Irreversible adsorption of methyl arsenic, arsenate, and phosphate onto goethite in arsenic and phosphate binary systems.

Replacement of one anion from goethite with another provides useful insight into the irreversible adsorption of the first added anion in a binary system. The objective of this study was to investigate the irreversible adsorption of dimethylarsinate (DMA), monomethylarsonate (MMA), arsenate, and phosphate onto goethite at pH 4 in phosphate and arsenic binary systems by adding two anions sequentially. The density of irreplaceable phosphate or arsenic on goethite decreases to a limit with an increase in the initial concentration of the other anion. This limit is the density of MMA, arsenate, and phosphate that irreversibly adsorbs onto goethite, which depends on the adsorption density of these species in the adsorption phase. The highest limit of phosphate that cannot be replaced with DMA, MMA, and arsenate is respectively 1.9, 0.5, 0.8 micromol m(-2). The limit of irreplaceable DMA is zero, and the highest limit of irreplaceable MMA and arsenate is 0.9 and 1.1 micromol m(-2), respectively. The results indicate that the irreversible adsorption of one specific anion in arsenic and phosphate binary systems is affected not only by the adsorption density of this anion before the addition of the other anion but also by the nature of the other.

Proton-arsenic adsorption ratios and zeta potential measurements: implications for protonation of hydroxyls on the goethite surface.

Successfully modeling the surface charge of goethite and anion adsorption on goethite using a surface complexation model (SCM) alone cannot verify the assumptions of this model. In this study, the assumptions of 2-pK triple layer model (TLM) and two-site 1-pK basic stern model (BSM) were assessed with respect to their ability to interpret both the proton-anion adsorption ratios of dimethylarsinate (DMA), monomethylarsonate (MMA), and arsenate and their effect on the zeta-potential. The proton-DMA adsorption ratio is around 0.9 at pH 4.25 and 1.1 at pH 6.75 at DMA surface coverage ranging from 0 to 2 micromol m(-2), and the zeta-potential is independent of DMA adsorption at these two pH values. The proton-MMA adsorption ratio increases to 1.5 at pH 4 and 2.1 at pH 6.75 as the MMA surface coverage decreases to 0.5 micromol m(-2). The zeta-potential is less dependent on MMA adsorption at a surface coverage range of 0 to 1.8 micromol m(-2), and it then decreases with a further increase in the MMA surface coverage at pH 4 and 6.75. The proton-arsenate adsorption ratio decreases to 2 as the arsenate surface coverage approaches zero, and the zeta-potential decreases linearly with the increasing arsenate surface coverage at pH 4 and 6.75. Neither the 2-pK TLM nor the 1-pK BSM give a consistent interpretation of both the proton-arsenic adsorption ratio and the effect of arsenic on the zeta-potential. The results suggest that the 1-pK MUSIC model in which each type of surface hydroxyls has its own intrinsic proton-affinity constant and only one type of surface hydroxyls is involved in DMA, MMA, and arsenate adsorption is preferably pursued. The protonation degree of reactive hydroxyls estimated from proton-arsenic adsorption ratios is 0.2 at pH 4 and 0 at pH 6.75 in 0.001 M NaNO(3).

The quality of QSAR models: problems and solutions.

Assessment of the quality of goodness-of-fit and the confidence in predictivity (prediction power) are the main terms used to define the statistical quality of QSAR models. Three parts of this assessment can be defined as: (1) Measure of goodness-of-fit. (2) Validation of model stability. (3) Predictivity analysis. Currently there are no mandatory requirements for the validation methods to be used and rules for the quantitative confidence estimates. To compare the statistical quality of QSAR models it is necessary to have an overall statistical quality index which will depend on the goodness-of-fit, validation and predictivity results together. To do so it is necessary to define the set of mandatory parameters for all three parts of assessment listed above and develop the approach for overall quality estimates based on these parameters. It is also necessary to include into the overall index the penalty mechanism for parameter absence. The goal of the present study is to analyse parameters for all three parts of the QSAR model statistical quality assessment and investigate the flexible weighting approach for the overall statistical quality index development. Due the different statistical parameters traditionally used for assessment of goodness-of-fit it is necessary to create the mechanism, which allows flexible set of parameters to be used for the overall statistical quality index. Only after approval by scientific community and regulatory boards the final set of mandatory parameters can be selected.

Effect of replacing a hydroxyl group with a methyl group on arsenic (V) species adsorption on goethite (alpha-FeOOH).

Arsenate and methylated arsenicals, such as dimethylarsinate (DMA) and monomethylarsonate (MMA), are being found with increasing frequency in natural water systems. The mobility and bioavailability of these arsenic species in the environment are strongly influenced by their interactions with mineral surface, especially iron and aluminum oxides. Goethite (alpha-FeOOH), one of the most abundant ferric (hydr)oxides in natural systems, has a high retention capacity for arsenic species. Unfortunately, the sorption mechanism for the species is not completely understood, which limits our ability to model their behavior in natural systems. The purpose of this study is to investigate the effect of replacing a hydroxyl group with a methyl group on the adsorption behaviors of arsenic (V) species using adsorption edges, the influence of the background electrolyte on arsenic adsorption, and their effect on the zeta potential of goethite. The affinity of the three species to the goethite surface decreases in the order of AsO4=MMA>DMA. The uptake of DMA and MMA is independent of the concentration of background electrolyte, indicating that both species form inner-sphere complexes on the goethite surface and the most charge of adsorbed DMA and MMA locates at the surface plane. Arsenate uptake increases with increasing concentrations of background electrolyte at pH above 4, possibly due to that the charge of adsorbed arsenate is distributed between the surface plane and another electrostatic plane. DMA and lower concentrations of MMA have small effect on the zeta potential, whereas the zeta potential of goethite decreases in the presence of arsenate. The small effect on zeta potential of DMA or MMA adsorption suggests that the sorption sites for the anions is not important in controlling the surface charge. This observation is inconsistent with most adsorption models that postulate a singly coordinated hydroxyls contributing to both the adsorption and the surface charge, but supports the thesis that the charge on the goethite surface comes primarily from protonation of the triply bound oxygen atoms on the surface.

Microbial cellulose decomposition in soils from a rifle range contaminated with heavy metals.

The objective of this study was to assess the effects of heavy metals on microbial decomposition of cellulose in heavy metal-contaminated soils using a cotton strip assay. The assay is a measure of the potential of soil microorganisms to decompose the plant polymer, cellulose. Cellulolytic activity in soil was assessed by determining the reduction in tensile strength of the buried cotton strips over a 25- and 45-day period. Soils were obtained from a rifle range that contain high levels of lead, copper and zinc. The site has been used for approximately 50 years, resulting in metal levels of up to 30,000 mg/kg of lead, 4000 mg/kg of copper and 600 mg/kg of zinc in the most contaminated soils. All the metal-contaminated soils had lower degradation rates than the uncontaminated soils tested. Among the contaminated soils, however, the heavy metal concentration was not the major factor in determining the loss in tensile strength of the cotton strips, where cellulose decomposition was governed by other soil physicochemical properties. Soil with a higher cation exchange capacity, readily oxidisable material and volatile solids content had the greatest loss in tensile strength of cotton strips. Microbial adaptation to the presence of high concentrations of soil heavy metals and reduced bioavailability of metals is the likely explanation for this phenomenon.

Competitive adsorption of phosphate and arsenate on goethite.

The competitive adsorption of phosphate and arsenate on goethite was investigated to better understand the bonding mechanisms for the two ions. The anions were added both simultaneously and sequentially. When added simultaneously, the two ions were adsorbed about equally, with the total surface coverage being slightly greater than for either ion alone. When added sequentially, the extent of exchange for the first ion depended on the equilibration time before the second ion was introduced--the longer the equilibration time the greater the exchange. There is a nonexchangeable fraction for both ions that is approximately equal to the initially adsorbed amount of each ion. The results suggest a two-phase reaction on the surface, with the first phase being a rapid surface complex formation on the goethite surface, followed by the slower buildup of a surface precipitate on the adsorbed layer. The exchangeable ions are in the surface precipitate. These results are incompatible with a surface complexation model (SCM) for anion adsorption on geothite and indicate that the actual reactions are more complicated than the reaction assumed in a SCM.

Integrated hydrometallurgical process for production of zinc from electric arc furnace dust in alkaline medium.

In this study, a novel and integrated hydrometallurgical process for the production of zinc powder from electric arc furnace (EAF) dust in alkaline medium is reported. The dust is firstly hydrolysed in water, and then fused in caustic soda at 350 degrees C for 1h, followed by leaching in alkaline solution in which both zinc and lead are effectively extracted. Zinc powder is then produced by electrowinning from the leach solution after the lead is selectively removed by precipitation using sodium sulphide as precipitant. The EAF dust tested contained 25% Zn, 1.8% Pb and 33% Fe. It was found that 38% of zinc and 68% of lead could be extracted from the dust when leached directly in caustic soda solution. Leaching of zinc increased to 80% when dust was directly fused with caustic soda followed by alkaline leaching. However, the leaching further increased to 95% when the dust was hydrolysed first with water before fusion. Zinc powder with a purity of 99.95% was then produced by electrowinning from the lead depleted solution. Stainless electrodes were used as both anode and cathode.

Distinguishing Adsorption and Surface Precipitation of Phosphate on Goethite (alpha-FeOOH).

The reaction between phosphate and goethite changes from adsorption into surface precipitation with no discernible changes in the adsorption isotherm. Distinguishing the two processes, by plotting the loss of phosphate from solution versus final phosphate concentration or based on theoretical calculations, is difficult. This paper presents a method for distinguishing between the two processes based on the change in zeta potential with increasing adsorption. During adsorption, the incoming phosphate results in a more negative surface charge as the more acidic phosphate ion replaces a less acidic surface hydroxyl. The amount of negative charge imparted to the surface should vary linearly with surface coverage for adsorption. Phosphate that is bound to a surface precipitate, on the other hand, imparts a much smaller negative charge to the surface, since there is no change in the character of the surface due to the additional phosphate. Zeta potential measurements of phosphated goethite at varying solution pH values and surface coverages are used to determine the transition point from adsorption to surface precipitation. The transition occurs at dissolved phosphate concentrations much lower than those calculated for phosphate in equilibrium with goethite and iron phosphate. Copyright 2000 Academic Press.