Soil-water Suspensions Research Articles

Recombinant luminescent bacterial sensors for the measurement of bioavailability of cadmium and lead in soils polluted by metal smelters

Environmental hazard of heavy metals in soils depends to a large extent on their bioavailability. The approach used in this study enables the determination of bioavailable metals in solid-phase samples. Two recombinant bacterial sensors, one responding specifically to cadmium and the other to lead and cadmium by increase of luminescence (firefly luciferase was used as a reporter) were used to determine the bioavailability of these metals in soil–water suspensions (a contact assay) and respective particle-free extracts. Fifty agricultural soils sampled near zinc and lead smelters in the Northern France containing up to (mg/kg) 20.1 of Cd, 1050 of Pb and 1390 of Zn were analysed. As the soil matrix interferes with the assay, recombinant luminescent control bacteria lacking the metal recognizing protein and corresponding promoter (thus, being not metal-inducible) but otherwise comparable to the sensor bacteria (the same host bacterium and plasmid encoding luciferase) were used in parallel to take into account the possible quenching and/or stimulating effects of the sample on the luminescence of the sensor bacteria. Both, chemical and sensor analysis showed that only μg/l levels of metals were extracted from the soil into the water phase (0.1% of the total Cd, 0.07% of Pb and 0.5% of Zn). However, 115-fold more Cd and 40-fold more Pb proved bioavailable if the sensor bacteria were incubated in soil suspensions (i.e., in the contact assay). The bioavailability of metals in different soils varied (depending probably on soil type) ranging from 0.5% to 56% for cadmium and from 0.2% to 8.6% for lead.

Construction and use of specific luminescent recombinant bacterial sensors for the assessment of bioavailable fraction of cadmium, zinc, mercury and chromium in the soil

Recombinant luminescent bacterial sensors for the detection of zinc [MC1061(pzntRluc)] and chromate [AE104(pchrBluc)] were constructed. The sensors carry firefly luciferase gene as a reporter under the control of zinc-inducible regulatory unit from Zn resistance system in chromosomal DNA of Escherichia coli or chromate-inducible unit from Ralstonia metallidurans CH34 megaplasmid pMOL28. The response of the sensors was calibrated using Zn 2+, Cr 2O 7 2− and CrO 4 2−, respectively. The detection limit of the zinc sensor for zinc was 40 μM (2.6 mg l −1 of Zn) and that of chromate sensor for dichromate 30 nM (1.6 μg l −1 of Cr) and for chromate 50 nM (2.6 μg l −1 of Cr), respectively. The specificity of the two above-mentioned sensors constructed in this study and a mercury-inducible bacterial sensor MC1061(pmerBR BSluc) constructed by us earlier [Anal. Chem. 73 (2001) 5168] was determined by using different heavy metal compounds. The zinc and mercury sensors were not completely specific to the target metals. The zinc sensor was co-inducible with cadmium and mercury and the mercury sensor with cadmium. The chromate sensor was inducible not only by chromate but also with Cr 3+. The bacterial sensors constructed were used for the estimation of bioavailable fraction of heavy metals in soils spiked with different amounts of zinc, cadmium, mercury and chromate. Both soil–water suspensions and soil–water extracts were analyzed. The results showed that the majority of heavy metals remained adsorbed to soil particles: only 0.6% of Cd, 1.3% of Hg, 2% of Zn and 46% of Cr (VI) were available to the bacterial sensors in soil–water extracts. However, when the soil–water suspensions were analyzed, approximately 20 times more cadmium and 30 times more mercury (12 and 40%, respectively) was available for the sensor bacteria whereas the available fractions of zinc and chromate in soil–water extract and suspension were similar. Thus, this study showed that in case of cadmium and mercury (but not zinc and chromium) the particle-bound metals were also partially bioavailable.

Organosmectites as sorbent and carrier of the herbicide bentazone

Sorption isotherms of bentazone on diverse organoclays (OCls) were measured at aqueous concentrations from 25 to 1500 μM. Sorption on organophilic OCls, OCls with large quaternary alkylammonium groups, i.e. hexadecyltrimethylammonium (AHDT) and dioctadecyldimethylammonium (ADOD), were compared to that on sorptive OCls, which have large primary alkylammonium (octadecylammonium, AC18) and small quaternary alkylammonium (phenyltrimethylammonium, PTA) groups. The organophilic OCls showed much higher sorption (AHDT K d=682–1789; ADOD K d=838–1728) than the sorptive OCls (AC18 K d=38–40; PTA K d=0), suggesting hydrophobic bentazone–organocation interactions as the main mechanism responsible for sorption. The incubation of the two organophilic OCls with soil contaminated with bentazone showed that availability of the herbicide decreased almost instantaneously from 124 μg g −1 to 1 μg g −1. Bentazone sorbed on the OCl, and thus immobilized, was partially extractable with CaCl 2/methanol solution suggesting its potential bioavailability. Two OCls, with high and low sorptions, were assayed as herbicide carriers by preparing bentazone–OCl complexes and monitoring their herbicide release in water and soil–water suspensions. These bentazone–OCl complexes released 20–80% of their bentazone content, depending of the sorbent capacity and the bentazone–OCl interaction. Results of this work show that by varying the sorptive properties of OCls, they can be used to immobilize pesticide in a contaminated soil and to protect soil and water by using OCl as pesticide carriers in slow release formulations.

Effects of surfactant addition on the biomineralization and microbial toxicity of phenanthrene.

Surfactants are known to increase the apparent aqueous solubility of polycyclic aromatic hydrocarbons and may thereby enhance their bioavailability. In this study the effects of four surfactants on the mineralization of phenanthrene by Pseudomonas aeruginosa in liquid culture and in soil-water suspensions was studied in batch reactors over a 15-week study period. In the absence of surfactant, liquid cultures mineralized approximately 50% of the phenanthrene added within seven weeks following a one-week lag period and an initial mineralization rate of 0.04 mg/d. Mineralization in soil-water suspensions proceeded without any measurable lag period. The initial mineralization rate was lower (0.006 mg/d), but mineralization continued to >70% over the fifteen week period. In general, the addition of very low concentrations of surfactant (<0.001%) to liquid cultures did not impact mineralization significantly. At higher surfactant concentrations (>CMC) all surfactants were seen to be inhibitory. In soil-water systems, the rate of phenanthrene mineralization was decreased even at surfactant doses that did not produce significant solubilization. In summary, none of the surfactants enhanced the mineralization of phenanthrene by P. aeruginosa in liquid culture or in soil-water suspensions. In order to rank surfactant toxicity, microbial toxicity tests were performed measuring the light output of bioluminescent bacteria as affected by the presence of surfactants. Additional toxicity testing indicated that the presence of solubilized phenanthrene increased the toxicity of the surfactant by a 100-fold suggesting that the toxicity of solubilized substrate needs also to be considered in the application of surfactant-amended remediation.

Modeling Arsenic(III) Adsorption and Heterogeneous Oxidation Kinetics in Soils

Arsenite [As(III)] is a soluble and toxic species of arsenic that can be introduced into soil by geothermal waters, mining activities, irrigation practices, and disposal of industrial wastes. We determined the rates of As(III) adsorption, and subsequent oxidation to arsenate [As(V)] in aerobic soil–water suspensions using four California soils. The rate of As(III) adsorption on the soils was closely dependent on soil properties that reflect the reactivity of mineral surfaces including citrate–dithionite (CD) extractable metals, soil texture, specific surface area, and pH. Heterogeneous oxidation of As(III) to As(V) was observed in all soils studied. The recovery of As(V) from As(III)‐treated soils was dependent on levels of oxalate‐extractable Mn and soil texture. After derivation of rate equations to describe the changes in soluble and recoverable As(III) and As(V) in soil suspensions, soil property measurements were used to normalize the empirically derived rate constants for three soils. The fourth soil, which had substantially different soil properties from the other three soils, was used to independently test the derived soil property–normalized model. The soil property–normalized consecutive reaction model gave a satisfactory description of the trends seen in the experimental data for all four soils. Understanding the effects of soil properties on the kinetics of chemical reactions of As(III) and As(V) in soils will be essential to development of quantitative models for predicting the mobility of As in the field.

Polychlorinated biphenyl desorption from low organic carbon soils: Measurement of rates in soil-water suspensions

The desorption of 13 PCB congeners from four soils has been investigated using the gas purge technique. The soils from PCB spill sites had been in contact with Aroclor 1242 1254 mixtures for 3 or more years; thus, sorption equilibrium is presumed to have been obtained. Soils were “engineered” ground cover materials used at utility industry substations and consisted of fine rock chips and sandsilt-clay fractions with organic carbon < 0.2%. The congeners contained from three to five chlorine atoms. Agreement of measured Henry's Law constants for the 14C-labeled congeners 24′, 22′55′ and 22′44′55′ with literature values established the proper function of the gas purge technique for measurement of congener release rates from soil-water suspensions. For all 13 congeners and all soils: 1) the labile fraction was typically 80 to 90% of the total congener concentration, 2) the majority of the labile fraction was desorbed or released within 48 hours of contact with water, and 3) the release of the remaining nonlabile fraction persisted for over 6 months with complete release estimated to be 1 to 2 years. Release rate constants, k d, decreased with increasing chlorine number with typical values for labile and nonlabile fractions ranging from 1.4 to 0.5 d −1 and 0.008 to 0.0006 d −1, respectively.

Polychlorinated biphenyl sorption by soils: Measurement of soil-water partition coefficients at equilibrium

The sorption of PCB congeners 24′ (lUPAC number 8), 22′55′ (52) and 22′44′55′ (153) by eight soils differing in their organic carbon content (2.3 to 0.2% by Wt.) and expandable clay mineral content (17 to 53 % by wt.) has been investigated for 0.1 g/L soil-water suspensions. The time to reach sorption equilibrium, i.e. sorption-diffusion equilibrium, for a highly aggregated montmorillonitic soil containing 0.9% organic carbon for congeners 8, 52, 153 was determined to be 3, 5, and 10 months, respectively, using a gas-purge technique. For these sorbate-sorbent contact times soil-water sorption equilibrium partition coefficients on a soil-organic carbon basis (K oc) were measured and for these congeners ranged varied from 2.4 × 10 4 to 6.8 × 10 6 mL/g. Based on these data a correlation relation was derived, log K oc = 1.07 log K ow − 0.98, for estimating K oc from known values of octanol-water partition coefficient (K ow). From literature K ow values this regression equation was used to estimate K oc for 46 of the major PCB congeners in Aroclor 1242, 1254 and 1260.

Acetogenic capacities and the anaerobic turnover of carbon in a kansas prairie soil

To assess the anaerobic capacities of a temperate grassland soil, a Kansas prairie soil was incubated anaerobically as either soil-water (1:2) suspensions or as soil microcosms at 78% soil water-holding capacity. Prairie soil formed acetate and CO(inf2) as the two main initial carbonaceous products from the anaerobic turnover of endogenous organic matter. Metabolic capacities of soil suspensions and microcosms were similar. Rates of acetate formation from endogenous organic matter in soil-water suspensions incubated at 40, 30, and 15(deg)C approximated 3.3, 2.4, and 1.1 (mu)g of acetate per g (dry weight) of soil per h, respectively. Supplemental H(inf2) and CO(inf2) were subject to consumption with the apparent concomitant synthesis of acetate in both soil suspensions and soil microcosms. In soil microcosms, rates of H(inf2)-dependent acetogenesis at 30 and 55(deg)C were nearly equivalent. The uptake of supplemental H(inf2) was not coupled to methanogenesis under any condition examined. These anaerobic activities were relatively stable when soils were subjected to either aerobic drying or alternating periods of O(inf2) enrichment. On the basis of the formation of nitrogen (N(inf2)), denitrification was engaged during anaerobic incubation periods; nitrous oxide (N(inf2)O) was also formed under certain conditions. Although extended incubation of soil induced the delayed methanogenic turnover of acetate, acetate was subject to immediate turnover under either O(inf2)- or nitrate-enriched conditions. These studies support the following concepts: (i) obligately anaerobic bacteria such as acetogenic bacteria are stable to periods of aerobiosis and are active in the anaerobic microsites of oxic soils, and (ii) acetate synthesized in anaerobic microsites of oxic terrestrial soils constitutes a trophic link to both aerobic and anaerobic microbial communities.

Measurement of soil pH: Problems and solutions

Abstract pH measurements in soil systems present unique challenges in terms of the interpretation of the values obtained. The principles behind the glass/calomel electrode system are discussed as a backdrop to pH measurements in both pure solutions and soils. The influence of the liquid junction potential and salt concentration on the pH values of soil water suspensions are discussed in detail from which it emerges that the current practice of measuring soil pH in stirred soil suspensions is likely to result in the greatest errors being incurred. This is due to the large liquid junction potential of uncertain magnitude developed in such systems. Measurements in salt solutions, such as M KCl and 0.1 M CaCl2, reduce the magnitude of the liquid junction potential substantially and make the pH values obtained more reproducible and consistent. The position of the calomel electrode salt bridge is crucial in measuring soil pH and should always be positioned in the clear supernatant solution.

Ultrasonic dispersion of soil in water - the effect of suspension properties on energy dissipation and soil dispersion

The application of ultrasonic energy to soil-water suspensions for particle size analysis has been widely adopted and more recently applied to aggregate stability assessment. However, suspension properties have been reported to affect both the energy applied and the degree of aggregate breakdown. This paper investigates the effect of suspension concentration, suspension volume, gas saturation, depth of ultrasonic probe insertion and particle size distribution on the power (energy per unit time) applied by an ultrasonic probe. It also investigates the effect of suspension concentration, suspension volume, gas saturation and method of wetting the soil on the dispersion produced by ultrasonic energy applications. Where the power applied was expressed per unit soil mass, the quantity of dispersed material released by ultrasonic action was not affected by either the volume or concentration of the suspension. However, decreasing the dissolved gas concentration in suspension decreased the dispersion produced. The method of soil wetting significantly affected initial disruption but had no effect on the maximum amount of &lt;2 and &lt;20 �m material produced by sonification. The power applied by the ultrasonic probe was found to decrease with suspension temperature, increase with dissolved gas concentration and increase with the depth of probe insertion. Recommendations are made on the range of suspension properties that should be used for standard measurements of aggregate stability assessment using ultrasonic energy.

Comparison of alternative farming systems. III. Soil aggregate stability

AbstractQuantitative studies are needed to separate the real and supposed benefits of alternative farming practices. Our objective was to learn how conventional and alternative practices on adjacent farms in central Iowa affected the water stability of soil aggregates. We collected samples of Clarion loam (fine-loamy, mixed, mesic Typic Hapludoll) from adjacent 16 ha fields in fall 1990 and spring 1991. Aggregate stability was determined by wet-sieving and by measuring turbidity of soil-water suspensions. The combined effects of alternative practices resulted in greater water stability of soil aggregates, higher soil organic matter content, and lower bulk density compared with conventional practices. The components of the alternative system that were mainly responsible for these differences were: rotations that included oat and hay crops; ridge-tillage; and additions of 45 Mg/ha of a mixture of animal manure and municipal sludge during the first 3 years of each 5 year rotation. The more favorable soil physical conditions, shown by increased water stability of soil aggregates, presumably will improve soil water regimes and reduce long-term soil erosion losses from the alternatively managed fields.

Nonpoint sources

Nonpoint sources

Sorption and microbial degradation of naphthalene in soil-water suspensions under denitrification conditions

The microbial degradation of naphthalene under denitrification conditions in soil-water suspensions was dependent on solute partitioning between soil and water. Soil-associated naphthalene was in equilibrium with aqueous-phase solute, with the rate of naphthalene degradation being mixed order with respect to aqueous concentration. The rate of degradation was modeled by coupling Michaelis-Menten kinetics for aqueous-phase solute with an intraaggregate radial diffusion model for naphthalene sorption and desorption with soil. It was shown by modeling and confirmed by experiment, for the soil suspension particle sizes employed in these tests, that the naphthalene sorption-desorption process was reversible and rapid compared to the rate of microbial degradation. The maximum rate of microbial degradation was proportional to the soil-to-water ratio and independent of nitrate concentration for initial nitrate concentrations greater than several hundred micrograms per gram of soil. Nitrate reduction was described by utilizing the total mass removal of naphthalene and a stoichiometric conversion factor. A coupled solute desorption-degradation model is presented for microbial degradation of hydrophobic organic compounds that are desorbed from porous soil aggregates, assuming sorbed solute is inaccessible to microorganisms and that the rate of solute release from the solid is rapid compared to the rate of retarded intraaggregate diffusion.

Surfactant Solubilization of Polycyclic Aromatic Hydrocarbon Compounds in Soil-Water Suspensions

Polycyclic aromatic hydrocarbons (PAH) are hydrophobic compounds readily sorbed onto soil; the addition of surfactants to soil-water systems may solubilize PAH compounds by incorporation of the PAH in surfactant micelles. The solubilization of anthracene, phenanthrene and pyrene was evaluated in soil-water suspensions with several nonionic and anionic surfactants. The most effective surfactants were nonionic octyl- and nonyl- phenylethoxylates with 9 to 12 ethoxylate units. At soil-water mass ratios of about 1:7 to 1:2, greater than 0.1% by volume surfactant dose was required in order to initiate solubilization, with doses of 1% by volume resulting in 70 - 90% solubilization. The soil-water partition coefficient for anthracene was reduced by two orders of magnitude with 1% by volume phenylethoxylate surfactant. The solubilization of anthracene, phenanthrene and pyrene in soil-water systems occurs at surfactant doses, [Csurf], much greater than the clean water surfactant critical micelle concentration. A surfactant-soil solubilization coefficient is derived as Caq(s)/S, where Caq(s) is the observed aqueous solubility of PAH in the presence of surfactant, and S is the specific sorption of PAH onto soil. Caq(S)/S is shown to be proportional to Csurf-CMCeff, where CMCeff is the effective critical micelle concentration in the presence of soil. Additional information is needed on the sorption characteristics of surfactant on soil in order to describe more fully the PAH-surfactant solubilization process.

Fluorite Solubility in Limed Acid Sandy Soils

AbstractThe solubility of fluorite (CaF2) in acid sandy soils was investigated via Ca2+ and F‐ activity measurements. Fluorite was added to three acid sandy soils previously limed to various pHs between 4.0 and 7.8. The equilibrium Ca2+ and F‐ activities in soil water suspensions were determined and compared to published and calculated ion activity products. The data indicated that CaF2 was the solid phase controlling F‐ activity in the suspension pH range of 5.5 to 7.0. However, below pH 5.0, the F‐ activity indicated supersaturation with respect to CaF2. The pKap for CaF2 prepared in the laboratory was calculated to be 10.50. The results revealed that liming fluoride‐contaminated acid sandy soils may reduce F‐ in the soil solution by precipitation of CaF2.

Título en español.

Durante un reconocimiento de los suelos hecho recientemente en el Valle de Lajas, se observaron tumores o promontorios de los cuales mana una suspensión de suelo en agua. El área en cuestión está ubicada al noroeste de la laguna Guánica y en general se halla cubierta de una vegetación halofítica. Las baterías de pizómetros localizadas en el área indican una alta presión ascendente en toda esta área. Durante épocas de lluvia o cuando se está regando la caña en la vecindad del área afectada, la presión es tan fuerte que el agua asciende a través del suelo trayendo gran cantidad del mismo en suspensión. Los estudios sobre la composición mecánica del suelo de los tumores o promontorios y del suelo de sitios cercanos a los promontorios indican que la mayoría de las partículas en movimiento consiste de arcilla, con cantidades menores de limo y arena muy fina. Sin embargo, este movimiento está limitado, en su mayor grado, a las 48 pulgadas superficiales. Los niveles de materia orgánica y nitrógeno son más altos en el suelo de los sitios cercanos a los tumores que en el de los tumores mismos, pero en ambos casos hay una marcada reducción bajo las primeras 24 pulgadas de profundidad. La capacidad para el intercambio de cationes de los suelos de ambos sitios es moderadamente alta y predomina el sodio intercambiable con poco calcio, magnesio y potasio. El alto porcentaje de sodio intercambiable, conjuntamente con el bajo contenido de sales totales del suelo de los tumores, según lo indica la conductividad de los extractos del suelo, explican el alto grado de dispersión en que naturalmente se encuentra este suelo. Los porcentajes de saturación de humedad de estos suelos son extremadamente altos y en muchos casos pasan del cien por ciento. Esto se debe a su alto contenido de arcilla con estructuras cristalinas del tipo 2:1, la cual se expande y contrae.

Seasonal changes in soil salinity at Tintinara, South Australia

Measurements have been made of seasonal changes in soil salinity and soil water at Tintinara in the Coonalpyn Downs of South Australia. Five separate sites representing different soil profiles were selected for study. All soils contained saline water-tables and four of the five had varying depths of sand over finer-textured subsoils; the fifth soil was fine-textured throughout. The investigations were conducted over a 12-month period, salinity and water determinations being made every 8 weeks. Data on soil salinity are presented as: (1) Conductance of soil pastes, determined on replicates and statistically analysed. (2) Percentage of total soluble salts, derived from conductivities of 1:5 soil-water suspensions determined on composite sample. (3) Percentage of chlorides expressed as NaCl, determined on composite samples. All sets of data show that soluble salts moved upwards through the soil profile during spring and summer and down during the rainy season. Where the summer water-table was within 4 ft of the surface large amounts of saline material accumulated in the top inch of soil; in one case there was an increase of from 0.019 per cent. NaCl in winter to 3.2 per cent. in summer. Data are also presented showing that the salinity of the ground-waters increased to a maximum of 2-3.5 per cent. total salts in spring or summer. Seasonal soil water changes are recorded and discussed in relation to salinity changes. The climatic, soil, and ground-water factors influencing salt movements are discussed and the agronomic implications of seasonal salinity changes are stressed. Suggestions on the basis of the results obtained are made respecting the sampling of saline soils.

The errors involved in pH determination in soils.

Errors of replication of pH values of 1: 5 soil-water suspensions are shown to differ significantly between routine observers and to be larger when duplicate determinations are made upon different days rather than on the same day. For the routine technique employed in these laboratories the 5 per cent. fiducial limits of a single determination do not rise above ±0.09 pH units due to the above causes. Errors due to soil variation over small distances in the field may show 5 per cent. limits as high as ±1.3 pH units. The causes of the errors which may arise within the measuring system are considered and details are given of errors in soil systems due to the suspension effect and to lack of equilibrium between the soil and aqueous phases. Absence of equilibrium may give differences as high as 1.0 unit when measurements are made upon sedimenting alkaline suspensions; no errors occur due to this cause below pH 5. The presence of salts does not modify the differences observed. The suspension effect is relatively small. It is recommended that where possible, pH measurements be made upon soil systems with the glass electrode in the suspension and the reference electrode in the dialysate or supernatant liquid. The description and use of a suitable electrode arrangement is given in an appendix. Generally pH measurements can be considered to no greater accuracy than ±0.1 unit and quite often circumstances do not justify this precision.

Indicator Significance of Some Shrubs in the Escalante Desert, Utah

1. Chemical effects of native plants on desert soils were studied. 2. Data are presented which indicate that the pH values of saturated soil pastes and 1: 10 soil-water suspensions, particularly of the surface soil, generally are higher under shadscale and invariably are higher under greasewood than in the adjacent bare areas or under plants such as sagebrush. 3. The pH value of the bare soil is usually highest in greasewood and lowest in sagebrush areas, with shadscale areas either intermediate or similar to those of sagebrush. 4. The exchangeable-sodium percentage of the soil is somewhat higher under shadscale and very much higher under greasewood than in the adjacent bare areas or under other plants such as sagebrush. 5. The soluble-salt content of the soil was appreciably higher under shadscale and greasewood than in adjacent bare soil or under other shrubs. 6. The increases in pH, exchangeable-sodium percentage, and, to some extent, salt content of soil which result from the growth of greasewood are associated with the size and presumably the age of the plant. That is, the larger the plant, the higher the pH, soluble-salt content, and exchangeable-sodium percentage as compared with soil in adjacent bare areas or under plants such as sagebrush. 7. Chemical analyses of plant parts and organic litter in relation to the zonation of pH and exchangeable-sodium percentage are discussed; and a mechanism which would account for the increase in pH, soluble salts, and exchangeable sodium is proposed.