Effective capping of dissolved sulfide generated in Ulva prolifera-rich marine sediments by iron-rich red soil.

Bloom-induced macroalgal enrichment on the seafloor can substantially facilitate dissolved sulfide (DS) production through sulfate reduction. The reaction of DS with sedimentary reactive iron (Fe) is the main mechanism of DS consumption, which however usually could not effectively prevent DS accumulation caused by pulsed macroalgal enrichment. Here we used incubations to investigate the performance of Fe-rich red soil for buffering of DS produced from macroalgae (Ulva prolifera)-enriched sediment. Based on our results, a combination of red soil additions (6.8 kg/m2) before and immediately after pulsed macroalgal deposition (455 g/m2) can effectively cap DS within the red soil layer. The effective DS buffering is mainly due to ample Fe-oxide surface sites available for reaction with DS. Only a small loss (4 %) of buffering capacity after 18-d incubation suggests that the red soil is capable of prolonged DS buffering in macroalgae-enriched sediments.

Assessing the impacts of differential depositional settings and/or anthropogenic perturbations on sulfur and iron diagenesis in sediments of the Bohai Sea and North Yellow Sea.

Natural processes and human activities exert important impacts on elemental cycling in coastal sediments, which has not been well documented. Sediments in the Bohai Sea and North Yellow Sea were investigated to assess the impacts of the Yellow River inputs and/or anthropogenic perturbations on diagenesis of iron and sulfur. Labile iron (0.5 M HCl-extractable iron) in the sediments is low due to iron-poor nature of source materials. Dynamic regimes and low availability of labile organic carbon (OC) result in relatively low sulfide contents in deltaic sediments. However, low but continuous supply of labile OC exported from an anthropogenically impacted bay could substantially elevate sulfide burial in sediments near the bay. Neither offshore oil exploitations nor frequent algal blooms in the seas have detectable influences on iron and sulfur diagenesis in the sediments. The sediments are capable of quickly consuming porewater sulfide by reaction with reactive iron under the current conditions.

Diagenesis of sulfur, iron and phosphorus in sediments of an urban bay impacted by multiple anthropogenic perturbations.

Solid-phase speciation and porewater chemistry measured by the diffusive gradients in thin films (DGT) technique were used to understand the diagenesis of sulfur (S), iron (Fe), and phosphorus (P) in sediments of Jiaozhou Bay (China), which has been impacted by multiple anthropogenic perturbations. Despite water eutrophication, sediments of the bay are low in organic carbon and sulfide, but high in unsulfidized Fe(II). Dissimilatory iron reduction (DIR) prevails in sediments of the bay, and there is no evidence for responses of S and Fe diagenesis to the water eutrophication, which is largely attributable to unique depositional and diagenetic regimes in association with multiple anthropogenic perturbations. Good coupling of porewater Fe2+ and P in the porewaters suggests that P mobilization is driven mainly by DIR. Low Fe2+/P ratios in porewaters imply that oxidative regeneration of Fe oxides within the upper sediments is incapable of efficiently scavenging upward diffusing P.

Sulfur, iron, and phosphorus geochemistry in an intertidal mudflat impacted by shellfish aquaculture.

Dissolved sulfide, iron (Fe), and phosphorus (P) in a mudflat (Jiaozhou Bay, China) impacted by shellfish aquaculture were measured in situ by the diffusive gradients in thin films (DGT) technique. A combination of porewater and solid-phase chemistry was used to characterize the interplays of Fe and S, and their control on P mobilization. Below the subsurface layer, two times higher fluxes (FDGT) of dissolved Fe2+ from porewater to the DGT device than those of dissolved sulfide indicate that dissimilatory iron reduction (DIR) dominates over sulfate reduction (SR). Spatial coupling of dissolved Fe2+ and P points to P release driven mainly by reductive dissolution of Fe. Much higher FDGT values of dissolved Fe2+ relative to dissolved P imply that oxidative regeneration of Fe oxides at the sediment-water interfaces (SWIs) of the transitional mudflat serves as an effective "iron curtain" of upward diffusing P. In the mudflat sediments of DIR prevalence, the accumulation of total reduced inorganic sulfur (TRIS) is dampened, which can largely ascribed to enhanced oxidative loss of sulfide and/or limited availability of degradable organic carbon in the dynamic regimes. Low dissolved sulfide concentrations in the sediments leave the majority of reactive Fe unsulfidized and thus abundantly available to buffer newly produced sulfide.

In situ, high-resolution DGT measurements of dissolved sulfide, iron and phosphorus in sediments of the East China Sea: Insights into phosphorus mobilization and microbial iron reduction.

Dissolved sulfide, iron (Fe), and phosphorus (P) concentrations in sediments of the East China Sea were simultaneously measured in situ by diffusive gradients in thin films (DGT) technique. The results, by combination with solid-phase Fe speciation, were used to characterize the interplays of Fe, S and P. Diverse distributions of dissolved sulfide among the sites are attributable to highly dynamic diagenetic regimes and varying availability of labile organic carbon (OC). The DGT technique provided high-resolution evidence for coexistence of microbial iron reduction (MIR) and sulfate reduction in localized zones, and for Fe-coupled P mobilization. Measured Fe2+/P ratios suggest that Fe2+ reoxidatiion at the oxic zones can serve as an efficient scavenger of P. Empirical estimation indicates that MIR plays an important role in anaerobic OC mineralization in the sediments, which is a combined result of low availability of labile OC, high reactive Fe content, and unsteady diagenetic regimes.

Spatial distribution of organic and pyritic sulfur in surface sediments of eutrophic Jiaozhou Bay, China: clues to anthropogenic impacts.

Anthropogenic perturbations exert important impacts on sulfur geochemistry in marine sediments. In the study, chemical extraction was used to quantify four sulfur pools, i.e., pyrite, humic-acid sulfur (HA-S), fulvic-acid sulfur (FA-S), and residual organic sulfur (ROS), in surface sediments of eutrophic Jiaozhou Bay. Results show that riverine inputs are the main control on organic matter (OM) distribution in the sediments. OM enrichment in the eastern coast is mainly due to discharges of anthropogenic wastes. Spatial coupling of pyrite and FA-S vs. TOC points to the impacts of OM enrichment on formation and preservation of pyrite and FA-S. Poor spatial coupling of HA-S vs. TOC is due to low fractions of diagenetic OS in the pool. ROS is mainly from riverine inputs and anthropogenic OS has been superimposed on this pool. Spatial coupling among TOC, pyrite-S and FA-S is a sensitive indicator of anthropogenic impacts on benthic processes of the bay.

Reductive reactivity of iron(III) oxides in the east china sea sediments: characterization by selective extraction and kinetic dissolution.

Reactive Fe(III) oxides in gravity-core sediments collected from the East China Sea inner shelf were quantified by using three selective extractions (acidic hydroxylamine, acidic oxalate, bicarbonate-citrate buffered sodium dithionite). Also the reactivity of Fe(III) oxides in the sediments was characterized by kinetic dissolution using ascorbic acid as reductant at pH 3.0 and 7.5 in combination with the reactive continuum model. Three parameters derived from the kinetic method: m 0 (theoretical initial amount of ascorbate-reducible Fe(III) oxides), k' (rate constant) and γ (heterogeneity of reactivity), enable a quantitative characterization of Fe(III) oxide reactivity in a standardized way. Amorphous Fe(III) oxides quantified by acidic hydroxylamine extraction were quickly consumed in the uppermost layer during early diagenesis but were not depleted over the upper 100 cm depth. The total amounts of amorphous and poorly crystalline Fe(III) oxides are highly available for efficient buffering of dissolved sulfide. As indicated by the m 0, k' and γ, the surface sediments always have the maximum content, reactivity and heterogeneity of reactive Fe(III) oxides, while the three parameters simultaneously downcore decrease, much more quickly in the upper layer than at depth. Albeit being within a small range (within one order of magnitude) of the initial rates among sediments at different depths, incongruent dissolution could result in huge discrepancies of the later dissolution rates due to differentiating heterogeneity, which cannot be revealed by selective extraction. A strong linear correlation of the m 0 at pH 3.0 with the dithionite-extractable Fe(III) suggests that the m 0 may represent Fe(III) oxide assemblages spanning amorphous and crystalline Fe(III) oxides. Maximum microbially available Fe(III) predicted by the m 0 at pH 7.5 may include both amorphous and a fraction of other less reactive Fe(III) phases.

Reactive iron and its buffering capacity towards dissolved sulfide in sediments of Jiaozhou Bay, China.

Reactive iron (Fe) oxides in marine sediments play a critical role in removal of free sulfide. In this study, 0.5 and 6 N HCl-extractable Fe, acid volatile sulfide (AVS), and pyrite were examined in sediments at three sites of eutrophic Jiaozhou Bay to investigate the interactions of sulfur and Fe and possible influences of eutrophication on free sulfide removal. The results indicate that formation and accumulation of AVS and pyrite are limited by low availability of labile organic matter, despite eutrophication of the bay water. Quick buffering of free sulfide proceeded mainly via consumption of 0.5 N HCl-extractable Fe (labile Fe), however, the consumption did not result in a depletion of the Fe pool. High residual buffering capacity enables a quick removal of free sulfide in porewater, and thereby it is difficult for sulfide to accumulate and to cause detrimental effects on benthic organisms at the present steady state. Significant effects of eutrophication on Fe and sulfur geochemistry is restricted only to the estuarine sediments which were subject to direct wastewater discharges, whereas no such effects were observed in other sediments of the bay.

Decolorization of methylene blue by delta-MnO2-coated montmorillonite complexes: emphasizing redox reactivity of Mn-oxide coatings.

Delta-MnO(2) coatings on clay substrates tend to be poorer in crystallinity as compared with their discrete counterparts, which may be of environmental significance for adsorption and oxidation of contaminants. Discrete delta-MnO(2) particles and three delta-MnO(2)-coated montmorillonite complexes with varying MnO(2) loadings (4.8-34.9%) were synthesized, and oxidative decolorization of methylene blue (MB) by the synthetic materials was investigated in batch systems. Results showed that oxidative decolorization of MB increased with increasing loading of Mn-oxide coatings, whereas oxidation capacity of the coatings, on the basis of unit mass of MnO(2), tended to decrease. Initial reaction rate of MB oxidation by both delta-MnO(2) coatings and their discrete counterpart increased linearly with increasing Mn-oxide loadings, but the rate of the former was higher than that of the latter. An increase in humic acid concentration displayed a progressively enhanced promotive effect on MB decolorization, whereas the promotive effect was greatly suppressed at lower pH.

Adsorption of phosphate on hydroxyaluminum- and hydroxyiron-montmorillonite complexes.

One hydroxyaluminum-montmorillonite complex (HyAl-Mt), two hydroxyiron-montmorillonite complexes (HyFe-Mts) with different iron contents, and three hydroxyiron/aluminum-montmorillonite complexes (HyFeAl-Mts) with various Fe:Al molar ratios were synthesized. Behavior and kinetics of phosphate (P) sorption on selected Mt-complexes mentioned above were investigated under acidic conditions. The results indicated that the intercalations of polymeric HyFe and/or HyAl ions in interlayers of Na-saturated montmorillonite (Na-Mt) caused significant changes in surface properties of the Na-Mt, such as cation exchange capacity, specific surface area, pH at zero point of charge. In pH range tested (3.0-6.5), P adsorption on the Mt-complexes decreased with increasing pH, whereas the effect became weaker with increasing Fe contents in the Mt-complexes. The adsorption capacities of the HyFeAl-Mts were greater than those of the HyAl-Mt and HyFe-Mt, which could be attributed to decreasing crystallinity of Fe and Al oxides in the HyFeAl-Mts. The equilibrium adsorption of P on the Mt-complexes could be well described using the Langmuir isotherm, and the kinetics of P adsorption could be well described by both the pseudo-second-order and Elovich models. An increase in Fe contents in the Mt-complexes could enhance the initial kinetic rate of P adsorption, as suggested by the Elovich models. It is inferred that a great number of Fe-related active sorption sites have been located on the outer surfaces of the HyFe-Mt, as indicated by extremely high alpha value in the Elovich model. Previous studies focusing mainly on P sorption on HyAl-Mt complexes might have underestimated the contributions of Mt-complexes to P retention in acidic soils high in Fe contents.

Oxidative decolorization of methylene blue using pelagite.

Pelagite generally has large surface area and high adsorbing and oxidizing reactivity due to highly amorphous nature, and high reducing potential of Mn (hydro)oxide phases present in it. In the present study, pelagite, collected from the East Pacific Ocean, was tested as a potential oxidant for decolorization of methylene blue (MB) in a batch system under air-bubbling and motor-stirring conditions. The effects of suspension pH (3.0-10.0), MB concentration (10-100 mgL(-1)) and loading (0.2-3.0 gL(-1)), and particle size (100-200 mesh) of pelagite on kinetics of MB decolorization were assessed. Results show that in typical concentration range of dye wastewaters (10-50 mgL(-1)), pelagite can be used as a highly efficient material for oxidative degradation of MB. MB decolorization was through a surface mechanism, that is, formation of surface precursor complex between MB and surface bound Mn(III, IV) center, followed by electron transfer within the surface complex. Iron (hydro)oxide phases present in the pelagite did not play an important role in MB decolorization. Suspension pH exerted double-edged effects on MB decolorization by influencing the formation of surface precursor complex, and reducing potential of the system. Kinetic rate of MB decolorization is directly proportional to saturation degree of available reaction sites by MB adsorption. At the initial and later stages, the kinetics for MB decolorization with respect to MB concentration, pelagite loading, and particle size could be described separately using two pseudofirst rate equations, except at very high pelagite loading (3.0 mgL(-1)). Accumulation of Mn(2+) and probably some organic intermediates exerted marked inhibitory effect on MB decolorization. Vigorous dynamic condition was favorable for MB decolorization. The presence of oxygen could enhance MB decolorization to a limited extent.

Removal of an anionic dye by adsorption/precipitation processes using alkaline white mud.

Alkaline white mud (AWM) has been investigated as a low-cost material for removal of an anionic dye, acid blue 80. The effects of contact time, initial pH of dye solution, AWM dosage, and the presence of inorganic anion sulphate or phosphate ion on removal of the dye were evaluated. The results show that AWM could be used as an effective material for removal of acid blue 80 in a pre or main process, particularly at high dye concentration (>300 mgL(-1)), reaching maximum removal efficiency of 95%. At low dye concentration, surface adsorption is mainly responsible for the dye removal, while chemical precipitation of the dye anions with soluble Ca(2+) and Mg(2+) may play a dominant role for the dye removal at high concentration, producing much less sludge than conventional adsorption method. Solution pH has only a limited effect on the dye removal due to high alkalinity and large pH buffer capacity of AWM suspension and thereby pH is not a limiting factor in pursuing high dye removal. The presence of SO(4)(2-) could reduce the dye removal by AWM only when SO(4)(2-) concentration is beyond 0.7 mmolL(-1). The dye removal may be significantly suppressed by the presence of phosphate with increasing concentration, and the reduction in the dye removal is much larger at high dye concentrations than at low ones.

Study on degradation of methyl orange using pelagite as photocatalyst.

Pelagite of East Pacific Ocean as a low-price, very high reserve photocatalyst, has been successfully used for the first time to realize total degradation and decoloration of methyl orange within 120 min through photocatalysis. The photocatalysis and degradation performance of pelagite for methyl orange under the effect of various factors has been studied. Results show that pelagite has satisfactory photocatalysis effect in degradation of organic compounds.

Sorption of an anionic dye by uncalcined and calcined layered double hydroxides: a case study.

Layered double hydroxides (LDHs) with a Mg/Al molar ratio of 2:1 were synthesized by using a co-precipitation method and their calcined products (CLDHs) were obtained by heating Mg/Al-LDHs at 500 degrees C. Sorption of a weak acid dye, Brilliant Blue R (BBR), by LDHs and CLDHs indicated equilibrium time required for BBR sorption by CLDHs was less than 12h, regardless of initial concentration of BBR, whereas BBR sorption by LDHs was longer than 20 h. Sorption capacity of CLDHs was much larger than that of LDHs. Therefore, CLDHs could be used to remove anionic dyes of relatively high concentrations while LDHs may only be used to remove anionic dyes of low concentrations. Isotherms for BBR sorption by CLDHs and LDHs were well described using the Freundlich and Langmuir equations, respectively. When the initial pH of BBR solutions was lower than 8.0, the final pH of the solution after sorption was enhanced and stabilized at 10.6-10.8. The effect of initial pH (<8.0) on BBR removal was negligible, which would be environmentally important for precipitation/co-precipitation of co-existing metal cations. The effects of both Cl- and SO4(2-) on BBR sorption by CLDHs were minimal; but the presence of CO(3)2- markedly reduced BBR removal. Thermal regeneration for re-use of LDHs and CLDHs after BBR sorption was feasible only within the first two cycles, after which the regenerated materials suffered from a large loss in their sorption capacities.

Interactions between variable-charge soils and acidic solutions containing fluoride: an investigation using repetitive extractions.

The reaction between two variable-charge soils and acidic solutions containing F was investigated with a repetitive extraction method. When added F concentration was 10(-4) mol/L, F did not markedly enhance solution pH in the whole prolonged extractions, in comparison with F-free acidic solution extractions. Most of the added F was adsorbed on soil surfaces and Al-F complexes were the dominant F species in solution. With increasing extractions, the fraction of Al-F slightly increased, arising from dissolution and/or desorption of Al. In comparison with F-free acidic solution extractions, F-induced Al dissolution did not significantly increase Al release, probably because of the modest reactivity of metal-F surface complexes at terminal sites at low F loading. The gradual decrease in Al release in the following extractions was due to the gradual depletion of readily reactive Al-containing mineral phases. In contrast to the low F loading, at an F concentration of 10(-3) mol/L, the pH was enhanced dramatically in the initial extraction and a high pH was maintained in the following extractions. In the initial extraction, the increase in negative surface charges and solution pH seemingly depressed proton-induced Al dissolution and enhanced readsorption of some positively charged Al-F complexes, resulting in low amounts of Al and F in solution. In the following several extractions, F-induced Al dissolution and desorption of Al-F complexes substantially enhanced the amounts of Al and F, and the fraction of Al-F complexes in solution. Several interconnected mechanisms such as ligand exchange, the release of OH(-) ions from soluble hydroxylated Al groups, desorption of Al as Al-F complexes, and F-induced breakdown of soil minerals were responsible for the alteration in pH, Al release, and the fraction of Al-F complexes in the later extractions. A molecular-level interpretation is needed in order to address the different impacts of varying F concentration levels on soil chemistry and environments.