Sulfate Desorption Research Articles

Effect of trace impurities in perchloric acid on blank voltammetry of Pt(111)

Trace impurities of nitrate and sulphate species have been detected as the cause for the charge quench of existing voltammetric peaks and the appearance of new voltammetric peaks in the blank voltammetry of Pt(111) in high-purity perchloric acid. Cyclic voltammetry of Pt(111) in high concentrations of HClO4 (1 M) electrolyte display anomalous reduction peaks at 0.5 and 0.32 VRHE. The reduction of nitrates to nitric oxide, resulting in the reduction peak at 0.32 VRHE, has been detected using in situ Fourier transform infrared spectroscopy. The peak at 0.5 VRHE, previously assigned in the literature to specific adsorption of perchlorate anions catalytically dissociating to chloride anions, is consistent with sulphate desorption as elucidated by spiking HClO4 electrolyte with trace quantities (10 ppm) of sulphuric acid. The presence of kosmotropic sulphate anions was further confirmed by the positive onset potential shift of the sharp OHads butterfly peak at ∼0.8 VRHE with increasing concentrations of HClO4 (0.01–1 M). Although commonly overlooked, we show here the marked deleterious impact these trace impurities have on the blank voltammetry of Pt(111), and provide recommendations for minimising this impact.

The Effect of Biochar on Sulfate Desorption Kinetics by Selected Soil in Sudan Savanna, Nigeria

Biochar is regarded as a promising soil amendment that maximizes soil productivity to boost food security. Limited data is available on the influence of biochar on sulfate desorption behavior in soil. To fill the knowledge gap, this research attempts to investigate sulfate sorption and desorption kinetics with soil parent materials mixed with biochar. Understanding the effects is of great importance in selecting a fit diagnosis and fertilization of S to ensure sustainable crop production and environmental protection. Maize stalk biochar was prepared and used for the study. The variation between soil desorption data (with and without biochar) was examined using a two-way analysis of variance (ANOVA) with fully randomized designs (CRD). The results obtained showed that the studied Biochar (BC) had no significant (P = 0.05) impact on the release of adsorbing S, independent of time and soil constituents. The findings led to the conclusion that the rate of desorption of adsorbed sulfate desorption by the studied soil parent material is not primarily controlled by biochar. Therefore, it is recommended to test the compatibility of BC to release the adsorbed sulfur, before applying it as an amendment to the soil. Linking the practice of on-farm sulfur management to OM management is also recommended.

Suitable Models for Describing Sulphate Desorption Kinetics in Selected Bauchi North Soils of Varying Parent Materials in the Nigerian Sudan Savanna

The modelling of sulphate desorption data is critical for a proper S diagnosis and fertilizer formulation to ensure profitable crop production. Five (5) models such as first-order, second-order, Elovich, fractional power, and parabolic diffusion were used to test the best model describing sulphate desorption kinetics in some soils from Bauchi-north, Sudan Savanna, Nigeria. To achieve this, soil samples were collected from three different parent materials namely Basement complex rock and two sedimentary rocks (Kerri-Kerri Formation and Chad Formation). The study showed that the parabolic diffusion and first-order models were found to describe S desorption data satisfactorily, characterized by relatively high R2 values and lowest S.E values by soil parent materials, respectively. While, the second-order, Elovich and Fractional power equations failed to describe the kinetics desorption of sulphate in all the studied soils, as judged by their high SE values. Therefore, the better fit of S desorption data to the first-order equation is an indication of probable ligand exchange of sulphate ion during the desorption process, and to parabolic diffusion equation suggests that diffusion-controlled phenomena are rate-limiting steps. Based on the findings, we concluded that the rate of sulphate desorption kinetics in these soils is mainly controlled by diffusion-controlled phenomena, which is critical for sulphate-based fertilizer formulations and applications.

Selectivity adsorption of sulfate by amino-modified activated carbon during capacitive deionization

ABSTRACT Capacitive deionization (CDI) is an environmentally friendly desalination technique with low energy consumption. However, unmodified carbon electrode materials have poor sulfate selectivity and adsorption capacity. In this work, to improve sulfate selectivity, we prepared activated carbon materials loaded with different amino contents by grafting amino groups via acid treatment for different times. In the competitive ion adsorption experiments, the sulfate selectivity of AC was only 0.64 and the amino-modified AC increased by 1.98-2.52 times due to the formation of stronger hydrogen bonds between the amino group and sulfate. AC-NH2-4 had the best selectivity and the sulfate selective coefficient was 2.25. The desorption of sulfate was 92.46% within one hour. In addition, the surface of the amino-modified activated carbon showed significantly improved electrochemical properties and better capacitance. The specific capacitance of amino-modified AC in different electrolyte solutions was consistent with the competitive adsorption results. The specific capacitance of amino-modified AC in Na2SO4 electrolyte solution was the highest. The modified electrode material also had the advantages of a higher adsorption capacity and excellent regeneration performance after continuous electric adsorption–desorption cycles. Therefore, it may have development potential to selectively adsorb sulfate in practical applications.

Sulfur accumulation in soil in a forested watershed historically exposed to air pollution in central Japan

Forest soils that have accumulated anthropogenic sulfur (S) as a result of historical air pollution may gradually or suddenly release inorganic sulfate either by desorption of inorganic sulfate or by mineralization of organic S, even after mitigation of previous air pollution. The release of inorganic sulfate from soil to surface water leads directly to watershed acidification. Thus, whether the organic S compounds that have accumulated in soils are resistant to microbial activity, due to formation of organo-mineral complexes, is essential information for management of forest soil ecosystem services. A nationwide air pollution monitoring survey by the Japanese Ministry of the Environment identified acidification of both surface water and soil in the Lake Ijira watershed of central Japan, presumably sourced from the Chukyo Industrial Area which lies upwind of the watershed. We compared the amount of S accumulated in the Ijira soils (Dystrudepts) with that in comparable soils in other watersheds in central Japan that were less affected by industrial air pollution. To determine how much mineral-associated organic S had accumulated in the Ijira soils, we measured pyrophosphate- extractable S. We used high-resolution x-ray fluorescence (HRXRF) spectroscopy with a double-crystal type high energy resolution spectrometer to estimate the oxidation states of both mineral-associated and free organic S. We also measured S isotope compositions to explore S dynamics in the soils. We found that more S had accumulated in the Ijira soils than in the comparable soils from other watersheds in central Japan. Concentrations of total S in the Ijira soils varied among soil horizons, reaching more than 1000 mg S kg−1 in some horizons. Mineral-associated organic S accounted for more than half of the total accumulated S. High-resolution x-ray fluorescence spectrum analyses showed that mineral-associated organic S was retained at both high and low oxidation states. The δ34S values of total S in the deeper horizons tended to be higher than those of the uppermost horizons of the Ijira soils and correlated positively with concentrations of extractable Al and Fe oxides (which characterize pedogenic minerals). Our results suggest that about half of total accumulated S in Ijira soils is stabilized in pedogenic minerals and the remainder comprises free organic S and adsorbed inorganic sulfates. There might have been repetitive mobilization and immobilization of S and the degraded organic S might have greater affinity with pedogenic minerals. Further, the ratio of total C to mineral-associated organic S correlated negatively with concentrations of extractable Al and Fe oxides, suggesting that organic S is more resistant to mineralization than organic C because S is preferentially associated with the pedogenic minerals. Monitoring of the chemical states of S compounds in soils historically exposed to air pollution is recommended to assist in prediction of the fate of the S compounds that have accumulated in them.

Acid mine drainage treatment with novel high-capacity bio-based anion exchanger

Aminated peat (termed PG-Peat) produced using polyethylenimine and glycidyltrimethylammonium chloride was used for the removal of sulphate from real acid mine drainage (AMD) in batch and column mode sorption studies. In the batch tests, the highest sulphate removal capacity achieved was 125.7 mg/g. PG-Peat was efficient and rapid in sulphate removal from AMD even at low temperatures (2–5 °C), achieving equilibrium within a contact time of 30 min. The PG-Peat column treating real AMD showed even higher sulphate uptake capacity (154.2 mg SO42−/g) than the batch sorption studies. The regenerative and practical applicability of PG-Peat was also tested in column set-ups using synthetic sulphate solutions (at pH 5.8 and pH 2.0). The sulphate uptake capacity obtained was higher in column mode when the solutions were treated at acidic pH (2.0) compared to pH 5.8. This could be attributed to the presence of cationized amine groups on PG-Peat under acidic pH conditions. Almost complete sulphate desorption was achieved with NaCl in the column that treated synthetic sulphate solution at pH 5.8, while the lowest desorption rates were observed in the column that treated acidic synthetic sulphate solution (pH 2).

Modification of Y‐Zeolite with Zirconium for Enhancing the Active Component Loading: Preparation and Sulfate Adsorption Performance of ZrO(OH)2/Y‐Zeolite

AbstractNowadays, several techniques have been utilized to remove sulfate from industrial sewage. The method using ZrO(OH)2 to adsorb sulfate is highly efficient and environmentally friendly, however, nano‐powdered ZrO(OH)2 is hard to be recycled owing to its great loss in the adsorption process, which brings enormous difficulties to industrial applications. In this research, to prevent adsorbents from losing, Y‐zeolite was modified with zirconium, including the incorporation of zirconium as heteroatoms into zeolite frameworks and ion‐exchange of Zr4+ into the cages of zeolite, and then used as carrier to load ZrO(OH)2 to fabricate adsorbent via the stepwise impregnation treatment. The as‐prepared supported adsorbent ZrO(OH)2/Y‐zeolite has a high SO42− adsorption capacity, a strong adsorption selectivity to SO42− and good adsorption‐regeneration circulation performances. The saturated SO42− adsorption capacity of 284.22 mg/g can be obtained under the following adsorption conditions: SO42− concentration of 10 g/L, the pH value of 1.5, the temperature of 25 °C. After five adsorption‐regeneration circulations, its adsorption capacity has decreased only about 10%. The adsorption‐ desorption of sulfate over ZrO(OH)2/Y‐zeolite adsorbent proceeds according to the ion‐exchange mechanism between sulfate and the surface hydroxyl groups.

Competitive Adsorption and Subsequent Desorption of Sulfate in the Presence of Various Anions in Soils

In this experiment we investigated the influence of various anions including oxalic acid encountered as solution phase in soil on the adsorption and desorption of sulfate in Chungwon Bt soil. The effect of chloride and nitrate on the adsorption of sulfate was not significant, suggesting that sulfate was better able to compete for adsorption sites at concentrations studied, in contrast to the large reduction in the amount of chloride adsorbed in the presence of sulfate. The results of competition for sorption sites between sulfate and anion showed that the simultaneous presence of two anions in solution was effective in reduction of competing anion at a maximum value of adsorption, due to the similar adsorption mechanism for anion competition. Therefore, the variation in the buffer power of the acids will produce a change in the strength and amount of adsorption and the competitive ability.

Sulphate sorption and desorption characteristics of selected Malawi soils

The soils selected for this study represented major soil types in Malawi. They exhibited a wide range of physical and chemical properties. Their pH values ranged from 4.7 to 6.7 and their contents of clay fraction (<0.002 mm) ranged from 170 to 500 g kg-1, organic matter from 6.7 to 39.3 g kg-1, free Fe2O3 from 18.9 to 44.7 g kg-1, and free Al2O3 from 17.8 to 45.6 g kg-1. The soils varied widely in their SO4 sorption behaviour. Soil pH was negatively and significantly correlated (P<0.05) with the bonding energy (k) of SO4 by the soil, Langmuir sulphate sorption maximum (b) and maximum buffering capacity (MBC). Soil organic matter was positively and significantly related with k, b and MBC at the same level of significance (P<0.05). Free Al2O3 and Fe2O3 were positively and significantly related (P<0.001) with b and MBC. Free Fe2O3 and Al2O3 were positively and significantly correlated with k at P<0.01 and P<0.001 respectively. About 90 and 91% of the total variations in the sulphate sorption maxima were accounted for by free Al2O3 and Fe2O3 respectively. Key words: Sulphate, sorption, desorption, Langmuir, hysteresis, affinity.

Removal of SO2from Flue Gas Using Basic Aluminum Sulfate Solution with the Byproduct Oxidation Inhibition by Ethylene Glycol

This paper presents a novel wet flue gas desulfurization (FGD) technology based on a basic aluminum sulfate (BAS) desorption regeneration process, in which ethylene glycol (EG) was first employed to inhibit the byproduct oxidation. The operating parameter effect on SO2 absorption efficiency and oxidation efficiency of sulfite was thoroughly examined in a lab-scale bubbling column. The results indicated that both the amount of aluminum and basicity play important roles on the desulfurization time with above 90% absorption efficiency. The BAS-based desulfurization process was more suitable for a low temperature and low gas flow rate. High inlet SO2 concentrations may contribute to the mass-transfer rate of SO2, and the SO2 absorption efficiency remained above 90% when the pH value was over 3.10. With the addition of 1% (v/v) EG in BAS solution, the oxidation efficiency dropped dramatically from 86 to below 10% (in 120 min). On the basis of the two-film theory, a model of the SO2 absorption process was developed and the mass-transfer characteristics were analyzed. The calculation results indicated that the SO2 absorption process for this system was decided by a combination of both the gas- and liquid-phase diffusion controls.

Sulfate Adsorption onto and Desorption from Silicon Dioxide Films

Sulfate ion adsorption onto and desorption from silicon dioxide surface has been investigated using a quartz crystal microbalance with dissipation monitoring capability (QCM-D). A viscoelastic model was used to calculate adsorption/desorption amount. Experimental results indicate that the adsorption from Na2SO4 and K2SO4 solutions at a pH of 6.8 results in the formation of a multilayer sulfate film with co-adsorption of Na+ or K+ ions. Sulfate ion uptake varied linearly with sulfate concentration in the range 0.01 M to 0.6 M. The first order rate constant for sulfate desorption with DI water is approximately 0.04 s−1 and 0.12s−1 for flow rates of 0.2 ml/min and 1.0 ml/min, respectively.

Sulfate Adsorption onto and Desorption from Silicon Dioxide Films

Sulfate ion adsorption onto and desorption from silicon dioxide surface has been investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D) capability. A viscoelastic model was used to calculate adsorption/desorption amount. The sulfate ion uptake at pH 6.8 varied linearly with sulfate concentration in the range 0.01 M to 0.6 M. Based on the hydrated radius of sulfate ion, the adsorption (uptake) amount indicated multilayer adsorption. The first order rate constant for sulfate desorption with DI water is approximately 0.04 s-1 and 0.12s-1 for flow rates of 0.2 ml/min and 1.0 ml/min, respectively.

Sulfate Adsorption onto and Desorption from Silicon Dioxide Films

Sulfuric acid, especially in a mixture with hydrogen peroxide, is a commonly used chemical in the surface preparation of silicon. Removal of adsorbed sulfate based contaminants by DI rinsing is an important step in processing steps using sulfuric acid. Fundamental information on the rate and extent of adsorption and desorption of sulfate on surfaces is very scarce. This presentation will discuss sulfate adsorption onto and desorption from SiO2 surfaces using a quartz crystal microbalance with dissipation monitoring capability (QCM-D). The amount of adsorption of sulfate was estimated from frequency shifts of a SiO2 coated AT-cut quartz crystal. The quartz crystal, with a fundamental frequency of 5 MHz, consists of a quartz disk of 14mm diameter, and gold electrodes that are sputtered on both sides of the quartz disc. The SiO2 coating, 300 nm thick, was deposited by PVD onto the active side of quartz crystals. Figure 1 shows a real time frequency recording of a SiO2 coated crystal exposed to flowing 0.2 M sodium sulfate solution. The frequency and dissipation change were measured for different harmonics of the fundamental crystal frequency at the same time. The divergent overtones and high dissipation factor indicated a “soft” adsorbed layer, rendering the Sauerbrey relation invalid for mass calculation from frequency changes. Hence, a viscoelastic model was used for conversion of frequency changes to mass changes. It can be seen the sulfate adsorption onto and desorption from SiO2 surface happens fairly quickly as marked in the figure. At 22 C, the rate constants of desorption are about 0.06 s-1 and 0.16–0.18 s-1 for flow rates of 0.2 ml/min and 1.0 ml/min respectively. The adsorption isotherm of sulfate is shown in figure 2. The equilibrium adsorption increases linearly with sulfate concentration at the range of 0.1 M to 0.6 M. Figure 1

Effect of phosphate status and pH on sulphate sorption and desorption

SummaryFor soils from tea estates in northern India, sulphate sorption was of a similar magnitude to, and sometimes exceeded, phosphate sorption. Only a small part of this relatively large sulphate sorption was caused by the low pH of these soils. Most was caused by increased negative charge as a result of prior reaction over many decades with phosphate fertilizers. This decreased sorption of both phosphate and sulphate, but the effect on phosphate was larger. This is compatible with a model in which the mean location of the charge on the adsorbed phosphate ions is closer to the surface than for sulphate. On soils of low phosphate status, sulphate desorption curves showed hysteresis; on soils of high phosphate status, they did not. Further, on soils of high phosphate status, displacement of sulphate by phosphate solutions was faster. We interpret these observations as showing that, for low phosphate status soils, sulphate ions penetrated the surface, but for high phosphate status soils it did not because the pathways by which sulphate diffuses into the adsorbing material were blocked. We also show that, with increasing soil phosphate status, phosphate solutions were less effective in displacing sorbed sulphate. We think this also occurred because reaction with phosphate had decreased the affinity for phosphate more than it decreased the affinity for sulphate.

Surface Energetics of Protein Adsorption on to Chromatographic Supports

Protein separation behavior during adsorption chromatography is governed by system thermodynamics and kinetic factors. Hydrophobic interaction chromatography (HIC) is widely utilized since many important biological products present a quite hydrophobic character. In this work, the interaction between a set of model proteins (n = 9) and a commercial adsorbent (Phenyl Sepharose FF, high substitution, GE Healthcare) was studied via extended DLVO (XDLVO) calculations. Psychochemical properties of both separand and adsorbent were gathered by contact angle determination and zeta potential measurements. Proteins were subjected to the mentioned measurements in the hydrated and the dehydrated state, so as to simulate protein properties in a low vs. high salt concentration milieu, respectively. In HIC, protein adsorption usually take place at high concentrations of ammonium sulphate (up to 1.7M) and protein desorption occurs by decreasing salt concentration in the mobile phase. The mentioned XDLVO approach allowed the calculation of the free energy of interaction vs. distance profiles between the interacting surfaces, in the aqueous environment provided by the operating mobile phase. XDLVO calculations were correlated with the actual chromatography behavior of the studied model proteins. This correlation revealed that these proteins can be segregated in two main groups, according to surface energy calculations and elution position during chromatography: i) strong binding showing a deeper secondary minimum energy >|0.20| kT ii) and weak binding having a small secondary minimum energy <|0.12| kT, thus calculations were able to predict early or late elution from a gradient chromatography experiment; the more the calculated interaction energy, the stronger will be protein binding and the later will be the elution time. The knowledge generated from these studies will generate a better understanding of real downstream bioprocess behavior which could, in turn, facilitate process design and optimization.

Assessing Mg and Ca depletion from broadleaf forest soils and potential causes – A case study in the Morvan Mountains

Compared to conifers, broadleaf forests are less prone to soil and water acidification, because (1) they often have soils with larger exchangeable base cation pools, (2) the pollution scavenging capacity of deciduous trees is lower, and (3) they grow more slowly and are often less intensively managed. Since the 1980s, atmospheric deposition acidity has strongly decreased in forest ecosystems, so that the present acidification status of broadleaf forests should be improving. We used a 35year-old beech plot in the Morvan Mountains (Burgundy, France) to question past and present acidification processes in broadleaf ecosystems.Soil exchangeable Mg, Ca and K pools measured from samples collected in 1974 and 2001 were compared and input–output budgets were computed over the 2003–2008 period. The objectives were (i) to assess Mg and Ca pool size changes over 1974–2008, (ii) to discuss the potential causes of these changes and, (iii) discuss the limits of conventional methods to study nutrient pool size changes (soil data comparison and nutrient budgets).Soil exchangeable Mg pools decreased during the 1974–2001 period while Ca and K pools remained constant, and very small. Soil solution monitoring and input–output budgets over the 2003–2008 period suggested an ongoing loss of exchangeable Ca and Mg, partly due to the desorption of sulphate from the soil which induced Mg, Ca and K depletion. Given the very low concentration in exchangeable base cations, and assuming no change in soil spatial variability, we computed that resampling soils at 10years intervals may not unequivocally demonstrate a gain/loss of exchangeable base cations. Foliar Mg concentrations were continuously below the deficiency level, K and N concentrations decreased but Ca did not.From this, we discuss the validity/limits of the different approaches used, how the ecosystem can cope with such low levels of nutrients, and the fluxes and processes within the ecosystem that should be investigated in this perspective.

Effective adsorption of sulfate ions with poly(m-phenylenediamine) in aqueous solution and its adsorption mechanism

Sulfate adsorption by poly(m-phenylenediamine)s (PmPDs) with various oxidation states synthesized through chemically oxidative polymerization was investigated. Series of sorption experiments were conducted, and the adsorption mechanism and the relationship between oxidation state and adsorption performance were studied with the characterization of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), pH tracking and energy calculation. The results show that the adsorption performance in acidic solution is improved with the decrease of oxidation state of poly(m-phenylenediamine) (PmPD). The rate constant is as high as 425.5 mg/(g·min) in the short equilibrium time of 30 min. The estimated highest adsorptivity of sulfate ions is 95.1%. According to the Langmuir equation, the adsorbance is 108.5 mg/g. The sulfate desorption efficiency is about 95% and the accumulative adsorbance is up to 487.95 mg/g in 5 cycles.

Soil Acid-Base Chemistry of a High-Elevation Forest Watershed in the Great Smoky Mountains National Park: Influence of Acidic Deposition

Understanding the acid-base chemistry of soil and the soil processes related to the release or retention of sulfate and nitrate is important in order to predict watershed recovery from long-term acid deposition. Soils were sampled from the Noland Divide Watershed (NDW), a small, high-elevation watershed in the Great Smoky Mountains National Park receiving high rates of acid deposition over several decades. Soil samples were measured for chemical properties related to acidification and used to conduct sulfate adsorption and nitrogen (N) incubation experiments. Shallow soil was higher in acidic and basic ions than deeper soils, and the mean effective cation exchange capacity was 8.07, 5.06, and 3.57 cmolckg �1 in the A, Bw, and Cb horizons, respectively. In all three soil horizons, the base saturation was equal to or below 7% and the ratio of Ca/Al was below 0.01, indicating that the NDW is very sensitive to acid deposition. Based on results from sulfate adsorption isotherms, the NDW has not reached its maximum sulfate adsorption saturation and is likely able to retain further additions of sulfate. Desorption of sulfate from NDW soils is expected if sulfate concentrations in soil solution drop below 50 μeq L �1 but is highly dependent on soil pH and organic carbon content. Total soil organic N was 500 times greater than inorganic N in the A soil horizon, and net N mineralization and nitrification remained constant during a 28-day incubation indicating a large reservoir of N substrate for soil microbes. Nitrogen experiment results suggest that nitrate export from the watershed is largely controlled by biological processes rather than by nitrate deposition flux. Soil data collected in this study contributes to our understanding of biogeochemical processes affecting the response of acid-impacted ecosystems such as the NDW to future changes in atmospheric deposition.

Site-to-site variability and temporal trends of DOC concentrations and fluxes in temperate forest soils

Here, we report site-to-site variability and 12–14 year trends of dissolved organic carbon (DOC) from organic layers and mineral soils of 22 forests in Bavaria, Germany. DOC concentrations in the organic layer were negatively correlated with mean annual precipitation and elevation whereas air temperature had a positive effect on DOC concentrations. DOC fluxes in subsoils increased by 3 kg ha−1 yr−1 per 100 mm precipitation or per 100 m elevation. The highest DOC concentrations were found under pine stands with mor humus. Average DOC concentrations in organic layer leachates followed the order: pine>oak>spruce>beech. However, the order was different for mean DOC fluxes (spruce>pine>oak>beech) because of varying precipitation regimes among the forest types. In 12 of 22 sites, DOC concentrations of organic layer leachates significantly increased by 0.5 to 3.1 mg C L−1 yr−1 during the sampling period. The increase in DOC concentration coincided with decreasing sulfate concentration, indicating that sulfate concentration is an important driver of DOC solubility in the organic layer of these forest sites. In contrast to the organic layer, DOC concentrations below 60 cm mineral soil depth decreased by <0.1–0.4 mg C L−1 yr−1 at eight sites. The negative DOC trends were attributed to (i) increasing adsorption of DOC by mineral surfaces resulting from desorption of sulfate and (ii) increasing decay of DOC resulting from decreasing stabilization of DOC by organo-Al complexes. Trends of DOC fluxes from organic layers were consistent with those of DOC concentrations although trends were only significant at seven sites. DOC fluxes in the subsoil were with few exceptions small and trends were generally not significant. Our results suggest that enhanced mobilization of DOC in forest floors contributed to the increase of DOC in surface waters while mineral horizons did not contribute to increasing DOC export of forest soils.

The effects of a whole-watershed calcium addition on the chemistry of stream storm events at the Hubbard Brook Experimental Forest in NH, USA

Patterns of storm runoff chemistry from a wollastonite (calcium-silicate mineral, CaSiO 3) treated watershed (W1) were compared with a reference watershed (W6) at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire (NH), USA to investigate the role of Ca 2+ supply in the acid–base status of stream chemistry. In the summer of 2003, six storm events were studied in W1 and W6 to evaluate the effects of the wollastonite treatment on the episodic acidification of stream waters. Although mean values of Ca 2+ concentrations decreased slightly from 33.8 to 31.7 μmol/L with increasing stream discharge in W1 during the events, the mean value of acid neutralizing capacity (ANC) was positive (1.2 μeq/L) during storm events, compared to negative values (− 0.2 μeq/L) in W6. This pattern is presumably due to enhanced Ca 2+ supply in W1 (20.7 to 29.0% of dissolved Ca 2+ derived from the added wollastonite) to stream water as a result of interflow along shallow flowpaths. In addition, the application of wollastonite increased pH and dissolved silica (H 4SiO 4) concentrations, and decreased the concentration of inorganic monomeric Al (Al i) in W1 in comparison with W6 during storm events. Despite an increase in SO 4 2− concentration, likely due to desorption of sulfate from soil after the treatment, the watershed showed an increase in ANC compared to the reference watershed, serving to mitigate episodic acidification.