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• Ferrous citrate significantly enhances the oxidation of Mn(II) and As(III) by AGS. • Bacteria rapidly degrade Fe(II)-citrate complex, producing abundant colloidal iron. • Colloidal iron (both II and III) stimulates extracellular proteins to generate ·O 2 −. • Extracellular ·O 2 − oxidizes Mn(II) to Mn(III/IV), which efficiently oxidizes As(III). Microbial manganese (Mn) oxidation plays a crucial role in shaping the fate of various elements, including arsenic (As). However, this process faces challenges in wastewater environments due to its inherent inefficiency and instability. In our initial research, a serendipitous discovery occurred: the addition of citrate to Fe(II)-containing wastewater stimulated the oxidation of Mn(II) by aerobic granular sludge (AGS). Subsequent experiments in four sequencing batch reactors (SBRs) over a 67-day period confirmed this stimulatory effect. The presence of Fe(II)-citrate led to a remarkable twofold increase in the oxidation of Mn(II) and As(III). The removal efficiency improved from 21±4 % to 87±7 % for Mn(II) and from 77.1 ± 1.8 % to 93.6 ± 0.2 % for As(III). The verification experiments demonstrated that the simultaneous addition of manganese-oxidizing bacteria (MnOB) and Fe(II)-citrate is an effective strategy for enhancing the oxidation and removal of Mn(II) and As(III) by AGS. Through a combination of genomic analysis, cell-free filtrate incubation, and bacterial batch cultivations (including monitoring the time-course changes of 17 substances and 2 free radicals), we elucidated a novel Mn(II) oxidation pathway in Pseudomonas , along with its stimulation method and mechanism. First, bacteria rapidly degrade citrate possibly via the citrate-Mg2+:H+ symporter (CitMHS) and the tricarboxylic acid (TCA) cycle, resulting in the formation of colloidal Fe(II), colloidal Fe(III), and biogenic iron (hydr)oxides (FeOx). Then, colloidal Fe(II) and colloidal Fe(III) stimulated extracellular proteins to produce superoxide radicals (·O 2 −). These radicals were responsible for oxidizing Mn(II) into Mn(III), ultimately forming biogenic manganese oxides (MnOx). Finally, MnOx effectively oxidized As(III) to the less toxic As(V). This innovative approach for bacterial Mn(II) oxidation holds promise for treating Mn(II) and As(III) in water and wastewater. Furthermore, the mechanism by which colloidal iron stimulates extracellular proteins to produce ·O 2 −, thereby facilitating Mn(II) oxidation, may widely occur across various engineering and natural ecosystems. [Display omitted] [ABSTRACT FROM AUTHOR]

In many parts of the global ocean, iron bioavailability is a critical factor controlling primary productivity. Because colloidal Fe (cFe) makes up a substantial proportion of the surface water Fe inventory, it is critical to evaluate the biogeochemical behaviour and availability of this quantitatively important Fe pool. Here we present and review a compilation of over 1300 soluble and corresponding dissolved Fe concentration measurements from which operationally-defined colloidal Fe data have been determined. Partitioning between cFe and the smaller soluble Fe (sFe) size fraction shows a high degree of variability in the surface ocean caused by the range of factors affecting size fractionation (e.g., Fe sources, ligand controls, thermodynamic controls, biological utilisation). Evaluation of the seasonality of sFe and cFe partitioning indicates that a strong decoupling between their respective biogeochemical cycling occurs in the spring time. This suggests that a seasonal component (e.g., seasonal biological growth, seasonal changes in Fe supply) is responsible for a proportion of the observed surface ocean variability. Below depths of 200 m, the partitioning becomes more constant (49 ± 17%). To better understand the bioavailability of the cFe pool, future work should complement traditional size-fractionated data with molecular level investigation into the functional group chemistry of organic Fe colloids and the mineralogy and surface chemistry of inorganic Fe colloids. [ABSTRACT FROM AUTHOR]

Abstract: Experiments were conducted using a semi-continuous batch protocol to leach iron from aerosol samples. In this procedure, aerosol samples were leached in an all-Teflon flow-through reaction chamber by multiple (3–4) aliquots (~40 mL) of leaching solution per predetermined leaching period. Several types of aerosols and leaching solutions were tested. The percent iron solubility obtained from aerosols collected in two urban regions was lower (~1%) than the solubility from samples collected in the open ocean (6–13%). This regional variability was greater than the variability obtained when leaching the same aerosols using different leaching solutions. A review of field and laboratory studies from the literature also suggests that different types of aerosols produce more variability in aerosol iron solubility estimates than do different leaching protocols. Colloidal iron was the dominant fraction of aerosol derived dissolved iron in all the experiments carried out during this study, suggesting that the dissolution of aerosol iron in seawater is dominated by the colloidal iron fraction. [Copyright &y& Elsevier]

Abstract: Peat bogs have the ability to produce strong chelate ligands (humic and fulvic acids) which enhance the weathering rates of iron-silicate minerals and greatly increase the solubility of the essential trace metal iron in river water. Fluvial networks link peat bogs with the ocean, and thus terrestrial-derived fulvic-iron complexes fuel the ocean''s biological productivity and biological carbon pump, but understanding this role is constrained by inconsistent observations regarding the behaviour of riverine iron in the estuarine mixing zone, where precipitation reactions remove iron from the water column. We applied a characterization of the colloidal iron carriers in peatland-draining rivers in North Scotland, using field-flow fractionation (FFF), in combination with end-member mixing experiments of river water sampled near the river mouth and coastal seawater using a 59Fe radiotracer method. According to our results, the investigated river contributed “truly dissolved” Fe concentrations of about 3300nmolL−1 to the ocean which is nearly two orders of magnitude higher than the dissolved iron contribution of the “average world” river (∼40nmolL−1). Thus we conclude that peatland-draining rivers are important sources of dissolved iron to the ocean margins. We propose highly electrostatic and sterical stabilized iron-organic matter complexes in the size range of <2kDa to be responsible for iron transport across the estuarine mixing zone. [Copyright &y& Elsevier]

The finding of various forms of iron, and less commonly of manganese, in numerous reverse osmosis (RO) membrane autopsies we have performed, have led us to call attention to the diverse forms of iron and manganese species in natural waters. Correlation of elemental composition analyses of foulants with contaminants in RO feedwater and pretreatment steps led to successful solutions of fouling problems by the modification of pretreatments, such as substitution of oxidation filtration strategies for iron and manganese reduction with sequestration with appropriate antiscalants. Cases where iron and manganese fouling failed to respond to sequestration with antiscalants, the existence in the RO feedwater of pre-existing colloidal iron and manganese particles are suspected. To fully control such fouling, speciation of the different forms of iron and manganese that exist in natural, industrial and wastewaters are central to process design, troubleshooting and RO system operation and maintenance. We have undertaken a literature review of iron and manganese colloidal chemistry as it pertains to the RO membrane process. This paper reviews the background for the association of iron with manganese in water treatment, natural sources of colloidal iron and manganese, early works on methods for the reduction of iron and manganese in traditional municipal waterworks where colloidal particles may be formed and not removed, speciation of forms of iron and manganese in water treatment, and the effects of colloidal iron and manganese on RO operation. The overall conclusion is that methods for the speciation and quantitation of colloidal forms of iron and manganese need to be fully developed and employed for the validation of methods useful for the control and/or removal of such foulants in RO feedwaters. [ABSTRACT FROM AUTHOR]

Abstract: During the Sub-arctic Ecosystem Response to Iron Enrichment Study (SERIES), the addition of ferrous iron to high-nitrate low-chlorophyll (HNLC) waters near Ocean Station PAPA (OSP: 50°N, 145°W) produced a phytoplankton bloom and CO2 drawdown, as evidenced by decreasing CO2 fugacity (fCO2). We analyzed five fractions or phases of iron: soluble (<0.03μm), dissolved (<0.22μm), total dissolved (acidified dissolved, <0.22μm), labile (unfiltered), and total (acidified, unfiltered). From these, we also calculated non-labile iron, colloidal iron (0.03–0.22μm), and both labile and non-labile particulate iron (>0.22μm). Here, we describe iron distributions and the evolution of iron phases in the upper ocean during the experiment. We also present an iron budget accounting for horizontal and vertical dilution. At the time of our first sampling eight hours after fertilization was completed, total iron reached 8.6nmolL−1 and dissolved iron was approximately 3nmolL−1. Early in the experiment the dissolved iron phase decreased the most rapidly and by late day 6 the integrated dissolved iron (8.6μmolm−2) represented less than 10% of the initial addition (90–95μmolm−2). However at this same time the total integrated iron at the centre of the patch was still 52μmolm−2 or almost 60% of the calculated initial addition. By day 12,45% of the added iron (from both injections) could be accounted for in the patch. The half-life of total iron in the patch for the first injection was estimated to be less than 5 days if dilution is not considered, but more than 13 days if dilution is taken into account. The most notable change in iron percentages from one form to another occurred early in the first week of the experiment where the predominant phase shift was from the colloidal portion of dissolved iron to labile particulate iron that could have been biologically induced or simply aggregation of oxyhydroxides. This was immediately followed by a physical event resulting in a reduction in the non-labile particulate iron due to sinking out of the patch. The second infusion did not change the relative concentration of the various pools of iron as might be expected, but this was likely due to the fact that it was a much smaller injection than the first. The most pronounced change after the second infusion was the reduction in the labile particulate pool which coincided with one of the largest decreases in silicate observed during the entire experiment. In general the gradual decrease in the fraction of the 10m colloidal iron as well as episodic losses of, or shifts in, integrated colloidal iron are thought to be the result of adsorption of colloidal iron to the plankton cell surfaces as well as aggregation of oxyhydroxides but could also be the result of utilization of colloidal iron by mixotrophic phytoplankton. [Copyright &y& Elsevier]

Fouling of 50 kDa MWCO OF membranes by iron colloid solutions was studied under constant pressure conditions and constant flux conditions at pH 5 and 7 and at various iron concentrations. By using measurements of both deposit density and changes in the transmembrane zeta potential, threshold fluxes were found that were significantly lower than the critical flux based on the increase in membrane resistance. Results are consistent with the initial phase of fouling involving pore blockage followed by cake filtration-like behavior. The dependence of fouling rate on iron colloid concentration can best be understood by realizing that only a small fraction of the iron colloid is of a size that contributes to fouling. [Copyright &y& Elsevier]