SO4 Adsorption Research Articles

Ammonia recovery from simulated anaerobic digestate using a two-stage direct contact membrane distillation process.

Ammonia is a key inorganic contaminant in wastewater and an important nutrient element for agriculture. Herein, a two-stage direct contact membrane distillation (DCMD) system was developed and investigated for ammonia recovery from a synthetic anaerobic digestate. In the 1st stage DCMD (DCMD-1), both ammonia and water moved across MD membrane to realize ammonia separation, while in the 2nd stage (DCMD-2), only water migrated and as a result ammonia was concentrated. It was found that increasing the initial feed solution pH could enhance ammonia removal in the DCMD-1 from 16.0±2.0% (no pH adjustment) to 84.2±1.9% (pH 12). A higher feed solution temperature increased both ammonia flux and water flux. The optimal condition was determined as an initial feed pH of 12, a feed temperature of 60°C, and the 0.6M H2 SO4 adsorption solution. With the addition of the DCMD-2, the ammonia concentration was improved from 3gL-1 to 7.8±0.2gL-1 , which was further enhanced to 26.3±3.0gL-1 after five batches of operation. These results have demonstrated the feasibility of a two-stage DCMD system for ammonia recovery from anaerobic digestate and warrant further investigation of several key issues that may advance this technology. PRACTITIONER POINTS: A two-stage membrane distillation system is developed to remove and recover ammonia from anaerobic digester effluents. The system uses ammonia/ammonium equilibrium to separate ammonia in the 1st stage and then concentrate it in the 2nd stage. A high initial pH of the feed solution plays a key role in achieving high ammonia removal. Minimizing the volume of permeate solution can increase the ammonia concentration.

Adsorption of Antimonate, Sulfate, and Phosphate by Goethite: Reversibility and Competitive Effects

Core Ideas Sb(V) adsorption by goethite decreases with increasing pH and is unaffected by ionic strength. Sb(V) adsorption is hysteretic. Adsorbed Sb(V) generates a downward shift in the goethite isoelectric point. The adsorption of Sb(V) decreases in the presence of phosphate, but is unaffected by sulfate. Inner sphere mono‐ and bidentate Sb(V) complexation by goethite is predicted. Goethite is a sink for antimonate Sb(OH)6– [Sb(V)] in soil and sediments. The pH‐ and ionic strength (I)‐dependent mass distribution of Sb(V) between adsorbed and solution phases at equilibrium may be used to indirectly infer an adsorption mechanism (inner vs. outer sphere surface complexation) and to develop predictive surface complexation models. The objectives of this study were to characterize the adsorption of Sb(V) by goethite as a function of pH, I, and in the absence or presence of competing ligands, SO4 or PO4. Ligand adsorption was at a maximum in strongly acidic suspensions and decreased with increasing pH. Adsorption envelopes of Sb(V) where not influenced by I, but increasing I decreased SO4 and increased PO4 retention. In strongly acidic suspensions, SO4 and PO4 retention was not influenced by I. Ligand adsorption decreased the isoelectric point (IEP) of goethite from 8.5 to 6.3 [Sb(V)] and 4 (PO4). The adsorption of SO4 did not affect the IEP, although SO4 reduced the zeta potential of goethite when pH < IEP. Antimonate and PO4 adsorption was hysteretic; SO4 adsorption was reversible. Sulfate did not impact Sb(V) retention; adsorbed Sb(V) generated greater negative surface charge, decreasing SO4 adsorption. Both Sb(V) and PO4 competed for adsorption sites, resulted in reduced retention of both in competitive systems. The triple layer formulation of the charge distribution multisite complexation model, coupled with inner sphere surface complexation, was used to describe the adsorption envelopes for Sb(V), SO4, and PO4, and to predict adsorption in competitive systems.

Groundwater quality modeling using neuro-particle swarm optimization and neuro-differential evolution techniques

Abstract Recently, the capabilities of artificial neural networks (ANNs) in simulating dynamic systems have been proven. However, the common training algorithms of ANNs (e.g., back-propagation and gradient algorithms) are featured with specific drawbacks in terms of slow convergence and probable entrapment in local minima. Alternatively, novel training techniques, e.g., particle swarm optimization (PSO) and differential evolution (DE) algorithms might be employed for conquering these shortcomings. In this paper, ANN-PSO and ANN-DE models were applied for modeling groundwater qualitative parameters, i.e., SO4 and sodium adsorption ratio (SAR). Three statistical parameters including root mean square error (RMSE), mean absolute error (MAE), and the coefficient of determination (R2) were used for assessing the models' capabilities. The results showed that the ANN-DE presents more accurate results than ANN-PSO in modeling SAR and electrical conductivity (EC).

Predicting sulphate adsorption/desorption in forest soils: Evaluation of an extended Freundlich equation

Sulphate adsorption and desorption can delay the response in soil acidity against changes in acid input. Here we evaluate the use of an extended Freundlich equation for predictions of pH-dependent SO4 adsorption and desorption in low-ionic strength soil systems. Five B horizons from Spodosols were subjected to batch equilibrations at low ionic strength at different pHs and dissolved SO4 concentrations. The proton coadsorption stoichiometry (η), i.e. the number of H+ ions co-adsorbed for every adsorbed SO42− ion, was close to 2 in four of five soils. This enabled the use of a Freundlich equation that involved only two adjustable parameters (the Freundlich coefficient KF and the non-ideality parameter m). With this model a satisfactory fit was obtained when only two data points were used for calibration. The root-mean square errors of log adsorbed SO4 ranged from 0.006 to 0.052. The model improves the possibility to consider SO4 adsorption/desorption processes correctly in dynamic soil chemistry models.

Amorphous Aluminum Hydroxide Control on Sulfate and Phosphate in Sediment-Solution Systems

We experimentally determined the adsorption characteristics of natural, freshly precipitated Al(OH)3 for SO4 and PO4. The fresh Al precipitate occurred in stream sediment of Jachymov Stream (Czech Republic). The Al-rich sediment strongly adsorbed added PO4 prior to acidification experiment; this sorbed PO4 was released only after substantial dissolution of the sediment, at pH < 3.67. Sorption of P by Al(OH)3 appears to be an important control on dissolved PO4 concentration in surface waters, unless there is a large excess of PO4. Acidification of the sediment-solution system caused protonation of the sediment surface, thereby increasing the adsorption capacity for SO4. Maximum SO4 adsorption occurred at pH 4.2, below which dissolution of the sediment offset the increasing anion adsorption capacity, and formation of AISO4 + inhibited the increasing SO4 adsorption capacity. This research demonstrates that there are important pH thresholds for anion adsorption in freshwaters below which dissolution of the Al(OH)3 substrate reduces total capacity for anion adsorption. In freshwaters, with sufficient concentrations of suspended Al(OH)3, or in Al(OH)3-rich sediment, PO4 mobility will be severely restricted. Suspended Al(OH)3 in acidified surface waters cannot strongly influence SO4 concentrations because of the considerably higher total SO4 concentrations compared to the available surface area.

Changes in water-extractability of soil inorganic phosphate induced by chloride and sulfate salts

One of the principal experimental variables which effect the results of phosphorus (P) sorption studies is the ionic composition, in addition to both species and concentrations of the contacting solution. In spite of the realization that ionic species, concentrations and their compositions effect P sorption and/or desorption, most of the salt-related studies are confined to Cl- (anion) in association with different cations. While the knowledge about the comparative response of P to Cl- and SO4(2-) ions was lacking, the current study was conducted to evaluate the comparative effects of anions (in association with cations) on inorganic P release and P fractions in the soil. The test soil was amended with livestock compost manure (OP); KH2PO4 (IPk) or Ca(H2PO4)2 (IPc) at a rate of 1ppm. Soil was subjected to one salt and nine subsequent water extractions and different P fractions were measured. Four salt types, NaCl, Na2SO4, KCl and K2SO4, were used at levels of 0.5 M. Irrespective of P sources, P release was substantially increased in the salt-pretreated soil as compared to the non-saline soil. Sulfate salts released more P in subsequent water extractions than Cl-. Phosphorus release decreased for salt types with Na2SO4 > NaCl > K2SO4 > KCl and for P sources with OP approximately IPk > Control (without P application) > IPc, respectively. No previous study was found to compare the results of more P release by SO4(2-) than Cl- salt. Most of the previous studies focused on anion sorption capacities, but the mechanism for their adsorption is not fully known. Most of the authors suggested that the mechanisms of SO4(2-) and PO4(3-) adsorption are similar, and that both ions compete for the same sorption sites (Kamprath et al. 1956, Couto et al. 1979, Pasricha and Fox 1993). Although adsorbed SO4(2-) does not compete strongly with PO4(3-), there is likely to be some competition for sorption between these anions which may cause comparatively more P release by SO4(2-) than Cl- salts. Higher P release by Na-saturation could be due to the release of P associated with oxide surfaces or due to dissolution of Ca-P phases (Curtin et al. 1987). Study clearly showed that not only cations species differ for P desorption capacity, but associated anions also play a vital role in the fate of P under saline environments. Synergetic effects exist between Na and SO4(2-) ions which enhanced the P release. This study has also confirmed the fact that P from organic sources is available as well as from inorganic P sources. However, P release depends more on the type of P source applied than on total P. It is highly recommended that more than one anion species must be used in the research plans for evaluating the P response in a saline environment. The results have important implications from the point of view of research, as most of the researchers focus on different cations only for evaluating P response to salts from an environmental point of view. However, our study has made it clear that anions in association with cations differed for their effects on P release.

Mutual Interactions of Sulfate, Oxalate, Citrate, and Phosphate on Synthetic and Natural Allophanes

In soil environments, particularly at the soil–plant interface, organic and inorganic ligands may compete with each other for sorption sites onto soil components. The interaction between two or more ligands has a great importance for understanding the adsorption/desorption processes of a given ligand in soil. We studied the mutual interaction of sulfate (SO4), oxalate (OX), citrate (CIT), and phosphate (PO4) in binary and ternary systems on synthetic and natural allophanic samples at different pH values (4.0–7.0). The adsorption capacity of SO4, CIT, and OX (in the order listed) was much lower than that of PO4, but CIT decreased the zero point charge (ZPC) more than OX, meanwhile PO4 increased the ZPC, whereas SO4 did not change the ZPC of the sorbents. The adsorption of SO4 was strongly inhibited in the presence of equimolar amounts of OX, CIT, or PO4 even at low surface coverage of all the ligands. The lower the final pH of the binary systems, the greater the inhibition of SO4 adsorption. The capacity of CIT to prevent SO4 adsorption (ranging from 22–63%) was similar or slightly greater than that of PO4 (ranging from 25–48%) but clearly higher than OX (ranging from 20–42%). On the contrary, SO4 very poorly prevented the adsorption of the other ligands (usually lower than 10%). The effectiveness of the other ligands in retarding or inhibiting SO4 adsorption on an Andisol sample containing 40% allophane decreased with increases in the reaction time. However, even after 15 d of reaction CIT (38%) prevented SO4 adsorption more than PO4 (27%) or OX (12%) did. Our results seem to demonstrate that changes in the electric potential of the surfaces after the addition of anions, as well as competition for adsorption sites, affected the reduction in adsorption of SO4 and the other ligands. However, competition in adsorption with SO4 for common adsorption sites was greater for the organic ligands than for PO4, because the surface coverage of PO4 was particularly low with respect to that of SO4, CIT, and OX.

Fate of Applied Sulfate in Volcanic Ash‐Influenced Forest Soils

Forests in the Inland Northwest, USA, commonly show SO4 deficiency, suggesting limited SO4 availability in the soils. Regional soils, which lie between the east slopes of the northern Cascade Range and the west slopes of the northern Rocky Mountains, are influenced to varying degrees by poorly crystalline aluminosilicates and ferrihydrite, and are often classified as Andisols (Andosols). Research has shown that SO4 retention is greatly influenced by Fe and Al oxides. However, little is known of the sorption behavior exhibited by poorly crystalline andic soils of the region. In this study we investigated the mineralogy and SO4 sorption capacity of ash‐influenced soils found in the Inland Northwest. Batch SO4 adsorption experiments showed that up to 40% of added SO4 was adsorbed. Furthermore, there were positive correlations between soil SO4 adsorption capacity and increasing ash influence as measured by (i) the andic soil parameter %Alo + 0.5%Feo (R2 = 0.89), (ii) P retention (R2 = 0.91), and (iii) NaF pH (R2 = 0.48). Soil pH, total organic C (TOC), and percentage of clay showed insignificant or inconclusive relationships with SO4 adsorption. Release of adsorbed SO4 was significantly lower in volcanic ash‐influenced soils as compared with non‐ash soils, indicating a greater affinity for SO4 These results indicate that poorly crystalline aluminosilicates and Fe oxides significantly influence the amount of SO4 present in forest soil solutions. Successful nutrient management plans must recognize the sorption behavior of these andic soils.

The effects of H2SO4 and (NH4)2SO4 treatments on the chemistry of soil drainage water and pine seedlings in forest soil microcosms

Abstract. An experiment comparing effects of sulphuric acid and reduced N deposition on soil water quality and on chemical and physical growth indicators for forest ecosystems is described. Six H2SO4 and (NH4)2SO4 treatment loads, from 0 – 44 and 0 – 25 kmolc ha-1 yr-1, respectively, were applied to outdoor microcosms of Pinus sylvestris seedlings in 3 acid to intermediate upland soils (calc-silicate, quartzite and granite) for 2 years. Different soil types responded similarly to H2SO4 loads, resulting in decreased leachate pH, but differently to reduced N inputs. In microcosms of calc-silicate soil, nitrification of NH4 resulted in lower pH and higher cation leaching than in acid treatments. By contrast, in quartzite and granite soils, (NH4)2SO4 promoted direct cation leaching, although leachate pH increased. The results highlighted the importance of soil composition on the nature of the cations leached, the SO4 adsorption capacities and microbial N transformations. Greater seedling growth on calc-silicate soils under both treatment types was related to sustained nutrient availability. Reductions in foliar P and Mg with higher N treatments were observed for seedlings in the calc-silicate soil. There were few treatment effects on quartzite and granite microcosm tree seedlings since P limitation precluded seedling growth responses to treatments. Hence, any benefits of N deposition to seedlings on quartzite and granite soils appeared limited by availability of co-nutrients, exacerbated by rapid depletion of soil exchangeable base cations. Keywords: acidification, manipulation, nitrogen, ammonium, deposition, soil, drainage, pine, microcosms, forest

Implementation of sulfate adsorption in the SAFE model.

An SO4(2-) adsorption submodel has been implemented in the dynamic soil chemistry model SAFE. The submodel calculates pH-dependent SO4(2-) and H+ adsorption to the soil, as well as the net surface charge development due to uneven adsorption of SO4(2-) and H+, using the empirical equations derived from an electrostatic model (Extended Constant Capacitance Model, ECCM) of SO4(2-) adsorption. The resulting new SAFE model was applied on a roof experiment plot in the Norway spruce [Picea abies (L.) H. Karst.] stand at Solling, Germany, where atmospheric S and N deposition was artificially reduced by the roof construction. The model performance was compared with the previous versions that used a pH-independent Freudlich model of SO4(2-) adsorption or assumed no SO4(2-) adsorption. With the ECCM-based SO4(2-) adsorption submodel, SAFE simulated soil solution SO4(2-) concentration and base saturation better, in comparison with measured data, than with the previous SO4(2-) adsorption formulations. Through the model application, also, need of additional improvement was suggested, such as calibration of mass transfer coefficients.

COMPETITIVE ADSORPTION OF SULFATE AND OXALATE ON GOETHITE IN THE ABSENCE OR PRESENCE OF PHOSPHATE

A study was carried out to examine the competition in adsorption of sulfate (SO4) and oxalate (OX) on goethite as affected by pH, concentration of the ligands, order of anion addition, and the presence of phosphate (PO4). The purpose was to obtain useful information about the factors that influence the adsorption of ligands, which have different affinity (PO4 >> OX > SO4) for this variable charge mineral. More OX than SO4 was adsorbed for the pH values between 3.0 and 7.5. When equimolar amounts of SO4 and OX were added, the molar ratio of adsorbed OX to adsorbed SO4 ranged from 2.25 to 3.78 in the pH range between 3.5 and 6.0. The sequence of anion addition influenced the adsorption of SO4 and OX only at pH <5.0. Each ligand prevented the adsorption of the other anion mainly when it was added first to goethite (SO4before OX or OX before SO4 systems). In the SO4before OX system, the greatest inhibition of OX adsorption by SO4 occurred at pH 3.0 (34%). There were many indications that at low pH values, some SO4 ions were so strongly adsorbed by the positively charged surface of goethite that they were able to compete efficaciously with OX. At pH 4.0 and at an initial OX/SO4 molar ratio of 3.33, SO4 adsorption decreased by 72.0%, whereas at an initial SO4/OX molar ratio of 3.33, the OX adsorption decreased by 38.7%. The addition of PO4 facilitated SO4 and, to a lesser extent, OX desorption. In the pH range of 3.0 to 4.5, 85 to 25% of the OX adsorbed when added first with SO4 remained adsorbed on the goethite after the addition of equimolar amounts of PO4. In this range of pH, even large amounts of PO4 did not desorb OX from the surfaces of goethite completely, indicating that at low pH values, OX ligands form particularly strong complexes on some sites of goethite. At low pH values, the total amounts of PO4, OX, and SO4 adsorbed on goethite were significantly greater than the quantities of PO4 adsorbed when added alone.

Phosphorus retention by tropical soils as influenced by sulfate application

Soils with variable surface charge are reported to have high affinities for the adsorption of anions and also to selectively retain cations. A laboratory study was conducted to determine the influence of sulfate (SO42‐) anion on phosphate (PO43‐) sorption/desorption from tropical soils. Duplicate 1.0 g soil samples were shaken with 10 mL portions of varying concentrations of calcium sulfate (CaSO4lb2H2O) (0.0 to 1.5 x 10‐2 M). Subsequently, the samples were equilibrated with increasing concentrations of monocalcium phosphate [Ca(H2PO4)2#lbH2O] in 0.01M calcium chloride (CaCl2) to measure PO4 sorption and afterwards with 0.01M CaCl2 for PO4desorption. Results showed that PO4 sorption maxima increased as concentrations of SO4 in solution increased. In the most acidic soil, SO4 adsorption from the 0.015M CaSO4#lb2H2O solution increased PO4 sorption by 93%. There was a decrease in pH in the SO4 extracts; this would indicate that calcium (Ca) from the SO4 salt applied to the soils before P addition was specifically adsorbed by the exchange complex and originated new sites for binding PO4. On the average, about 20% of the sorbed PO4was removed with the CaCl2 treatment, suggesting that PO4 was specifically adsorbed by the variable charge soils.

Ionic Strength Effects on Sulfate and Phosphate Adsorption on γ‐Alumina and Kaolinite: Triple‐Layer Model

AbstractIt is known that PO4 is retained by soils through ligand exchange, i.e., inner sphere complexation, but the mechanism for SO4 adsorption at the mineral‐water interface has been in debate. By studying the effects of ionic strength on ion adsorption, it is possible to distinguish between inner and outer sphere ion surface complexes. This study was conducted to evaluate ionic strength effects on SO4 and PO4 adsorption on γ‐Al2O3 and kaolinite at varying solution pH (3–11), and to infer SO4 and PO4 adsorption mechanisms at the mineral‐water interface. The adsorption of SO4 on γ‐Al2O3 and kaolinite decreased monotonically with increasing solution pH and was markedly reduced by increasing the concentration of background electrolyte. On the other hand, PO4 adsorption on γ‐Al2O3 and kaolinite increased from pH 3 to 4 and decreased from pH 6 to 11, with an adsorption plateau between pH 4 and 6. Effects of change in ionic strength on PO4 adsorption varied with pH. At low pH, PO4 adsorption demonstrated a slight decrease with increasing ionic strength, whereas at high pH, PO4 adsorption increased slightly with increasing ionic strength, resulting in a crossover point where there was no ionic strength effect. The triple‐layer model (TLM) was applied to model the adsorption of SO4 and PO4 with both inner and outer sphere complexes using the FITEQL 3.1 computer program. Sulfate adsorption was better modeled by assuming outer sphere complex formation, while PO4 adsorption was better modeled by assuming inner sphere complex formation.

Sulfate Retention by Kaolinitic Alfisols from Nigerian Savanna

AbstractThe low amount of available SO4 in savanna Alfisols necessitates investigation of mechanism of SO4 retention and availability in these soils. The study examines the mechanism and SO4 retention capacity of kaolinitic Alfisols from Nigerian savanna. Soil samples from 0‐ to 20‐cm depth from three cultivated plots and a native site were equilibrated with varying amounts of SO4 (0–0.003 M) in three supporting electrolytes, 0.01 M CaCl2, KCl, and NaCl, and three ionic strengths, 0.0 1, 0.1, and 0.2 M KCl at pH 5.6 ± 0.2, above the point of zero salt effect (PZSE) and point of zero net charge (PZNC) of the soils determined by potentiometric titration and ion adsorption method, respectively. The results of the study indicated that the soils had high SO4 retention capacity ranging from 15 to 17 mmol kg−1 soil, and the sorption isotherms were described by a modified Freundlich equation. Sulfate sorption was, however, not significantly affected by ionic composition and strength. If the forces of SO4 adsorption were simply electrostatic, increasing ionic strength would have significantly decreased SO4 adsorption above the PZSE or PZNC of the soils. The study suggested that SO4 retention by kaolinitic Alfisols might involve inner‐sphere complexation despite the difficulty of making mechanistic, molecular level interpretation from gross solution data of whole soils.

Modeling pH‐Dependent Sulfate Adsorption in the Bs Horizons of Podzolized Soils

AbstractA model was developed that predicted the pool of adsorbed SO4 in the Bs horizons of northern Scandinavia as a function of pH and the solution SO4 concentration, assuming that 2 H+ are consumed for every SO2−4 adsorbed. To accomplish this, a sequential leach procedure with acid MgSO4 was used which facilitated the determination of SO4 adsorption capacity and the characterization of the adsorption isotherm. The Temkin equation showed the best fit within the range of SO4 concentrations used in this study. The variability between different soils in terms of adsorption capacities and isotherm slopes was modeled using linear and multiple regressions with other soil characteristics. Model results showed good agreement with other experimental observations in northern Scandinavia. The model, however, underestimated SO4 adsorption for soils from southern Scandinavia, which may be because of regional differences in, for example, adsorbent crystallinity. Furthermore, the results show that most pristine soils in northern Scandinavia retain 20 to 40% as much SO4 as they would retain if they were located in a severely acidified area. Therefore SO4 desorption following a decreasing atmospheric S deposition would only involve part of the adsorbed SO4. Only a small hysteresis effect was observed when the adsorption isotherm was compared to the desorption isotherm for one soil.

Role of SO4 adsorption and desorption in the long-term S budget of a coniferous catchment on the Canadian Shield

The hypothesis that SO4 desorption can explain apparent long term net SO4-S losses (5 kg·ha−1·yr−1 on average) at the Lake Laflamme catchment from 1982 to 1991 is examined. Field observations show that SO4 concentrations in the soil solution are strongly buffered during percolation through the Bf horizon. In the Bf horizon, SO4 exchange reactions between the adsorbed and aqueous compartments are rapid (hours). Most (≈60%) of the adsorbed SO4 may be readily desorbed with deionized water. These observations and the presence of an important adsorbed SO4-S reservoir in the Bf horizon (113 kg·ha−1) as compared with annual wet SO4-S deposition (7 kg·ha−1), suggest that on the short-term, adsorption and desorption reactions can control dissolved SO4 concentration in the Bf horizon. To examine whether SO4 adsorption/desorption could explain long-term SO4-S losses by the catchment, an aggregated Langmuir isotherm for the Bf horizon was used to calculate the catchment's resilience to changing SO4-S loads. The results indicate that the soil should adjust rapidly (within 4 years) to changing SO4-S loads and that SO4 desorption alone cannot explain long-term net SO4-S losses. Other possibilities, such as an underestimation of dry deposition or the weathering of S-bearing minerals also appear unlikely. Our results suggest a net release of SO4-S from the soil organic S reservoirs (1230 kg·ha−1) present in the catchment.

Reactions of Ground Magnesium Limestone and Gypsum in Soils with Variable‐Charge Minerals

AbstractUltisols and Oxisols in Malaysia are characterized by low pH and high Al, with their subsoils usually deficient in Ca, Mg, or both. An experiment, using polyvinyl chloride columns, was conducted to study chemical reactions in a representative Ultisol and Oxisol from Malaysia resulting from application of ground magnesium limestone (GML), gypsum, and their combinations. An increase in soil pH due to GML application in the Ultisol and the Oxisol resulted in an increase in negative charge on the soil surfaces in the zone of incorporation. Calcium was retained in that zone. In the gypsum‐treated soils, some Ca moved into the underlying horizons. Negative charge in the Oxisol increased from 2.8 to 3.7 cmolc kg−1 by an application of 8 t gypsum ha−1 due to specific adsorption of SO4 on Fe oxides. An increase in pH of the Oxisol due to SO4 adsorption and OH desorption was also observed. There was an accumulation of NO3 in the subsoils of the Oxisol and Ultisol as a result of gypsum treatment. The NO3 was adsorbed by the positively charged oxides. Leachate pH values were high at the beginning of the experiment, but the values decreased with time and were higher in the Oxisol than the Ultisol. This was attributed to a lower Al concentration in the leachate of the Oxisol. Effective alleviation of topsoil Al toxicity and subsoil Ca or Mg deficiency would require the application of 2 t GML together with 1 to 2 t gypsum ha−1.

Kinetics of Ion Removal from an Iron‐Rich Industrial Coproduct: II. Sulfate

AbstractAn iron‐rich material (IRM) contained copious quantities of salts, including Cl and SO4. A previous study on Cl removal hypothesized that SO4 played a major role in the chemistry of the IRM. Therefore, this study investigated the kinetics of SO4 removal from the IRM, which had a point of zero charge similar to those of some tropical soils. Stirred‐flow (SF) and column studies showed that SO4 removal was a time‐dependent chemical reaction(s). The SF technique facilitated modeling of the chemical kinetics of SO4 removal by providing a uniformly mixed system. The SO4 removal mechanism from the IRM to the ambient solution, in the SF system, was a zero‐order oxidation‐dissolution reaction of the Fe‐sulfide present as a minor constituent in the IRM. The computed value of the rate‐constant was 0.043 µmol min−1. In the column studies, the newly dissolved SO4 was present in the effluent, and was adsorbed on the IRM through a ligand (OH−) exchange mechanism. Calcium hydroxide dissolution, together with the SO4 adsorption, resulted in a pH increase with pore‐volume.

Response of Buried Mineral Soil Bags to Experimental Acidification of Forest Ecosystem

AbstractAt Bear Brook Watershed in Maine, H2SO4 and/or HNO3 was added to 15 by 15 m plots (three replicates per treatment) from 1988 through 1990. Total loading above ambient treatments for the 3 yr was 0 (control), 5100 (low S, low N), 10 200 (high S and N + S), and 11 950 (high N) molc ha−1 yr−1. Changes in soil chemistry were evaluated using tension lysimeters and buried mineral soil bags placed below the forest floor before treatments were initiated. Total soil S was highest in the high S treatment (355 mg S kg−1 soil), intermediate in the low S and N + S treatments (314 and 310 mg S kg−1 soil, respectively), and lowest in the low N, high N, and control treatments (278, 275, and 270 mg S kg−1 soil, respectively). Differences in total S were mostly attributed to phosphate‐extractable SO4, although smaller changes in ester sulfate and C‐bonded S were also detected. Low pH and elevated SO4 in solution resulted in increased SO4 in solution resulted in increased SO4 adsorption. There were no differences caused by treatments in N or C constituents in the soil bags, although N was strongly linked with C (r = 0.86). Nitric and sulfuric acid additions lowered soil pH (H2O) from 4.83 in the control to 4.67 and 4.70, respectively. Sulfuric acid treatments increased exchangeable Ca to 0.32 cmolc kg−1 compared with 0.24 cmolc kg−1 in the control, whereas HNO3 treatments decreased Ca to 0.18 cmolc kg−1. A strong correlation (r = 0.94) between exchangeable Ca and base saturation reflected the quantitative dominance of exchangeable Ca among base cations. Similarities in effects of the N + S and the low S treatments were due to identical additions of SO4 and the absence of a marked effect of NO3 additions on the mineral soil. Changes in the chemistry of the buried mineral soil bags can be explained by both the effects of H2SO4 and HNO3 additions as well as pedogenic processes.

Modeling Soil Sulfate Sorption Characteristics

AbstractKnowledge of soil SO4 adsorption characteristics is required for predicting the fate of atmospheric S depositions on terrestrial ecosystems. This study developed two models for predicting the SO4 sorption characteristics of field moist soils from routinely measured soil physical and chemical properties. Sixty‐two soil samples from the U.S. Northeastern (NE) and Southern Blue Ridge Province (SBRP) regions were used in model development. The first modeling approach used stepwise multiple regression to identify which combinations of soil properties best predicted each Langmuir constant. The Langmuir constant regression equations were then substituted back into the Langmuir model and used to predict SO4 sorption for each equilibrium SO4 concentration. This approach described SO4 sorption reasonably well for both regions (R2 of 0.57–0.89). In the second approach, a better fit of SO4 sorption was achieved (R2 of 0.81–0.92) using fewer soil properties by expressing the Langmuir constants as linear functions of selected soil properties and directly estimating the soil property coefficients by nonlinear regression. Soil properties used in the second model included: dithionite‐citrate‐extractable Al (Ald), total organic C (TOC), and extractable SO4 for the NE region; and Ald, TOC, clay content, pH, and CEC for the SBRP region. These results indicated that regional soil SO4 sorption characteristics could be predicted for field moist soils with the knowledge of only a few routinely measured soil properties.