Batch Dissolution Research Articles

Mechanism of dissolution of minium (Pb3O4) in water under depleting chlorine conditions

Long-term batch dissolution experiments were performed on solid phase Pb3O4 under drinking water conditions simulating a depleting chlorine regime. Analytical and spectroscopic results indicate Pb3O4 is first dissolved in the presence of chlorine through a nucleophilic attack to the Pb2+ centers in the solid, and is subsequently oxidized by free chlorine to form β-PbO2, with a concomitant decrease in soluble lead concentrations. Following chlorine depletion, Pb3O4 dissolution regulates the process, resulting in surface deposition of β-PbO2. Formation of lead carbonates controls the final observed lead concentrations for Pb3O4 dissolution. A dissolution model is presented to rationalize these observations.

Influence of pH and temperature on alunite dissolution: Rates, products and insights on mechanisms from atomistic simulation

The processes, rates, controlling factors and products of alunite (KAl3(SO4)2(OH)6) dissolution were assessed using batch dissolution experiments at pHs of c. 3, 4, 4.6, 7 and 8, and temperatures of c. 280, 293 and 313K. Alunite dissolution is roughly congruent at pH3, while at pH≥3.9 the process is incongruent, giving a lower Al/K ratio in solution than in the pristine alunite sample. The decrease in the Al/K ratio appears to be caused by precipitation of secondary aluminium sulfate/hydroxysulfate minerals coating the surface of the dissolving alunite, as inferred from SEM images and XPS determinations, but these minerals do not passivate the alunite surface for the time frame of the experiments (up to 400h). The lowest dissolution rates are obtained for pH4.6 and 280K. Both the temperature increase and any pH variation from that point lead to faster dissolution rates.Based on the potassium release to solution, the influence of pH and temperature on the alunite dissolution rate for pH of 4.8 and below can be expressed as;rateK=10-4.4±0.5aH+0.10±0.02e-32±3/RTwhere rateK is the alunite dissolution rate (in mol·m−2·s−1); aH+ is the activity of hydrogen ions in solution; R is the Universal gas constant (in kJ·mol−1·K−1) and T is temperature (in K).For pH of 4.6 and above, the alunite dissolution rate can instead be expressed as;rateK=10-2.5±0.8aOH-0.14±0.02e-39±4/RTwhere aOH- is the activity of hydroxyl ions in solution.In light of the calculated values for the activation energy under the two sets of pH conditions (32±3 and 39±4kJ·mol−1), alunite dissolution appears to be surface-controlled. Examination of the most stable solvated alunite surfaces obtained by atomistic computer simulations suggests that the least energetically favourable steps during alunite dissolution are the detachment of either Al atoms or SO4 tetrahedra from exposed surfaces. Thus, these processes are most probably the rate-determining steps in alunite dissolution.

Response to Comments upon, “Evidence and potential implications of exponential tails to concentration versus time plots for the batch dissolution of calcite”

where k2 is the rate constant of the back-reaction to dissolution, csat and c are concentrations of solute at saturation and any time, t, respectively, and the apparent surface area is a time-dependant function of time, A(t). Note that in the calcite experiments of Truesdale (2015), the actual surface area was constant. Note also that the rate constant of the forward reaction, k1, does not appear in the rate equation (Eq. 1) because this is a complex reaction in which the forward reaction and back-reaction are tied together, analogously to a bolas (Truesdale 2015). The discussion paper had three objectives. Firstly, to launch the hypothesis that new and re-worked results, together with an extended Shrinking-Object model, might explain the very long reaction times needed for saturation to be reached in batch dissolutions of calcite. Secondly, to explain that the hypothesis could have been formed around 1975, had the rigour of kinetics of that time (Lewis 1974), defined in Truesdale (2015) as Empirical Kinetics (Fig. 1), been consistently applied to mineral dissolution, generally. Thirdly, to show that the hypothesis might even cover the more complex examples of silicate and alumino-silicate mineral dissolutions as they too had not been investigated under Empirical Kinetics. Such a universal solution is badly needed if modelling of calcite behaviour in the environment at the large scale, e.g. the oceanic water column (e.g. Morse et al. 2007), and the catchment (e.g. Buhmann and Dreybrodt 1985),

Evidence and Potential Implications of Exponential Tails to Concentration Versus Time Plots for the Batch Dissolution of Calcite

Calcite batch dissolution kinetics behaviour has been described as complex for more than half a century. The dissolution of 100- to 250-μm particles of natural calcites at 20–35 °C in Tris buffer or alkali at pHs 8–10.6 reveals that the non-ideal, long tails to the plot of calcium concentration versus time, are probably exponential. Equivalent alternative evidence from elsewhere for both calcite and gypsum is supplied. Inadequate kinetics templating of earlier results is believed to have obscured this exponential character, but rectification could induce a renaissance for batch dissolution study of calcite as well as for silicate and aluminosilicate minerals, too. The work extends a programme of study of the shrinking object (SO) model which has already considered a variety of substances such as sucrose, sodium chloride, gypsum, and silica gel, through which it has operationally defined ideal dissolution as an exponential rise in concentration with time, at high solids loadings. The SO model is limited to a constant surface area of the solid during dissolution. However, the possibility of explaining non-ideal dissolution of calcite by including a reduction in the apparent surface area during a dissolution, is explored here. In this the model shifts from an initial mode under one reaction rate constant to a terminal mode under a second rate constant, and by this the SO model is shown to share properties with a decelerating bolas—the South American hunting tool. The bolas model is consistent with earlier suggestions elsewhere that non-ideality arises from the surface being either poisoned or otherwise changed during dissolution. This current investigation derives from Empirical Kinetics, which is defined here as the wisdom of kinetics collected by the middle of the last half of the twentieth century. With the preliminary success gained with the exponential tails, the paper overcomes much confusion in the literature by differentiating direct measurements of rates of dissolution made for the purposes of chemical kinetics, from those needed for biogeochemical field purposes.

Impact of trace mineral phases on the total solute flux from andesitic volcanics

Recent work on the weathering of high standing islands (HSI’s) of New Zealand (Goldsmith et al., 2008), Dominica (Goldsmith et al., 2010) Martinique and Guadeloupe (Rad et al., 2006) and portions of the Philippines (Schopka et al., 2011) shows weathering rates based on stream water chemistry for areas draining andesitic terrains are comparable to weathering rates determined for basaltic terrains, indicating that andesite weathering might be much more important in drawing down atmospheric CO2 than previously recognized. While an easily erodible parent material has been largely attributed to sustaining rates at these locations, little is known to known regarding its associated reaction kinetics. We conducted a series of batch dissolution experiments on andesitic material collected from ∼10,000year old tephra deposits from Dominica to determine the dissolution rate of major and trace mineral phases to better understand geochemical processes controlling weathering flux from these areas. Dissolution experiments were conducted over a range of pH (4 and 7) on bulk samples and mineral separates.The dissolution rates based on Si release from the Dominica tephra bulk samples were similar, and ranged from 0.04 to 0.13μmole Si/g-day in water, and ∼0.14 to 0.27μmole Si/g-day in dilute acid (initial pH ∼4). Although the bulk of the ash is predominately composed of vesicular felsic (Na–Al–Si) volcanic glass, reaction rates and stoichiometry indicate ash dissolution is dominated by the reactivity of trace Mg or Ca-bearing silicate phases (olivine, pyroxene or amphiboles) and Ca–phosphate phases (apatite), especially under slightly acidic conditions. Analysis of reacted phases by SEM shows little evidence of alteration of glassy material, whereas surfaces of Ca–Mg inosilicates, olivine and apatite show etched features indicative of dissolution. Results of the dissolution experiments suggest that, although these phases are relatively minor components of the ash, they contribute disproportionately to the overall weathering flux, and their reactivity may be particularly important in areas where physical weathering and erosion are constantly exposing new fresh surfaces available for chemical reaction.

Influence of aqueous Fe(III) on release of Mn(II) from low-molecular-weight organic acid-promoted dissolution of an Oxisol and γ-MnO2

Mn(II) release from the soil mineral dissolution is a critical process to Mn availability to and uptake by plants, but little was known about how this process was affected by the coexisting Fe-containing minerals and/or aqueous Fe(III). In this study, batch dissolution experiments were conducted to obtain the kinetic of Mn(II) release from the dissolution of soil and γ-MnO2 by oxalic acid or citric acid with the addition of aqueous Fe(III), which will shed light on the influence of Fe and root exudates on Mn geochemical behavior. The release of Mn(II) from the mineral dissolution of Oxisol by oxalic acid was inhibited in the presence of aqueous Fe(III). This result was attributed to the decrease in the concentration of uncomplexed oxalic acid, which was more efficient in the dissolution of manganese minerals than the complexed species. Such statement was mainly based on the complexation reaction between oxalic acid and aqueous Fe(III) as well as the subsequent adsorption of Fe(III)-oxalate complexes, as evidenced by the gradual decrease in the concentration of Fe(III) during the latter stage of the dissolution process. By contrast, the addition of aqueous Fe(III) or α-Fe2O3 could enhance the release of Mn(II) from the dissolution of soil Mn oxides by citric acid, probably resulting from the further reduction of soil Mn oxides by reductants such as Fe2+ and hydroxyl radicals generated from the auto-reduction of aqueous Fe(III)-citrate complexes. However, the dissolution of γ-MnO2 by citric acid was suppressed in the presence of aqueous Fe(III), owing to the inhibition of the auto-reduction of aqueous Fe(III)-citrate complexes when they were adsorbed onto the MnO2 surfaces. This hypothesis was confirmed by the kinetic results of Fe(III) adsorption onto γ-MnO2 surfaces that the aqueous Fe(III) concentration decreased rapidly, nearly to zero. Overall, the present study indicated the geochemical behavior of manganese would be significantly affected by aqueous Fe(III) due to the changes in aqueous species of organic acids and/or the coupling of Mn-containing minerals dissolution with the intramolecular reaction of Fe(III)-organic acid complexes.

Impact of Water Chemistry on Lead Carbonate Dissolution in Drinking Water Distribution Systems

Elemental lead is a known toxic metal that can pose threats to human health and can be found in a variety of sources including drinking water at very low level concentrations (i.e. μg/L range). Destabilization of the corrosion scale at the inner layer of pipeline is the major source of lead in drinking water. Chemical properties of the water passing through the distribution system such as pH, alkalinity, chlorine content, oxidation reduction potential (ORP) and natural organic matters will affect the formation and/or destabilization of the corrosion scale. This research examines the impact of pH values (7.0 - 9.5), temperatures (5°C vs 20°C) and alkalinity levels (moderate vs low), in the presence of chlorine, on dissolution of hydrocerussite and cerussite in drinking water by various sets of batch dissolution experiments. The results showed dissolution of cerussite and hydrocerussite was not impacted significantly by pH ranging 7.0 - 9.5. In addition, and somewhat surprisingly, cold temperature (5°C) and moderate alkalinity showed a great influence on decreasing the solubility of lead species.

Dependence of Stability of Sr<sub>5</sub>(AsO<sub>4</sub>)<sub>3</sub>OH on pH Value

The object of this work was to determine the solubility and stability of the synthetic Sr5(AsO4)3OH solid solution at 25°C and the different initial pHs (pH at 2, 6 and 9) by batch dissolution experiments. The results indicated that with the initial pH value of 2, 6 and 9, the reaction system reached equilibrium after 2880h, and by that time the pH value of the solution reached, 7.90, 8.74 and 8.27 respectively. The concentration of strontium and arsenic in the initial solution pH at 2 are higher than that of solutions pH at 6 and 9. While the concentration of strontium in neutral or alkaline solutions is low and stays unchanged.

Role of Pb(II) defects in the mechanism of dissolution of plattnerite (β-PbO2) in water under depleting chlorine conditions.

Destabilization of lead corrosion scales present in plumbing materials used in water distribution systems results in elevated lead concentrations in drinking water. Soluble lead release caused by changes in water chemistry has been linked to dissolution of lead carbonate and/or lead oxide solid phases. Although prior studies have examined the effects of varying water chemistry on the dissolution of plattnerite (β-PbO2), β-PbO2 dissolution under depleting chlorine conditions is poorly understood. This paper reports results obtained for long-term batch dissolution experiments for solid phase β-PbO2 under depleting chlorine conditions. Results indicate that the initial availability of free chlorine effectively depresses dissolved lead concentrations released from β-PbO2. However, the dissolved lead levels remained low (∼4 μg/L) even after free chlorine was depleted. Detailed spectroscopic characterization of solid samples collected during the β-PbO2 experiments indicates that changes in the electronic structure of PbO2 occurred during the dissolution. This further points out that Pb2+ defects present in crystalline β-PbO2 play a dominant role in the dissolution of this solid phase.

Lead Immobilization and Hydroxamate Ligand Promoted Chloropyromorphite Dissolution

The immobilization of lead, a major environmental contaminant, through phosphate amendments to form the sparingly soluble lead phosphate mineral chloropyromorphite [Pb5(PO4)3] (CPY) is an effective in situ strategy for soil remediation. An important question is the effect of microbial processes on this remediation. Here, we investigate the role of the microbial siderophore ligand desferrioxamine-D1 (DFO-D1) and its analog acetohydroxamic acid (aHA) in CPY lability using pH-dependent batch dissolution kinetics and model calculations. Both (0.01) M aHA and (0.00024) M DFO-D1 are similarly effective and enhance lead release from CPY by more than two orders of magnitude at pH > 6 compared to in the absence of ligands. This is consistent with model calculations of pH-dependent (aqueous) complexation of lead with hydroxamate ligands. More importantly, pH-dependent ligand sorption is predictive of its ligand promoted dissolution behavior. Our results suggest that organic ligands can significantly increase CPY lability at alkaline pHs in soils and sediments and that addition of P amendments to immobilize Pb as CPY may only be successful at acid pHs.

Metal oxyhydroxide dissolution as promoted by structurally diverse siderophores and oxalate

Siderophores, a class of biogenic ligands with high affinities for Fe(III), promote the dissolution of metal ions from sparingly soluble mineral phases. However, most geochemical studies have focused on quantifying the reactivity of DFOB, a model trishydroxamate siderophore. This study utilized three different siderophores, desferrioxamine B, rhizoferrin, and protochelin, with structures that contain the most commonly observed binding moieties of microbial siderophores to examine the siderophore-promoted dissolution rates of FeOOH, CoOOH, and MnOOH in the absence and presence of the ubiquitous low molecular mass organic acid oxalate by utilizing batch dissolution experiments at pH=5–9. Metal-siderophore complex and total dissolved metal concentrations were monitored for durations of one hour to fourteen days, depending on the metal oxyhydroxide identity and solution pH. The results demonstrate that MnOOH and CoOOH generally dissolve more quickly in the presence of siderophores than FeOOH. Whereas FeOOH dissolved exclusively by a ligand-promoted dissolution mechanism, MnOOH and CoOOH dissolved predominantly by a reductive dissolution mechanism under most experimental conditions. For FeOOH, siderophore-promoted dissolution rates trended with the stability constant of the corresponding aqueous Fe(III) complex. In the presence of oxalate, measured siderophore-promoted dissolution rates were found to increase, decrease, or remain unchanged as compared to the observed rates in single-ligand systems, depending on the pH of the system, the siderophore present, and the identity of the metal oxyhydroxide. Increases in observed dissolution rates in the presence of oxalate were generally greater for FeOOH than for MnOOH or CoOOH. These results elucidate potential dissolution mechanisms of both ferric and non-ferric oxyhydroxide minerals by siderophores in the environment, and may provide further insights into the biological strategies of metal acquisition facilitated by coordinated exudation of low molecular weight organic acids and siderophores.

Effects of pH and phosphate on CeO2 nanoparticle dissolution

As the result of rapidly grown nanotechnology industries, release of engineered nanoparticles (ENPs) to environment has increased, posing in a serious risk to environmental and human health. To better understand the chemical fate of ENPs in aquatic environments, solubility of CeO2 NPs was investigated using batch dissolution experiments as a function of pH (1.65–12.5), [phosphate] and particle size (33 and 78nm). It was found that CeO2 dissolution was only significant at pH<5 and inversely proportional to surface area. After 120h, the release of Ce was ∼3 times greater in large NPs than that in small NPs that is likely contributed by the difference in exchangeable Ce(III) impurity (small: 0.3mMkg−1, large: 1.56mMkg−1). When 100μM of phosphate was added, the dissolution rate of CeO2 NPs was decreased in small NPs by 15% at pH 1.65 and 75% at pH 4.5 and in large NPs by 56% at pH 1.65 and 63% at pH 4.5. The inner-sphere surface complexation of P that is revealed by the zeta potential measurements is effectively suppressing the CeO2 NP dissolution. Predicting the fate and transport of CeO2 NPs in aquatic environment, pH and P ligands might play important roles in controlling the solubility of CeO2 NPs.

Dissolution and rheological behaviour of hematite and quartz particles in aqueous media at pH 1

In this study we investigate isothermal, atmospheric acid dissolution behaviour of quartz and hematite minerals which constitute two of the predominant host gangue phases of typical low grade limonitic laterite ores. Batch dissolution tests were carried out on 57wt.% solid dispersions for 4h at pH 1 and 25 and 70°C to establish the influential role of oxide mineralogy/chemistry on rheology and leaching behaviour. The results show that the two minerals displayed a weakly temperature and time-independent, non-Newtonian rheological behaviour with low shear yield stresses (<4Pa) and viscosities (9–17mPas). Hematite dissolution rate was significantly higher compared with that of quartz under similar conditions. Quartz dissolution mechanism was substantially volume diffusion controlled at lower agitation rate (600rpm) whilst for hematite it was both volume diffusion and chemical reaction controlled. These mechanisms reflected activation energies of 17.7±0.9 and 28.5±1.4kJ/mol, respectively, for quartz and hematite. At 800 and 1000rpm, dissolution of both minerals was chemical reaction-controlled with similar activation energies (32.6±1.7 and 32.2±1.6kJ/mol). The findings underscore the need for higher agitation rates and elevated temperatures, to overcome both volume diffusion and chemical reaction limitations for enhanced acid leaching of these two fairly refractory oxides studied herein.

The Characteristic Dissolution and Physical Chemistry Parameter of Synthetic Pyromorphite

The Dissolution of Synthetic Pyromorphite was Studied at 25°C in a Series of Batch Experiments. in Addition, the Aqueous Concentrations from the Batch Dissolution were Used to Calculate the Solubility Product and Free Energy of Formation of Pyromorphite. the Results of the Fourier Transform Infrared Spectroscopy Analyses Indicated that the Synthetic, Microcrystalline Pyromorphite with Apatite Structure Used in the Experiments has Not Changed after Dissolution. the Mean KspValue was Calculated for Pb5(PO4)3Cl of 10-78.31 at 25°C; the Free Energy of Formation ΔGf0[Pb5(PO4)3Cl] was-3756.82kJ/mol.

Association of trace elements and dissolution rates of soil iron oxides

In this study, nine Oxisols and five Ultisols from Thailand were used to determine the association of major and trace elements with iron (Fe) oxides. The Fe oxides were concentrated and the association of elements (Al, Ca, Cu, Cr, Mg, Mn, Ni, Pb, P, Si, V, Ti, Zn) with Fe was evaluated using batch dissolution in 1 m HCl at 20°C. The dissolution behaviour of Fe oxide concentrates was determined using batch dissolution and flow-through reactors. In addition to Fe, both Al and Ti were present in significant amounts in the Fe oxide concentrates. Manganese was the most abundant trace element, and Cu, Zn, Pb and As concentrations were &lt;250 mg kg–1 in most samples. The dissolution behaviour of Fe-oxide concentrates indicated that Al, Cr and V were mostly substituted for Fe3+ in the structure of goethite and hematite. A significant proportion of Mn, Ni, Co, Pb and Si was also present within the structure of these minerals. Some Mg, Cu, Zn, Ti and Ca was also associated with Fe oxides. The dissolution kinetics of Fe oxide concentrates was well described by three models, i.e. the cube root law, Avrami–Erofejev equation and Kabai equation, with the dissolution rate constants (103k) corresponding to the three models ranging from 0.44 to 6.11 h–1, from 1.01 to 4.40 h–1 and from 0.03 to 4.12 h–1, respectively. The k constants of Fe oxide concentrates in this study were significantly and negatively correlated with the mean crystal dimension derived from [110] and [104] of hematite, the dominant mineral in most samples. The steady-state dissolution rate of a soil Fe-oxide concentrate (sample Kk) was substantially higher than for synthetic goethite under highly acidic conditions; this is possibly due to the greater specific surface area of sample Kk than the synthetic goethite.

Characterization, Dissolution and Solubility of Pyromorphite [Pb&lt;sub&gt;5&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl] at 25°C

Pyromorphite was synthesized by precipitation and characterized by various techniques. The batch dissolution experiment was conducted at 25°C and different initial pHs (2, 4 and 6). The solid phase showed no obvious change before and after dissolution. The aqueous phosphate concentration decreased quickly from 0.22 to 0.01mmol/L and then increased very slowly with time. After 2160h dissolution, it reached a steady-state value of about 0.13mmol/L. The aqueous lead concentration increased initially and reached a peak value after 72h dissolution.

Characterization, Dissolution and Solubility of Mimetite[Pb&lt;sub&gt;5&lt;/sub&gt;(AsO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt;Cl] at 25°C

Mimetite [Pb5(AsO4)3C was firstly synthesized by precipitation and characterized by various techniques, and then the batch dissolution experiment was conducted at 25°C and different initial pHs. XRD and SEM analysis indicated that the solid was indistinguishable before and after the dissolution. For the dissolution at the initial pH=2, the aqueous arsenate and lead concentration decreased firstly with time and reached a steady state after 480h. For the dissolution at the initial pH=4 and 6, the aqueous arsenate and lead concentrations varied between 0-0.0005mmol/L and 0-0.0008mmol/L, respectively.

Synthesis and dissolution kinetics of zirconia based ceramics

In this study, an ideal Nd2Zr2O7 pyrochlore and a defect fluorite ceramic were synthesised by coprecipitation, pressing and sintering. Detailed investigations by electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and powder X-ray diffraction (XRD) were used for the characterisation of the powders. In particular both powders were found to be single phase ceramics.All batch and dynamic dissolution experiments were conducted in C(HCl) = 0.1 M at 90 °C to compare the two types of Nd-zirconia structures and the experimental approaches. A good agreement between batch and dynamic dissolution experiments was observed for the pyrochlore with a Nd-based final dissolution rate in the range between 0.84 × 10−5 and 1.95 × 10−5 g m−2 d−1. The dynamic experiments indicate a strongly incongruent preferential release of Nd from the pyrochlore ceramic at the beginning of the experiment and a close to congruent dissolution at the end of the experiment.The dissolution behaviour of defect fluorite in dynamic experiments is observed to be very similar to the pyrochlore. An incongruent dissolution during the early stages is followed by a congruent steady state. The defect fluorite steady state dissolution rate is lower than the one determined for pyrochlore.

Mechanisms of goethite dissolution in the presence of desferrioxamine B and Suwannee River fulvic acid at pH 6.5

Siderophores are Fe3+ specific low MW chelating ligands secreted by micro-organisms in response to Fe stress. Low MW organic acids such as oxalate have been shown to enhance siderophore mediated dissolution of Fe3+ oxides. However, the effect of fulvic acid presence on siderophore function remains unknown. We used batch dissolution experiments to investigate Fe release from goethite in the goethite–fulvic acid–desferrioxamine B (goethite–SRFA–DFOB) ternary system. Experiments were conducted at pH 6.5 while varying reagent addition sequence. FTIR and UV–Vis spectroscopy were employed to characterise the Fe–DFOB, Fe–SRFA and DFOB–SRFA complexes. Iron released from goethite in the presence of SRFA alone was below detection limit. In the presence of both SRFA and DFOB, dissolved Fe increased with reaction time, presence of the DFOB–SRFA complex, and where SRFA was introduced prior to DFOB. FTIR data show that in the ternary system, Fe3+ is complexed primarily to oxygen of the DFOB hydroxamate group, whilst the carboxylate CO of SRFA forms an electrostatic association with the terminal NH3+ of DFOB. We propose that SRFA sorbed to goethite lowers the net positive charge of the oxide surface, thus facilitating adsorption of cationic DFOB and subsequent Fe3+ chelation and release. Furthermore, the sorbed SRFA weakens Fe–O bonds at the goethite surface, increasing the population of kinetically labile Fe. This work demonstrates the positive, though indirect role of SRFA in increasing the bioavailability of Fe3+.

Super-faradaic charge yields for aluminium dissolution in neutral aqueous solutions

Dissolution of elemental aluminium or iron in aqueous solutions is the essential reaction in electrocoagulation and other processes. These and several other such metals have been reported as exhibiting dissolution rates that are higher than expected from the current/charge passed, assuming electron stoichiometries of three (Al) or two (Fe). In batch dissolution experiments with aluminium electrodes, the addition of 30gm−3 humic acid to a neutral solution of 0.5molm−3 Na2SO4 and 8.8gm−3 NaCl led to an unexpected increase in the charge yield from 1.0 to ca. 1.5, i.e., the measured concentration of aluminium species in solution was 50% higher than expected from Faraday's law. This phenomenon may be explained when net currents are used for the determination of charge yields, whereas partial dissolution currents should be used. The latter may be significantly greater than the former when a concurrent reduction reaction, such as hydrogen evolution, occurs at the dissolving electrode.