Equilibrium Solution pH Research Articles

Impacts of soil organic matter, pH and exogenous copper on sorption behavior of norfloxacin in three soils

Norfloxacin sorption and the factors (soil organic matter (SOM), pH, and exogenous copper (Cu) influencing the sorption were investigated in a black soil (soil B), a fluvo-aquic soil (soil F), and a red soil (soil R). With increasing norfloxacin concentrations, sorption amount of norfloxacin increased in both the bulk soils and their SOM-removed soils, but the sorption capacity of SOM-removed soils was higher than that of their corresponding bulk soils, indicating that the process of norfloxacin sorption in soil was influenced by the soil properties including SOM. The sorption data in all bulk soils and SOM-removed soils were fitted to Freundlich and Langmuir models. The correlation coefficients suggested that the experimental data fitted better to Freundlich equation than to Langmuir equation. Furthermore, the data from soil F and SOM-removed F could not be described by Langmuir equation. The norfloxacin sorption amount decreased in soil B and soil F, whereas it increased in soil R as solution pH increased. The maximum K D and K OC were achieved in soil R when the equilibrium solution pH was 6. The norfloxacin sorption was also influenced by the exogenous Cu 2+, which depended on the soil types and Cu 2+ concentrations. With increasing Cu 2+ concentrations in solution, generally, sorption amount, K D and K OC for norfloxacin in soils increased and were up to a peak at 100 mg/L Cu 2+, and then the sorption amount decreased regardless of norfloxacin levels.

Influence of Soil Solution Salinity on Molybdenum Adsorption by Soils

Molybdenum (Mo) adsorption on five arid-zone soils from California was investigated as a function of equilibrium solution Mo concentration (0-30 mg L), solution pH (4-8), and electrical conductivity (EC = 0.3 or 8 dS m). Molybdenum adsorption decreased with increasing pH. An adsorption maximum was found near pH 4. Molybdenum adsorption as a function of solution pH was independent of solution salinity from pH 4 to 8. Molybdenum adsorption for five soils as a function of solution Mo concentration conformed to the Langmuir adsorption isotherm equation. The Mo adsorption maxima obtained with the Langmuir equation for both ECs were not statistically significantly different at the 95% level of confidence, with the exception of one soil. The constant capacitance model, a surface complexation model, was able to describe Mo adsorption as a function of solution Mo concentration and solution pH. Molybdenum adsorption was predicted using the soil chemical properties cation exchange capacity, organic carbon content, inorganic carbon content, and iron oxide content. These results are advantageous, as they indicate that, under agricultural conditions (pH 4-8), Mo adsorption can usually be described without consideration of changes in soil solution salinity.

Adsorption of methylene blue and orange II onto unmodified and surfactant-modified zeolite

Adsorption of cationic methylene blue and anionic orange II onto unmodified and surfactant-modified zeolites was studied using a batch equilibration method. The effects of equilibrium time, solution pH, and sorption temperature were examined. The results suggested that 2% sodium dodecyl benzenesulfonate (SDBS)- and 3% sodium dodecyl sulfate (SDS)-modified zeolites had higher adsorption capacities for methylene blue than the unmodified zeolite, while 2% cetylpyridinium bromide hexadecyl (CPB)- and 2% hexadecylammonium bromide (HDTMA)-modified zeolites were the best adsorbents for orange II. The adsorption conditions were optimized, and the mechanisms of adsorption are briefly discussed.

Zinc adsorption on goethite as affected by glyphosate

Cosorption of metals with herbicides on minerals affects their mobility and their environmental effect. Batch experiments were conducted to evaluate the interaction between Zn and glyphosate [ N-(phosphonomethyl)glycine (GPS; H 3L)] with regard to the effect of GPS on Zn adsorption on goethite. The herbicide GPS markedly affected Zn adsorption on goethite when they coexisted in a goethite suspension. When solution pH was not intentionally adjusted, addition of GPS decreased Zn adsorption on goethite, since the equilibrium solution pH was significantly decreased in the presence of GPS and correspondingly the negative surface charges of goethite decreased. Zinc adsorption on goethite in the presence and absence of GPS at different pH of the equilibrium solution was studied in order to know if pH was the only variable for Zn adsorption with coexisting GPS. At lower pH (pH < 5), the presence of GPS increased the adsorption of Zn, because Zn adsorbed on the sites of goethite via GPS bridge. However, at higher pH (pH>5), the presence of GPS decreased the adsorption of Zn on goethite, because GPS reacted with solution Zn to form water-soluble complexes that had lower affinity to the goethite surface in comparison with Zn itself. Zeta potential of goethite significantly decreased after adsorption of GPS, suggesting a chemical bond occurred between GPS and the mineral. FTIRs also show that GPS adsorbs on goethite by coordinating through caboxylate group.

Cosorption of zinc and glyphosate on two soils with different characteristics

Agricultural application of large amounts of glyphosate [N-(phosphonomethyl)-glycine] may affect soil metal behaviors to some extend, because glyphosate can react with many kinds of metals to form metal complexes. Cosorption of Zn and glyphosate on a Red soil (RS, Udic Ferrosols) and a Wushan soil (WS, Anthrosol) was studied. In comparison with the WS, the RS has less adsorption capacity for Zn and higher for glyphosate. The presence of glyphosate decreased Zn adsorption on the two soils, which are resulted from the decreased equilibrium solution pH caused by the added glyphosate, and also the formation of water-soluble complexes of glyphosate with solution Zn2+ that had lower affinity to soil surface in comparison with Zn2+ itself. Such effect is more significant on the RS than on the WS, mainly because of the less adsorption quantity of Zn on the former one. On the contrary, the presence of Zn increased the adsorption quantities of glyphosate on the RS and WS, which is resulted from the decreasing pH value of the equilibrium solution caused by Zn2+ exchange with H+ ions of soil surface. Such results suggest that glyphosate in field may increase the mobility and bioavailability of Zn and correspondingly increase its environmental risk.

Short-term effects of three animal manures on soil pH and Al solubility

A short-term (3-day) equilibration experiment was carried out to determine the effects of 3 animal manures (poultry, pig, and cattle) added at a rate of 20 mg/g to an Oxisol on pH and the concentrations of total (AlT) and monomeric (AlMono) Al in solution. The pH of equilibrium solutions from the control, cattle, pig, and poultry manure treatments were 4.0, 4.6, 5.2, and 6.3, respectively. AlMono concentrations decreased progressively as pH increased but concentrations of AlT were similar in the pig and poultry manure treatments. Subsequent equilibration experiments examined the solubility of AlMono and AlT when the 3 manures were added to the Oxisol or a 40 um AlCl3 solution and pH was maintained at 4.0, 4.5, 5.0, 5.5, 6.0, and 6.5. In both experiments, addition of manures generally tended to reduce the concentration of AlT in the lower pH range (4.0 and 4.5) but increase it in the higher range (pH 5.5 or 6.0 and above). Concentrations of AlMono, and the proportion of AlT present as AlMono, were reduced over the whole pH range. Results were explained in terms of the strong bonding ability of organic matter in both the solid and solution phases for Al. At lower pH, where Al is highly soluble, complexation by solid-phase organic matter results in a reduction in both AlT and AlMono. However, at higher pH, where Al solubility becomes limited, complexation of Al by soluble organic matter becomes an important mechanism and this results in an elevation in AlT, while concentrations of AlMono remain very low. It was concluded that the effects of manures in reducing concentrations of AlMono in soil solution are attributable to both an increase in pH and the complexing ability of organic matter for Al.

Influence of surface functional groups and solution pH on removal of organic compounds and a heavy metal by activated carbon

Influence of surface functional groups and solution pH on the adsorption of dissolved organic substances (aniline andnitrobenzene) and a heavy metal (Cd (II)) on activated carbon has been studied. Commercially available activated carbon (AC) and oxidized one with nitric acid (Ox-AC) were employed as adsorbents for the experiments. Adsorption ability of the adsorbents was evaluated by Langmuir- and Freundlich-type adsorption isotherms. Point of zero charge (pHpzc) of the activated carbon was dropped from 9.2 to 3.2 by the oxidation. Decrease in organic matter uptake was also observed for Ox-AC due to decrease in π electron density and formation of water molecular clusters on the carboxyl groups located on the pore mouth at higher pH region. When aniline was used as adsorbate, decline of adsorption probably owing to repulsive force between anilinium ion and graphene layer occurred at lower pH value. On the other hand, Cd (II) uptake by Ox-AC was rapidly increased as the equilibrium solution pH rose, whereas it was not for AC. Since Cd (II) adsorption sites were considered to be carboxyl groups at the edge of the graphene layers, the Cd (II) adsorption capability was greatly influenced by the charge of them on the Ox-AC.

Desorption of cadmium from goethite: Effects of pH, temperature and aging

Cadmium is perhaps environmentally the most significant heavy metal in soils. Bioavailability, remobilization and fate of Cd entering in soils are usually controlled by adsorption–desorption reactions on Fe oxides. Adsorption of Cd on soil colloids including Fe oxides has been extensively studied but Cd desorption from such soil minerals has received relatively little attention. Some factors that affect Cd adsorption on goethite include pH, temperature, aging, type of index cations, Cd concentrations, solution ionic strength and presence of organic and inorganic ions. This research was conducted to study the influence of pH, temperature and aging on Cd desorption from goethite. Batch experiments were conducted to evaluate Cd desorption from goethite with 0.01 M Ca(NO 3) 2. In these experiments Cd desorption was observed at 20, 40 and 70 °C in combination with aging for 16 h, 30, 90 and 180 d from goethite that adsorbed Cd from solutions containing initial Cd concentrations of 20, 80 and 180 μM. Following the adsorption step Cd desorption was measured by 15 successive desorptions after aging at various temperatures. At the lowest amount of initially adsorbed Cd and equilibrium pH 5.5, cumulative Cd desorption decreased from 71% to 17% with aging from 16 h to 180 d and the corresponding decrease at equilibrium pH 6.0 was from 32% to 3%. There was a substantial decrease in Cd desorption with increasing equilibration temperature. For example, in goethite with the lowest amount of initial adsorption at equilibrium pH 5.5, cumulative Cd desorption decreased from 71% to 31% with increase in temperature from 20 to 70 °C, even after 16 h. Dissolution of Cd adsorbed goethite in 1 M HCl, after 15 successive desorptions with 0.01 M Ca(NO 3) 2, indicated that approximately 60% of the Cd was surface adsorbed. Overall, dissolution kinetics data revealed that 23% to 88% Cd could not be desorbed, which could possibly be diffused into the cracks and got entrapped in goethite crystals. At elevated temperature increased equilibrium solution pH favoured the formation of CaCO 3 and CdCO 3 which reasonably decreased Cd desorption. Cadmium speciation showed the formation of calcite and otavite minerals at 40 and 70 °C due to increase in pH (>9.5) during aging. X-ray diffraction analysis (XRD) of these samples also revealed the formation of CaCO 3 at elevated temperatures with aging. While mechanisms such as Cd diffusion and/or entrapment into fissures and cracks in goethite structure with increase in temperature and aging are possible.

Effects of Anions on the Capacity and Affinity of Copper Adsorption in Two Variable Charge Soils

Effects of nitrate,(NO 3 − ) chloride (Cl−), sulfate (SO 4 2- , and acetate (Ac−) on Cu2+ adsorption and affinity of the adsorbed Cu2+ were evaluated in two Fe and Al enriched variable charge soils from Southern China. The maximum adsorption of Cu2+ (M, a parameter from the Langmuir isotherm model) in the presence of different anions decreased in the order Cl− > Ac− > NO 3 − > SO 4 2- for both soils. The clayey loamy soil (mixed siliceous thermic Typic Dystrochrept, TTD), developed on the Arenaceous rock, adsorbed less Cu2+ than the clayey soil (kaolinitic thermic Plinthudults, KTP), derived from the Quaternary red earths, regardless of anion type present in the medium. The affinity of adsorbed Cu2+ to both soils could be characterized by the Kd (distribution coefficient) values and successive extraction of the adsorbed Cu2+ with 1-mol NH4Ac L−1. The log10Kd value was smaller for the TTD soil than for the KTP soil and decreased in the order of Cl− > NO 3 − > SO 4 2- > Ac− at low initial Cu2+ concentrations (≤40 mg Cu2+L−1), whereas at 80 mg Cu2+L−1, the log10Kd value was similar for NO 3 − , SO 4 2- , and Ac−, but was slightly higher for Cl−. Complete extraction of Cu2+ adsorbed in the presence of Ac− was achieved. Influence of NO 3 − and SO 4 2- on the affinity of adsorbed Cu2+ was similar, but the effects of Cl− depended on the initial Cu2+ concentrations. The extracted percentage of the adsorbed Cu2+ in the presence of NO 3 − or SO 4 2- increased with increasing Cu2+ adsorption saturation. The presence of Cl−, NO 3 − , or SO 4 2- markedly decreased the equilibrium solution pH for both soils with increasing initial Cu2+ concentrations, and the delta pH values at the highest Cu2+ level were 0.5, 0.63, and 0.55 U for the TTD soil and 0.79, 0.84, and 0.93 U for the KTP soil, respectively for the three anions. The presence of Ac− had a minimal influence on the equilibrium solution pH because of the buffering nature of the NaAc/HAc medium which buffered the released protons. The effects of anions on Cu2+ adsorption and affinity of the adsorbed Cu2+ were dependent on anion types and were apparently related to the altered surface properties caused by anion adsorption and/or the formation of anion– Cu2+ complexes.

Application of Hydrofluoroethers as Diluent to Solvent Extraction of Zinc(II) Using Phosphorus Acid Extractants

Hydrofluoroethers have been applied as diluent for phosphorus acid extractants in the solvent extraction of zinc(II). They are promising alternatives to conventional organic diluents because of their non-flammability, chemical stability and non-toxicity. Hydrofluoroethers used in this study are a mixture of ethylperfluorobutylether and ethylperfluoroisobutylether and a mixture of methylperfluorobutylether and methylperfluoroisobutylether (HFE-7100). Among the extractants examined, two phosphorus acid extractants, di-(2-ethylhexyl)phosphoric acid (D2EHPA) and 2-ehtylhexyl phosphoric acid mono-2-ethylhexyl ester (EHPNA) were soluble in both hydrofluoroethers. Because of the high specific gravity of hydrofluoroethers and their low solubility in water, phase-separation was rapid and the resulting phases were clear. In the extraction of zinc(II) using the phosphorus acids, the system with D2EHPA gave higher extraction than that with EHPNA, while there was little difference in the extraction behavior between the two hydrofluoroethers. The effects of the equilibrium solution pH and the extractant concentration on the zinc extraction were examined, and the stoichiometric relationship for the extraction was proposed. The extraction equilibrium constants for zinc(II) were found to be higher than those with conventional chlorinated organic solvents such as chloroform and carbon tetrachloride. The backward extraction of zinc(II) was satisfactorily attained with a dilute acid solution, and the separation of several heavy metals was also demonstrated. Moreover, the elution of the extractants and hydrofluoroethers into the aqueous phase was very low, and this makes the present system more attractive.

Adsorption and cosorption of cadmium and glyphosate on two soils with different characteristics

Glyphosate [ N-(phosphonomethyl)glycine] (GPS; H 3G) is a widely used pesticide throughout the world. It affects metal behaviors in soil–plant system due to its functional groups, which react with metal ions to form metal complexes. Adsorption and cosorption of cadmium and glyphosate on a Wushan soil (WS soil, Anthrosol) and a Zhuanhong soil (ZH soil, Udic Ferrisol) as affect by solution pH were studied by means of batch adsorption experiments. It indicated that the adsorption quantity of Cd or glyphosate was highly relevant to soil characteristics. The WS soil had higher adsorption capacity of Cd than the ZH soil, due to its high organic matter content and cation exchange capacity (CEC). In contrast, the adsorption quantity of glyphosate on the WS soil was less than that on the ZH soil, because the WS soil has lower iron and aluminum oxides content but higher pH than the ZH soil. The herbicide glyphosate affected Cd adsorption on the two soils when they coexisted in a same soil solution, which was attributed to a glyphosate-induced pH-decrease and the corresponding decline in negative surface charges of the soil. Beside that, glyphosate reacted with solution Cd to form the water-soluble complexes that had lower affinity to soil surface in comparison with Cd itself. On the other hand, the presence of Cd in the soil solution also affected the adsorption of glyphosate on the soils. The presence of Cd increased adsorption quantity of glyphosate on the WS and ZH soils, which was resulted from the decrease of equilibrium solution pH caused by Cd 2+ exchange with H + ions of soil surface. In addition to that, glyphosate adsorption possibly takes place on sites where Cd was previously adsorbed and acted as a bridge between the soil and glyphosate.

Effect of Structural and Electrolytic Properties of Carboxyl-Containing Cellulose Cationites on the Sorption of Cephalexin from Aqueous Solutions

Sorption of antibiotic cephalexin from aqueous solutions by monocarboxyl cellulose (MCC) samples with various content of carboxyl groups and structure (the degree of crystallinity) is studied. It is disclosed that the sorption occurs mainly due to electrostatic interactions via the proton transfer from carboxyl group of MCC to the carboxylate ion of cephalexin. The sorption is characterized by Langmuir isotherms and described by the equation of localized stoichiometric sorption. Sorption of cephalexin increases with the content of carboxyl groups in the sorbent phase, which is explained by the peculiarities of the dissociation of MCC carboxyl groups. Structural characteristics of MCC samples do not substantially influence the sorption of antibiotic. Dependence of the sorption value on the pH of equilibrium solution has a maximum at pH ≈ 3.5, which is explained by the specific features of the electrolytic dissociation of cephalexin and MCC.

Chromium adsorption on high-performance activated carbons from aqueous solution

The adsorption of Cr(VI) from aqueous solutions by commercial and lab-made high surface area (HSA)-activated carbons was investigated. Physiochemical factors such as equilibrium time, temperature and solution pH that affect the magnitude of Cr(VI) adsorption were studied. The HSA-activated carbons showed high performance for Cr removal, and their adsorption capacity is several times larger than that of commercial carbons. Both micropores and mesopores have important contribution on the adsorption. However, desorption is more dependent on the mesoporosity of activated carbons. Therefore, regeneration is easier for the carbon with high mesoporosity. As a result, the adsorption capacity of mesoporous carbon could be recovered over 97%, whereas 54% for highly microporous carbon.

Conductivity and electrokinetic potential of microcrystalline cellulose particles in aqueous HCl and NaOH solutions

The conductivity of microcrystalline cellulose (MCC) dispersions in aqueous 10–5–10–2 M HCl and NaOH solutions was measured as a function of the particle volume fraction by the conductometry. The dependence of the relative conductivity of MCC particles on pH of equilibrium solutions was determined using the Wagner equation. The electrophoretic mobility of MCC particles in the aforementioned solutions was measured by the microelectrophoresis, and corresponding dependences of the particle ζ potentials on pH of aqueous HCl and NaOH solutions (pH 2–11) were calculated using the Smoluchowski and Henry equations of the electrophoresis theory. It was shown that, in the case of MCC, the Henry equation, which allows for the significant conductivity of the dispersed particles, as compared with the dispersion medium, makes it possible to calculate more accurate ζ potential values and, consequently, to derive the ζ(pH) dependence, which is satisfactorily consistent with the effect of the surface charge and solution ionic strength on the ζ potential in a wide pH range.

Effect of organic acids on adsorption and desorption of rare earth elements

Effect of citric, malic, tartaric and acetic acids on adsorption of La, Ce, Pr and Nd by and desorption from four typical Chinese soils was studied. Generally, adsorption capacities of rare earth elements (REEs) were significantly correlated with the cation exchange capacity (CEC) of soils. In the presence of acetic acids adsorption of REEs was similar to that in the presence of Ca(NO 3) 2. However, in the presence of citric, malic and tartaric acids adsorption of REEs by Heilongjiang, Zhejiang and Guangdong soils decreased to varying extents if compared with that in the presence of nitrate and acetic acid. The significance of suppression followed the order of citric acid > malic acid > tartaric acid > acetic acid , which was consistent with the order of stability of complexes of REEs with these organic acids. However, the adsorption increased with increasing equilibrium solution pH. For Jiangxi soil with low soil pH, CEC and organic matter these organic acids exerted an even more serious suppression effect on the adsorption of REEs. Another feature of the relationship between the adsorption of REEs and equilibrium solution pH was that the adsorption of REEs decreased with increase of pH from 2 to 4.5 and then slightly increased with further increase of pH. Desorption of REEs varied with soils and with organic acids as well. REEs were released easily from Heilongjiang, Zhejiang and Guangdong soils in the presence of organic acid. Generally, desorption of REEs decreased with increasing equilibrium solution pH. Effect of organic acids on desorption of REEs from Jiangxi soil was more complicated. In the presence of citric and malic acids no decrement and/or slight increase in desorption of REEs were observed over the equilibrium solution pH from 3 to 6.5. The reasons for this were ascribed to the strong complexing capacity of citric and malic acids and low soil pH, CEC and organic matter of Jiangxi soil.

Adsorption of Phenolic Compounds from Aqueous Solutions, by Activated Carbons, Described by the Dubinin−Astakhov Equation

It is shown that the adsorption of phenol and 3-chlorophenol from aqueous solutions, by basic active carbons and at an equilibrium solution pH below that of dissociation of the phenols, can be described by an equation of the DRK type with exponent n = 4. The principle of temperature invariance is fulfilled between 283 and 313 K, which means that predictions can be made on the basis of a characteristic energy, Es, and of affinity coefficients, βs, relative to phenol. This approach presents an advantage over the traditional Langmuir isotherm. Moreover, the comparison of the limiting amounts adsorbed by the different carbons suggests that phenol and 3-chlorophenol are adsorbed in a monolayer as observed for carbon blacks, except in the case of activated carbons with a low degree of activation in which molecular-sieve effects can take place. Similar conclusions are obtained from the enthalpies of immersion into the aqueous solutions and their comparison with graphitized carbon blacks.

Simultaneous removal of nonionic surfactant and heavy metal(II)

Simultaneous sorption of nonionic surfactant (alkylmonoethers ALM-10) and heavy metal cations (lead, nickel) by Purolite C 106 carboxylic acrylcationic exchanger has been investigated. Sorption isotherms and kinetic curves were measured using spectrophotometric determination for nonionic surfactant and complexometric titration for heavy metals. The sorption of metal cations, surfactant molecules and products of their interaction takes place simultaneously. In the absence of surfactant the sorption of metal cations determined experimentally corresponds to that calculated from the change in the equilibrium solution pH, assuming that two hydrogen cations are replaced by one metal cation. In the presence of surfactant the change in pH is lower than that without surfactant; the total sorption of metal determined experimentally is higher than that calculated from the change in pH. It is because the sorption of metal takes place in the form of not only free cations but also cations bonded to the surfactant. The sorption of metal bonded to the surfactant can be evaluated from both the total metal adsorbed which was determined experimentally and the sorption of the free cations. The total sorption of lead(II) is higher than that of nickel(II) (transition metal). Subsequently the action of lead(II) results in the decrease of the equilibrium sorption of surfactant while the action of nickel(II) on the equilibrium sorption of surfactant is insignificant. The sorption of metal cations decreases in the presence of surfactant.

METAL-INDUCED SULFATE ADSORPTION BY SOILS

Adsorption of SO 4 2− and its associated metal ion by four soils from Iowa, Chile, and Costa Rica was studied. The surface charge of each soil was characterized, and the relationship between SO 4 2− adsorption and pH was constructed for pH ranging from 3.5 to 7 at 0.5 increments by using 1 mmol/L (2.0 mmol c L −1 , or 50.0 mmol c kg −1 soil) each of seven SO 4 2− forms [Cs 2 SO 4 , K 2 SO 4 , (NH 4 ) 2 SO 4 , CaSO 4 , MgSO 4 , Al 2 (SO 4 ) 3 , and In 2 (SO 4 ) 3 ] made in 1 or 10 mmol/L NaCl. Results showed that SO 4 2− adsorption was greatest at the lowest pH values (ca. 3.5) and decreased with increasing equilibrium solution pH and ionic strength. The amounts of SO 4 2− adsorbed were three to four times greater in soils containing large rather than small amounts of Fe and Al hydrous oxides, regardless of the type and valence of the metal ion. The amounts of SO 4 2− adsorbed from solutions containing different metal iona were affected by the charge of the metal ion and, in general, followed the order: In 2 (SO 4 ) 3 > Al 2 (SO 4 ) 3 > CaSO 4 > MgSO 4 > Cs 2 SO 4 > K 2 SO 4 (NH 4 ) 2 SO 4 . The effect of the metal ion was more pronounced at higher pH values (pH > 5), and Iowa soils (low in Fe and Al hydrous oxides) were more affected by the increase in the valence of the metal ion and ionic strength than the soils from Chile and Costa Rica. The ratios of SO 4 2− /metal (M) adsorbed (SO 4 2− /M) by the four soils decreased sharply with increasing pH up to 4.6 to 6.0, above which no significant change in these ratios occurred. For all soils, a relatively linear relationship was obtained between log K d (SO 4 2− distribution coefficients) and pH when the metal ions were monovalent, regardless of the ionic strength. But the K d values for SO 4 2− varied among the soils, depending on the SO 4 2− form used

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.

SOLUBLE ALUMINUM IN ACIDIFIED ORGANIC HORIZONS OF FOREST SOILS

Concentrations of labile and total Al in soil extracts were measured as a function of equilibrium solution pH in six forest soil organic horizons acidified with HNO3 (0–20 cmol H+∙kg−1) under controlled conditions of ionic strength (0.05 M NaNO3), temperature (23 °C), and solution:soil ratio (25:1). Decreases of 0.1–0.2 in solution pH in the range of pH 2.4–4.5 resulted in increases and decreases in concentrations of labile and total Al. Organic acids and soluble C were the dominant Al-complexing ligands in solution, and acidification of each horizon decreased their concentration. Silicon and F concentrations in the equilibrium solutions were lower than that of C, but levels were similar to those of nonlabile Al. Nonlabile complexes were the dominant forms of soluble Al in horizons above pH 4, and labile Al comprised an increasing percentage of total Al as pH decreased, especially below pH 3. The results showed pH alone to be a poor predictor of log nonlabile (r2 = 0.06) or total Al (r2 = 0.33) concentration in these soils, but a good predictor of log labile forms (r2 = 0.77). Solution pH, however, was positively correlated with the log of the ratios of C-to-total Al (r2 = 0.78), C-to-nonlabile Al (r2 = 0.68), and C-to-labile Al (r2 = 0.88). The results should aid in predicting the changes in speciation and solubility of Al due to inputs of acid precipitation to forest soil organic horizons with variable pH and content of dissolved organic C. The observed variability in response of Al to acidification of organic horizons of forest soils indicated that it is not possible to generalize whether nonlabile and labile forms of Al will increase, decrease, or not change upon addition of mineral acid. Besides pH, concentrations of different Al-complexing ligands should be measured in equilibirum soil extracts. Key words: Acid precipitation, fluoride, silica, fulvic acid