Ca Sorption Research Articles

A study of silver sorption from aqueous solutions by solid-phase modified poly(n-thiocarbamoyl-3-aminopropyl silsesquioxane)

Sorption properties of poly(3-aminopropylsilsesquioxanes) modified with thiourea groups were studied to predict the possibility of using poly(n-thiocarbamoyl-3-aminopropylsilsesquioxanes) (TCPS) in separation and concentration techniques. The sorbents were synthesized by the sol-gel method followed by solid-phase modification using ammonium thiocyanate or thiosemicarbazide as modifiers. It was established that sorbents with the highest content of thiocarbamide functional groups (modified with ammonium thiocyanate) enabled quantitative and selective recovery of Ag (I) from strongly acidic solutions. The degree of Ag (I) recovery slightly decreases at pH>5, which is associated with increasing competitive sorption of Cu (II), Ca (II), Mg (II), Mn (II) and Fe (III). The highest recovery of Ag (I) for TCPS-0.63 and TCPS-0.68 was achieved at pH ranging from 4 to 8 (degree of recovery was 55–68% (TCPS-0.63); 21–26% (TCPS-0.68 )); at the same time, there was a predominance of competitive sorption of Ca (II), Mg (II), Mn (II), and Fe (III). Based on the aforementioned, it can be concluded that the sorbents modified with ammonium thiocyanate are characterized by high concentration of attached thiocarbamide groups that make them suitable for the selective recovery of Ag (I) ions from multicomponent solutions at pH 1–2.

Monitoring and inverse modelling of hydrogeochemical processes during managed aquifer recharge in Southwestern Bangladesh

Managed Aquifer Recharge (MAR) has been applied as Aquifer Storage, Transfer, and Recovery (ASTR) to provide fresh drinking water for local communities at 99 locations in southwest Bangladesh since 2009. Aerobic freshwater from ponds is filtered and subsequently infiltrated into anaerobic shallow brackish aquifers. At approximately 45% of these sites, relatively higher levels of Fe and As were observed in recovered water, which requires a better understanding of the hydrogeochemical processes that govern the Fe, Mn, and As levels in these MAR systems. Therefore, two representative sites with As above (74 ± 11 μg/L at site GMF11) and below (19 ± 6 μg/L at site JJS91), the Bangladesh drinking water standard of 50 μg/L were weekly monitored on hydrochemical changes from Dec 2017 to Dec 2018. Hydrogeochemical processes occurring during storage were quantified with inverse and forward geochemical mass balance models developed with PHREEQC. The following processes explained the changes in water quality: 1) mixing of infiltration water with native groundwater (∼90%:∼10%); 2) consumption of O2 by a) dissolved Fe2+ that subsequently precipitated as Fe(OH)3 at GMF11 and by b) dissolved and sedimentary organic matter (OM) at site JJS91; 3) reduction of SO4 coupled to the oxidation of OM at both sites; and 4) mixing corrosion and freshening induced cation-exchange (Ca sorption; Na desorption) triggering calcite and siderite dissolution at GMF11. Dissolution of these carbonate minerals occurred to a lesser extent at JJS91, while cation exchange (Na sorption; Ca desorption) suggested that the freshwater was displaced by brackish groundwater because of inadequate infiltration at JJS91. Distinct pH values in recovered water reflected the dominance of Fe2+ versus OM oxidation. Siderite dissolution led to 4.3 ± 3.1 and 1.0 ± 0.5 mg/L Fe in recovered water at GMF11 and JJS91, respectively. Elevated As and Mn levels in recovered water were caused for max. 20% by mixing with native groundwater and for min. 80% by mobilization processes, mainly by desorption of As from Fe-oxides and by the dissolution of Mn-bearing siderite. Recommendations are provided to improve recovered water quality.

Dyeing of Waste Cotton Fabric as Biosorbent of Heavy Metals from Aqueous Solution

Heavy metal pollution is a major problem in the environment. The impact of toxic metal ions can be minimized by different technologies. But among them, adsorption was found to be very efficient and common due to the low concentration of metal uptake and economically feasible properties. Cotton fabrics are of low cost and widely used, and very promising for the future. These are available in abundant quantity, are cheap and have low or little economic value. Waste cotton fabrics (WCF) are used in different metal detoxifications in water and waste water as absorbent for the sorption of Ca (II)and Mg (II) ions from aqueous solution. . The cotton fabrics were characterized by before and after the adsorption process. Cotton fabric has been used in two directions removal of heavy metals and dyeing .The results of maximum adsorption capacity was 42 mg g−1 for Ca (II) and 35 mg g−1 for Mg (II) at optimal pH (11). The dyeing behavior of the WCF before and after adsorption of metals (WCF-Ca (ll)) with reactive yellow145 (RY 145 ) has been studied and the build up of dye, measured as exhaustion percentage (E%) and ( K/S ). The overall fastness properties to light, perspiration, washing for the colored samples are ranging from very good to excellent. in this study shows significant benefit of eco–friend and cost-efficiency for removal of metals in potential river metal pollutions comparing with traditional adsorbents.

Surface complexation of Ca and competitive sorption of divalent cations on montmorillonite under alkaline conditions

In the geological disposal system of high-level radioactive and transuranic wastes, an increase in Ca concentration (arising from the alteration of cementitious materials) could affect the retention of other radionuclides due to competitive sorption. Batch sorption experiments were performed to investigate the sorption behavior of Ca and its competition with other divalent cations (Sr and Ni) on the edge sites of Kunipia F montmorillonite under alkaline conditions. Ca sorption increased with pH (when pH > 8) and it was more pronounced under high ionic strength conditions, indicating that Ca formed a surface complexation with the edge sites. The sorption behavior of Sr was similar to that of Ca. Meanwhile, the Ni sorption increased with pH, when this was >6 (for lower pH values than in the case of Ca and Sr), Ni tends to form surface complexes more readily than Ca and Sr. The results of a fitting analysis conducted based on the 2-site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model indicate that Ca and Sr sorb onto the ≡SW2OH sites and Ni sorbs onto the ≡SSOH sites. Additionally, the sorption of the CaOH+ and SrOH+ ionic pair species by cation exchange was implied at pH ~ 12. Competitive sorption experiments were also carried out to evaluate the effect of Ca on the sorption of Sr and Ni. Sr sorption decreased with Ca concentration in the alkaline pH region, whereas Ni sorption was not affected by Ca concentration. Overall, these results indicate that Ca and Sr sorb onto the same sites, while Ca and Ni sorb onto different sites; additionally, the competitive sorption seemed to depend on chemical similarities (e.g., hydrolysis behavior). The sorption model parameters obtained from the single-element batch sorption experiments successfully reproduced the results of the competitive sorption experiments.

Aggregation behaviors of sonicated tapioca starch with various strengths of Hofmeister salts under pre- and post-ultrasonic treatment

Tapioca starch granule was treated with various ionic strengths (IS 0.3, 0.6 and 0.9) of Ca+2 and Na+ ions prior to and following ultrasonication (400 W, 10 min at 60 °C). As a result, scanning electron microscopy (SEM), X-ray differaction, and fourier transform infrared spectroscopy (FT-IR) indicated the sorption of Ca+2 and Na+ ions at the granule surface and facilitation of their penetration into the interior of the starch granule in post ultrasonication compared with pre ultrasonication. Calcium and sodium ions incorporated into the granules affected the starch thermal stability (DSC) and pasting properties (RVA). Contrary to Na+ ions, Ca+2 reduced crystallinty (%) and Ratio 1047/1022 cm−1. The Ca+2 which was added after sonication led to an increase in pasting viscosity, which was reduced in the presence of Na+ ions (particularly after sonication) due to increased crystallinty (%)in starch granules.

Cationic collector conformations on an oxide mineral interface: Roles of pH, ionic strength, and ion valence

The adsorption of collectors at the solid–liquid interface to impart and enhance a mineral’s hydrophobicity is essential to mineral froth flotation. During adsorption, collectors must conform on the mineral surface with their nonpolar tails facing the aqueous phase to enhance the mineral’s hydrophobicity. This conformation is subject to conditions prevailing in the aqueous phase, such as the pH and nature of ions present. Since this conformation can affect the degree of hydrophobicity, molecular-level understanding of this phenomenon also is beneficial to improve hydrophobicity via adsorption layer manipulation. In this study, the structural conformational changes of cationic collector behenyl trimethyl ammonium chloride on a silica surface were studied using an in situ quartz crystal microbalance with dissipation (QCM–D) coupled with contact angle measurements. Two electrolytes, NaCl and CaCl2, were used to investigate the effects of ionic strength (IS) and ion valence. The QCM–D data at different pHs, both with and without a background electrolyte, revealed complex changes in the collector conformations and structure of the adsorption layer. Two distinct adsorption characteristics were also observed depending on ion valence: the presence of NaCl led to a thin, rigid layer (low dissipation), while a thick, soft layer (high dissipation) formed in the presence of CaCl2. Meanwhile, the increase in the IS of NaCl resulted in a more rigid adsorbed layer on the silica at both pHs. For CaCl2, the adsorbed layer became softer and highly dissipated at pH 5.7, whereas it became less soft at pH 10. This inconsistency in the rigidity with IS in the presence of CaCl2 was likely due to the strong sorption of Ca(OH)+ at a high pH. Interestingly, the imparted hydrophobicity decreased as the IS increased, with a greater extent of reduction at a high pH regardless of ion type. Moreover, it had a strong relationship with the ratio of dissipation and frequency change, with a lower contact angle at a higher ratio due to greater imbibition of water molecules and/or hydrated cations. These findings have implications for silica flotation systems wherein Ca2+ and Na+ cations are present in appreciable amounts.

Ion reactivity of calcium-deficient hydroxyapatite in standard cell culture media

Solution-mediated surface reactions occur for most calcium phosphate-based biomaterials and may influence cellular response. A reasonable extrapolation of such processes observed in vitro to in vivo performance requires a deep understanding of the underlying mechanisms. We therefore systematically investigated the nature of ion reactivity of calcium-deficient hydroxyapatite (CDHA) by exposing it for different periods of time to standard cell culture media of different chemical composition (DMEM and McCoy medium, with and without osteogenic supplements and serum proteins). Kinetic ion interaction studies of principal extracellular ions revealed non-linear sorption of Ca 2+ (∼50% sorption) and K + (∼8%) as well as acidification of all media during initial contact with CDHA (48 h). Interestingly, inorganic phosphorus (P i) was sorbed from McCoy medium (∼50%) or when using osteogenic media containing β-glycerophosphate, but not from DMEM medium. Non-linear sorption data could be perfectly described by pseudo-first-order and pseudo-second-order sorption models. At longer contact time (21 days), and with frequent renewal of culture medium, sorption of Ca 2+ remained constant throughout the experiment, while sorption of P i gradually decreased in McCoy medium. In great contrast, CDHA began to release P i slowly with time when using DMEM medium. Infrared spectra showed that CDHA exposed to culture media had a carbonated surface chemistry, suggesting that carbonate plays a key role in the ion reactivity of CDHA. Our data show that different compositions of the aqueous environment may provoke opposite ion reactivity of CDHA, and this must be carefully considered when evaluating the osteoinductive potential of the material.

Sorption of bivalent ions by a fibrous chelating ion exchanger and the structure of sorption complexes

Acid–base properties, kinetics and equilibrium sorption of Ca 2+, Cu 2+, Pb 2+, Cd 2+, Ni 2+ and Mn 2+ ions on fibrous chelating ion exchanger FIBAN X-1 containing iminodiacetic group have been studied. The structural and electronic characteristics of ion-polymeric complexes have been obtained by non-empirical quantum chemical calculations. The rate of sorption processes was extremely high; the half-process time of the sorption process was equal to 24 s. The isotherms of ion sorption are described by the Langmuir equation what allowed deriving equation for dependence of the distribution coefficient on the ion concentration and accurate extrapolation of these value to zeros concentration. The concentration range the Henry’s law applicability was determined theoretically and proved experimentally. The sorption selectivity series in a neutral medium was Cu > Ni > Pb > Cd > Mn > Ca. The same series follows the pH at which the sorption begins. Complete extraction of all heavy metal ions occurs at pH > 4. The quantum chemical calculations have shown that the selectivity of sorption is determined by the number of coordination bonds between the cation and fixed anion, the length of the bonds and degree of their covalence. The way of combining of these factors is individual for each specific cation and can be revealed by quantum chemical calculations.

Sorption of alkaline earth metal ions Ca 2+ and Mg 2+ on lyocell fibres

Ca 2+ and Mg 2+ content of cellulose fibres is of relevance for a wide range of applications e.g. textile processing, pulp/paper, food. Sorption of Ca 2+ and Mg 2+ ions were found on lyocell type regenerated cellulose fibres. Higher affinity was found for Ca 2+ ions compared to Mg 2+ ions. At pH 9, fibre saturation was observed at a calcium binding capacity of 18–20 mmol/kg. A carboxylic group content of 18 mmol COOH per kg fibre material was determined based on the Methylene Blue absorption. This indicates a 1:1 molar stoichiometry between the carboxylic groups present in the fibres and the bound Ca 2+ ions. Thus it is proposed that the salt in fibre shows the general composition (Cell-O − Ca 2+ X −), X − being an anion bound in the salt to achieve charge neutrality. The sorption of Ca 2+ also can be demonstrated by complex formation with 1,2-dihydroxy-9,10-anthraquinone (alizarin) which forms a red-violet Ca 2+-complex. Colour fixation thus can be used as an indicator for the Ca 2+-ions bound in the fibre.

Sorption of Ca 2+, Mg 2+, Na + and K + by clay minerals

The objective of this work is to compare the removal of Ca 2+, Mg 2+, Na + and K + by kaolinite and carbon-materials prepared from pyrolysis of sewage sludge mixed with kaolinite. Three carbon-based materials were prepared by pyrolysis of the sewage sludge, the sludge with a 5% of kaolinite and the sludge with a 10% of kaolinite at 650°C, leading to SL-650, SL-650–5 and SL-650–10, respectively. The addition of kaolinite to the sewage sludge increased the cation exchange capacity of the final carbon-materials prepared by pyrolysis process. The increment raised to 37.6% and 45.3%, respectively, in the SL-650–5 and SL-650–10 with respect to the SL-650. The kaolinite and the carbon-based materials were mixed with solutions with different concentrations of Ca 2+, Mg 2+, Na + or K + (100 and 200 mg L −1). Results shown that the highest was pH, highest was the percentage of Ca 2+ and Mg 2+ removal by kaolinite and carbon-based materials. Nevertheless, the removal of Na + and K + was negligible. Results show that kaolinite and carbon-materials prepared by pyrolysis of mixtures of sewage sludge and kaolinite can be effective to the to reduce the hardness of water.

Influence of pH on the interlayer cationic composition and hydration state of Ca-montmorillonite: Analytical chemistry, chemical modelling and XRD profile modelling study

The hydration state of a <2 μm fraction of Ca-saturated SWy-2 montmorillonite was characterised after rapid equilibration (3 hours) under pH-controlled conditions (0.1–12.6 pH range). The solution composition was monitored together with the interlayer composition and X-ray diffraction (XRD) patterns were recorded on oriented preparations. Experimental XRD patterns were then fitted using a trial-and-error procedure to quantify the relative proportions of layers with different hydration states. The montmorillonite is mostly bi-hydrated in basic and near-neutral conditions whereas it is mostly mono-hydrated at low pH. The transition from the bi-hydrated to the mono-hydrated state occurs through very heterogeneous structures. However, the proportion of the different layer types determined from XRD profile modelling and that derived from chemical modelling using Phreeqc2 code strictly coincide. This correlation shows that the hydration modification is induced by a H 3O +-for-Ca 2+ exchange at low pH, the two species being distributed in different interlayers. This layer-by-layer exchange process occurs randomly in the layer stack. Under alkaline conditions, results from XRD profile modelling and from near infrared diffuse reflectance spectroscopy (NIR-DRS) clearly demonstrate that there is no CaOH +-for-Ca 2+ exchange at high pH. The apparent increase in Ca sorption in smectite interlayers with increasing pH is thus probably related to the precipitation of Calcium-Silicate-Hydrate (CSH) phases, which also accounts for the decrease in Si concentration under high-pH conditions. This precipitation is thermodynamically favoured.

Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P CO 2 , and pH at 25°C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P CO 2 , and pH at 25°C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na + with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca 2+ in solution will decrease K ds. At the same time, to the extent that high P CO 2 s increase Ca 2+ levels, K ds will decrease with CO 2 levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well. U-carbonate complexation leads to desorption). This indirect CO 2 effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K ds.

Effect of Na+ on Ca2+-Binding on the Plasma Membrane of Barley Mesophyll Cells: an Electrophoretic Study

It is well known that Ca ions play an important role in mitigating the detrimental effects of high NaCl on plant growth (LaHaye and Epstein 1969, Greenway and Munns 1980, Kent and Lauchli 1985). Several reports suggest that external Na can displace Ca from the binding sites on the outer plasma membrane (PM) surface, which may be one of the primary responses to salt stress, leading to a disruption of membrane integrity and leakage of intracellular solutes; the mitigative effects of supplemental Ca may be related to the maintenance of membrane-associated Ca (LaHaye and Epstein 1969, Cramer et al. 1985, Lynch et al. 1987, Colmer et al. 1994). Cramer et al. (1985) and Lynch et al. (1987) attempted to determine whether Na displaced Ca from the plasma membrane of root cells of salt-sensitive glycophytes but failed to get any conclusive evidence, because they used an unsuitable fluorescent probe CTC for measurement of Ca bound to the cell surface (Lynch and Lauchli 1988). Recently, Yermiyahu et al. (1994), using Ca sorption technique, reported that Na ions reduced the amount of Ca sorbed to plasma membrane vesicles of salt-sensitive melon root cells. Our electrophoretic studies showed that Na ions could specifically (and nonspecifically) cause desorption of Ca from the outer plasma membrane surface of salt-sensitive tobacco cells (Murata et al. unpublished

Effects of simple organic acids on sorption of Cu 2+ and Ca 2+ on goethite

Effects of phthalic acid and chelidamic acid on sorption of Cu 2+ and Ca 2+ were examined in goethite ( α-FeOOH)/water suspensions. Single-sorbate sorption of Cu 2+ and Ca 2+ on goethite over a wide range of conditions (pH, I, sorbate/sorbent ratio) was described reasonably well by the Generalized Two Layer Model (GTLM). In the presence of the organic acids, particularly chelidamic acid, sorption of Cu 2+ was significantly enhanced at low pH values. Sorption of Cu 2+ in Cu 2+-phthalic acid and Cu 2+-chelidamic acid binary systems was quantitatively reproduced over a wide range of conditions (pH, Cu/organic acid concentration ratio) with the GTLM by using a unique set of Cu 2+ -organic acid ternary surface complexes for each binary system. Sorption of Ca 2+ remained unaffected in the presence of phthalic acid, but decreased in the presence of chelidamic acid due to the formation of nonsorbing Ca 2+-chelidamic acid solution complexes. These effects were predicted reasonably well by the GTLM. Although the exact sorption mechanisms are unconfirmed, available data suggest that enhanced sorption of Cu 2+ resulted from sorption of Cu 2+-organic acid complexes in a ligand-like manner, i.e., involving oxide surface sites and carboxyl groups of the organic acids not participating in Cu 2+ complexation. Results from this work suggest that the reported mineral-humate-cation interactions are consistent with the formation of cation-humate ternary surface complexes.

Instability of SiO2 Colloids and Sorption of Ca2+ Ions

AbstractSiO2 sols were made unstable by addition of Ca2+ ions. The resulting states of instability were classified as gelation, flocculation, and precipitation by means of observation, by checking the Tyndall effects on the supernatant or suspending solution, as appropriate, and by measuring the apparent densities of flocculated mass. The concentrations of free Ca2+ ions left in solution were measured by means of a Ca2+ ion selective electrode. The amounts sorbed onto SiO2 particles were then calculated by material balance. It was found that while the amount sorbed dictates the limit of stability, the SiO2 concentration in the mixture is an important factor deciding the state of instability. Depending on the SiO2 concentration, there were two distinct flocs with the apparent floc density of 6 ± 1 and 12 ± 1 mg SiO2/ml.

Sorption of calcium and sulfate by sandy soils from flue‐gas desulfurization gypsum and phosphogypsum

Abstract Increasing concern on sulfur dioxide (SO2) pollution has prompted many coal‐fired electric power generating plants to install SO2 scrubbing units which generate large amounts of flue‐gas desulfurization gypsum (FGDG). This source of gypsum can be an excellent amendment to supply calcium (Ca) and/or sulfate (SO4) to sandy soils which are deficient in these nutrients. In this study, reactions of FGDG, phosphogypsum (PG), reagent grade calcium sulfate (CaSO4), and calcium chloride (CaCl2) were investigated using a Candler fine sand (uncoated, hyperthermic, Typic Quartzipsamment), a typical well‐drained deep sand, and a Pineda fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf), a typical shallow poorly drained sand. Concentrations of various water‐soluble elements were similar in FGDG as well as PG, except the latter contained 11 and 15 g/kg phosphate (PO4) and fluoride (F), respectively, while these ions were undetectable in the former. In batch‐equilibration (4 h) technique using soilrsolution ratio of 1:2, pH of equilibrium solution decreased by 0.2 and 0.8 units, while ionic strength increased by 5‐ and 15‐fold in the Pineda and Candler sand, respectively, when using FGDG or PG solution as compared to using water for equilibration. Sorption of Ca by the two soils varied from 0.56 to 0.71 Cmolc/kg regardless of source of gypsum. As compared to sorption of SO4, that of Ca was greater by 2.5‐ and 2.6 to 4.3‐fold by the Candler and Pineda sand, respectively. With the exception of SO4 sorption by the Pineda sand, this study demonstrated that the reactions of FGDG with these sandy soils were very similar to those of PG or reagent grade CaSO4. Since much of the past research on agricultural use of gypsum was done using PG and CaSCU, we could conclude that similar response could be expected with use of FGDG.

Multicomponent cation exchange including alkalinization/acidification following flow through sandy sediment

Cation exchange associated with displacement of aqueous solutions is a complex physicochemical process. In this study column experiments were combined with numerical modeling to determine the controlling chemical processes for natural systems at near‐neutral pH. The experimental results showed the importance of two processes in addition to exchange between free cations: exchange of monovalent metal‐bicarbonate complexes and proton buffering. The former primarily results from changes in CO2 pressure, whereas the latter results from changes in aqueous pH. Neglecting these two processes had little effect when modeling the cation elution curves. However, the pH and alkalinity breakthrough curves were poorly fit, since nonreactive behavior of these variables was assumed. Inclusion of either specific sorption of Ca and Mg hydroxy complexes or transfer of Al hydroxy species to or from the exchanger sites resulted in a better fit. It is shown theoretically that the two additional processes have a major impact on calcite equilibria when considering calcareous systems.

Impact of Bacterial Biomass on Contaminant Sorption and Transport in a Subsurface Soil

The objective of this study was to investigate the impact of bacterial biomass on the sorption and transport of three solutes (quinoline, naphthalene, and Ca) in a subsurface soil. Miscible displacement techniques were employed to measure sorption and transport of the above compounds during steady, saturated water flow in sterile and/or bacterium-inoculated soil columns. The soil was inoculated with either a quinoline-degrading bacterium, Pseudomonas sp. 3N3A isolate, or its mutant isolate, B53, which does not degrade quinoline. In soil columns inoculated with the B53 and 3N3A isolates, quinoline sorption was reduced by about 60 and 20%, respectively. In contrast, Ca sorption was minimally reduced, which indicated that biomass did not significantly alter the cation-exchange capacity of the soil. Biomass impacts on sorption were solute specific, even when the sorption mechanism for both quinoline and Ca was similar. Thus, the differential response is attributed to biomass-induced changes in quinoline speciation; an increase in pH at the sorbent-water interface would result in a larger proportion of the neutral species and a decrease in sorption. Sorption of naphthalene was reduced by about 30%, which was attributed to accessibility of hydrophobic regions. Minimal biosorption of all solutes indicated negligible biofacilitated transport. Alteration of the soil surfaces upon addition of bacterial biomass reduced sorption of quinoline and naphthalene, thereby enhancing transport.

Chemical Effects of Repeated Equilibrations of Variable-Charge Soils with Phosphogypsum Solution

Surface applications of gypsum or phosphogypsum (PG) to acid soils have resulted in improved crop growth as a result of amelioration of subsoil acidity; however, the mechanism involved is not clearly understood. The effects of repeated equilibrations of soils with PG solution on soil solution and extractable ion composition were investigated in this study. Cultivated and woodland Cecil Bt horizon (0.6–0.8 m depth) soil materials (clayey, kaolinitic, thermic Typic Kanhapludult) were sequentially equilibrated five times in the laboratory with 2 g L−1 PG solution at 40% (v/w) soil moisture content. With an increase in the number of equilibrations, the pH of the displaced soil solution increased in the cultivated soil, and initially decreased and then increased slightly in the woodland soil. On the basis of predictions of saturation indices by using the MINTEQ speciation model, the composition of displaced soil solutions showed no evidence of CaSO4 precipitation regardless of the number of equilibrations. The cumulative sorption of Ca and SO4 after five equilibrations with PG solution was 2.80 and 1.69 cmolc kg−1, respectively, in the cultivated soil, and 4.43 and 4.41 cmolc kg−1, respectively, in the woodland soil. For the cultivated soil, 83 and 71% of the final cumulative sorption of Ca and SO4 was reached after the second equilibration while, for the woodland soil, the values were 45 and 46%, respectively. The retention of Ca and SO4 following the equilibration of the soils with PG solution was attributable to a salt-sorption mechanism, as evident from the sorption of Ca and SO4 in equal proportions with negligible release of cations or anions into the displaced soil solution. Successive equilibration of the soils with PG solution resulted in a decrease in extractable Al, an increase in extractable Ca, and a net increase in cation-exchange capacity. These changes were much greater in the woodland soil than in the cultivated soil.

EFFECTS OF CONCENTRATION OF SUPPORTING SOLUTION ON SOIL SOLUTION ALUMINUM IN VARIABLE CHARGE SOILS

Characterization of soil solution aluminum (Al) as a predictor of Al toxicity to plant roots growing in acid soils has received increasing attention during recent years. However, release of Al into soil solution may depend on several soil properties and the concentration of supporting solution employed for displacing soil solution. For this study we obtained subsoil (0.6–0.8 m depth) samples of Bladen (Typic Albaquult) and Wedowee (Typic Hapludult) series from Georgia and a sample Podzolic (Epiaquic Haplustult) soil from Australia. Soil solution was displaced by centrifugation following 4 h equilibration with deionized water or solutions of CaCl2.2H2O at 0.1, 1, 10, or 100 mM concentration. The pH and ionic strength of displaced soil solutions were not influenced by the concentration of CaCl2 in the supporting solution from 0 to 1.0 mM. However, the pH of soil solutions decreased while the ionic strength increased when the concentration of CaCl2 in the supporting solution was increased to 10 or 100 mM. Concentrations of Al in soil solutions displaced by water or 0.1–1.0 mM CaCl2 solutions were much greater in predominantly smectitic-Bladen soil than in kaolinitic-Podzolic or Wedowee soils, although exchangeable Al was 78% greater in the Podzolic soil and only 36% lower in the Wedowee soil than in the Bladen soil. Concentration of Al in soil solution rapidly increased along with a decrease in pH and an increase in ionic strength when soil solution was displaced using 10 or 100 mM CaCl2 solutions. The sorption of calcium (Ca) was very similar to the release of other cations into soil solution in the Bladen soil, but it was greater than the release of other cations into soil solution in the Podzolic and Wedowee soils. Greater sorption of Ca from the supporting solution in the Podzolic and Wedowee soils than in the Bladen soil, due to an increase in surface charges, may be partly responsible for lower concentration of Al in displaced soil solution in the former soils than in the latter soil.