organic matter enhanced degradation.     

Influence of soil texture and tillage on herbicide transport

Two long-term no-till corn production studies, representing different soil texture, consistently showed higher leaching of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] to groundwater in a silt loam soil than in a sandy loam soil. A laboratory leaching study was initiated using intact soil cores from the two sites to determine whether the soil texture could account for the observed differences. Six intact soil cores (16 cm dia by 20 cm high) were collected from a four-year old no-till corn plots at each of the two locations (ca. 25 km apart). All cores were mounted in funnels and the saturated hydraulic conductivity (Ksat) was measured. Three cores (from each soil texture) with the lowest Ksat were mixed and repacked. All cores were surface treated with 1.7 kg ai ha(-1) [ring-14C] atrazine, subjected to simulated rainfall at a constant 12 mm h(-1) intensity until nearly 3 pore volume of leachate was collected and analyzed for a total of 14C. On an average, nearly 40% more of atrazine was leached through the intact silt loam than the sandy loam soil cores. For both the intact and repacked cores, the initial atrazine leaching rates were higher in the silt loam than the sandy loam soils, indicating that macropore flow was a more prominent mechanism for atrazine leaching in the silt loam soil. A predominance of macropore flow in the silt loam soil, possibly due to greater aggregate stability, may account for the observed leaching patterns for both field and laboratory studies.     

Herbicide movement and dissipation at four midwestern sites

Abstract

This study was conducted to evaluate atrazine (2‐chloro‐4‐ethylamino‐6‐isopropyl‐1, 3, 5‐triazine) and alachlor (2‐chIoro‐N‐(methoxymethyl)acetamide) dissipation and movement to shallow aquifers across the Northern Sand Plains region of the United States. Sites were located at Minnesota on a Zimmerman fine sand, North Dakota on Hecla sandy loam, South Dakota on a Brandt silty clay loam, and Wisconsin on a Sparta sand. Herbicide concentrations were determined in soil samples taken to 90 cm four times during the growing season and water samples taken from the top one m of aquifer at least once every three months. Herbicides were detected to a depth of 30 cm in Sparta sand and 90 cm in all other soils. Some aquifer samples from each site contained atrazine with the highest concentration in the aquifer beneath the Sparta sand (1.28 μg L^‐1). Alachlor was detected only once in the aquifer at the SD site. The time to 50% atrazine dissipation (DT₅₀) in the top 15 cm of soil averaged about 21 d in Sparta and Zimmerman sands and more than 45 d for Brandt and Hecla soils. Atrazine DT₅₀ was correlated positively with % clay and organic carbon (OC), and negatively with % fine sand. Alachlor DT₅₀ ranged from 12 to 32 d for Zimmerman and Brandt soils, respectively, and was correlated negatively with % clay and OC and positively with % sand.     

Modelling of atrazine transport in the presence of surfactants

Laboratory experiments were conducted to examine the effect of detergents on transport of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] through loam and sandy loam soils under saturation conditions. The Convection Dispersion Equation (CDE) was used to model and quantify the effects of detergents on atrazine model parameters: the retardation factor (R), pore velocity (v) and dispersion coefficient (D). The transport parameters were estimated using moment technique and partition coefficient obtained from batch experiments and compared with best-fitted parameters, R and D, keeping pore velocity constant. Results indicated the CDE model was not successful in predicting atrazine transport in the presence of surfactants at high concentrations. In the case of anionic surfactant with Elora loam, the average predicted R and D from moment technique of 3.4 and 11.1 cm2/h, respectively were significantly different than fitted parameters (R = 39 and D = 227 cm2/h). The poor performance of CDE in the presence of surfactants results from physiochemical changes in herbicide solubility and retention to the soil matrix rather than changes in soil hydraulic properties since the predicted pore water velocities from moment technique were similar to those measured during leaching experiments. Nevertheless, BTC analysis with CDE showed that land application of anionic surfactant (sulphonic) significantly increased R and D and decrease v for both soils. Addition of sulphonic increased R of atrazine by 12 and 26 folds for loam and sandy loam soils, respectively. On the other hand non-ionic surfactants seemed to decrease R, especially in sandy loam soil, thus facilitating atrazine leaching through soil. Non-equilibrium conditions seemed to govern atrazine transport in the presence of surfactants; double peaks in breakthrough curves were observed, indicating a need for mathematical models to account for such phenomena. Atrazine dispersion and tailing seemed to be higher through Elora loam compared to Caledon sandy loam due to higher aggregation of the Elora soil.     

Persistence, degradation and leaching of coumaphos in soil

Dissipation, degradation and leaching of fresh 14C coumaphos, alkylated 14C coumaphos and aged residues of 14C coumaphos from vats were studied in alkaline sandy loam soil in soil columns in the field under subtropical conditions in Delhi for a year. Dissipation, degradation and bound residue formation was more in case of alkali treated coumaphos than fresh coumaphos. After 365 days total residues of fresh coumaphos accounted for 33.25% while that of alkali treated coumaphos was 19.12%. Bound residue formation was almost double in case of alkali treated coumaphos (18.95%) than fresh coumaphos (9.53%) after 150 days followed by release of bound residue in both the cases. The proportion of metabolites 4-methylumbelliferone, chlorferon and potasan collectively was 86.05% in fresh coumaphos extractable residues while the same was 91.74% in alkali treated coumaphos after 365 days. Aged residues from vats containing copper sulphate and buffer were found to be more persistent in soil as total residues remained were 95.58% in comparison with 83.09% total residues of aged residues from vats containing only buffer after 150 days of treatment. Copper sulphate seems to inhibit the degradatiion of coumaphos in soil by microorganisms. Chlorferon was the major metabolite in generally all the samples. Coumaphos did not leach below 10 cm in all the cases.     

Biogenic sulphide plays a major role on the riboflavin-mediated decolourisation of azo dyes under sulphate-reducing conditions

The effect of high concentrations of sulphate on the reductive decolourisation of different azo dyes by anaerobic sludge was studied in batch cultures. Sludge cultures were pre-incubated under sulphate-reducing conditions prior addition of dyes. Little or no effects of sulphate (5-10 g sulphate l(-1)) on the rate of decolourisation of Reactive Orange 14 (RO14), Direct Blue 53 (DB53) and Direct Blue 71 (DB71) were observed when no external redox mediator was provided. However, an increase in sulphate concentration, in the presence of riboflavin (20 microM), enhanced the decolourisation of all dyes. The first-rate constant of decolourisation (k) was increased up to 2-, 3.6- and 2-fold for RO14, DB53 and DB71, respectively, by supplying high sulphate concentrations, compared to the controls lacking sulphate, in the presence of the redox mediator. Sulphate reduction did not take place during the course of azo reductions, but was only evident before dye addition and after complete decolourisation, suggesting azo dyes reduction out-competed sulphate reduction for the available reducing equivalents. The experimental data suggest that reduction of azo dyes by riboflavin, which had been reduced by biogenic sulphide, was the major mechanism implicated during decolourisations, which was corroborated by abiotic incubations. Riboflavin greatly accelerated the abiotic reduction of RO14, so that the k value was increased up to 44-fold compared to the control lacking riboflavin.     

Metal (Cu, Cd and Zn) removal and stabilization during multiple soil washing by saponin

The influence of multiple saponin washing on copper, cadmium and zinc removal and stability in three types of soils (loamy sand, loam, silty clay) was investigated. Distribution of metals and their mobility measured as the ratio of exchangeable form to the sum of all fractions in soils was differential. After single washing the highest efficiency of metal removal was obtained in loamy sand (82-90%) and loam (67-88%), whereas the lowest in silty clay (39-62%). In loamy sand and loam metals had higher mobility factors (44-61% Cu, 60-76% Cd, and 68-84% Zn) compared to silty clay (9% Cu, 28% Cd and 36% Zn). Triplicate washing led to increase both efficiency of metal removal and percentage content of their stable forms.In consequence, fractional patterns for metals before and after treatment changed visibly as a result of their redistribution. Based on the redistribution index, the most stable metal (mainly in residual and organic fractions) after triplicate washing was Cu in loamy sand and loam. For silty clay contaminated with Cd, effective metal removal and its stabilization required a higher number of washings.