Pyrite oxidation by hexavalent chromium: investigation of the chemical processes by monitoring of aqueous metal species

Pyrite, an iron sulfide, occurs in many soils and sediments, making it an important natural reductant of toxic metal pollutants. This study investigated the processes leading to aqueous Cr(VI) reduction by pyrite in a closed thermostated (25 +/- 0.1 degrees C) system and under an argon atmosphere. Synthetic pyrite suspensions were reacted with a range of Cr(VI) solutions from 0 to 7 x 10(-4) M and at pH 2-8. Metal species concentrations were continuously monitored during a period lasting approximately 20 h. Preliminary experiments carried out in acidic media without Cr(VI) have shown that some pyrite dissolution occurred. Then, metal species concentration changes with time during pyrite oxidation by Cr(VI) solutions exhibited two distinct trends depending on the complete or incomplete Cr(VI) removal. As long as chromate existed in solution, the Cr-(Ill) to Fe(lIl) ratio was found to be an effective parameter to investigate the pyrite reaction stoichiometry with Cr(VI). Experimental values close to 2 suggest that sulfur compounds with oxidation states between 0 and 2 should be formed during pyrite oxidation by Cr(VI). If Cr(VI) was completely reduced from solution, then the pyrite oxidation by Fe(lll) ions took place to generate ferrous ions.     

Influence of heterotrophic microbial growth on biological oxidation of pyrite

The rate and extent of pyrite oxidation by the iron-oxidizing bacteria Acidithiobacillus ferrooxidans was limited by the growth of the heterotrophic microbe Acidiphilium acidophilum. In batch systems containing a mixture of both organisms, the maximum zero-order rate of ferric iron accumulation was about 1.4 mg of Fe3+ L(-1) d(-1) as compared to 9.4 mg of Fe3+ L(-1) d(-1) for pure cultures of A. ferrooxidans under the same conditions. Pyrite oxidation was limited in cases where both cultures of organisms were initially present as well as situations where the heterotrophic organisms were added to established, pyrite-oxidizing systems containing A. ferrooxidans. Results also indicated that organic carbon remaining in solution following heterotrophic bacterial growth reduced the rate of abiotic pyrite oxidation by the ferric ion. Furthermore, a cell-free solution of the residual organic carbon resulted in a lag of A. ferrooxidans growth in soluble ferrous medium. The residual organic carbon solution that accumulated during the growth of Aph. acidophilum had a diverse molecular weight distribution, indicating that different compounds could be responsible for the inhibition of chemical pyrite oxidation and the A. ferrooxidans growth lag observed. Titration of dissolved copper ions with residual dissolved organic carbon originating from Aph. acidophilum cultures indicated that a metal complexation mechanism could be responsible for the lower rates of pyrite oxidation observed. These data suggest that encouraging the growth of heterotrophic microorganisms under acid mine drainage conditions may be a feasible strategy for decreasing both the rate and the extent of sulfide mineral oxidation.     

聚合硅酸硫酸铝铁处理造纸中段废水

该文采用硫酸铝、硫酸铁和硅酸钠等为原料,制备聚合硅酸硫酸铝铁（polymeric aluminum ferric sulfate,PAFSS）絮凝剂,研究了n（Si）：n（Al）：n（Fe）、pH等因素对所制PAFSS在去除造纸废水的色度及COD_（Cr）时的影响,对PAFSS的物化特性进行了系统的研究。结果表明：n（Si）：n（Al）：n（Fe）、pH和反应温度对PAFSS性能有重要影响;聚硅酸与铝离子、铁离子及铝铁水解产物间存在一定的络合作用,这种作用对PAFSS聚集体的结构形貌、铝离子和铁离子的水解聚合过程及铝、铁水解产物的形态分布都有一定的影响。在n（Fe＋Al）：n（Si）=3：1、n（Al）：n（Fe）=4：1、pH=4和温度为40℃等条件下制得的PAFSS对废水色度和COD_（Cr）有很好的去除效果,并通过对PAFSS絮凝机理分析,可知PAFSS的絮凝作用主要是通过电中和和吸附架桥2方面实现的。     

Catalytic mechanisms of metmyoglobin on the oxidation of lipids in phospholipid liposome model system

The catalytic mechanism of metmyoglobin (metMb) on the development of lipid oxidation in a phospholipid liposome model system was studied. Liposome model system was prepared with metMb solutions (2.0, 1.0, 0.5, and 0.25 mg metMb/mL) containing none, diethylenetriamine pentaacetic acid (DTPA), desferrioxamine (DFO), or ferric chloride and lipid oxidation was determined at 0, 15, 30, 60, and 90 min of incubation at 37 °C. Metmyoglobin catalysed lipid oxidation in the liposome system, but the rate of lipid oxidation decreased as the concentration of metMb increased. The amount of free ionic iron in the liposome solution increased as the concentration of metMb increased, but the rate of metMb degradation was increased as the concentration of metMb decreased. The released free ionic iron was not involved in the lipid oxidation of model system because ferric iron has no catalytic effect without reducing agents. Both DFO and DTPA showed antioxidant effects, but DFO was more efficient than DTPA because of its multifunctional antioxidant ability as an iron and haematin chelator and an electron donor. The antioxidant activity of DTPA in liposome solution containing 0.25 mg metMb/mL was two times greater than that with 2 mg metMb/mL due to the increased prooxidant activity of DTPA-chelatable compounds. It was concluded that ferrylmyoglobin and DTPA-chelatable haematin generated from the interaction of metMb and LOOH, rather than free ionic iron, were the major catalysts in metMb-induced lipid oxidation in phospholipid liposome model system.     

Physical structures of lipid layers on pyrite

The physical structures of lipid layers on pyrite (FeS2), a ubiquitous sulfide mineral, were studied in air and in water by atomic force microscopy (AFM). Egg PC, a phospholipid that forms bilayer structures on atomically flat substrates, was investigated, and our experimental observations show that this lipid formed bilayers on an atomically rough pyrite surface, as inferred by a measured layer thickness of 5.0 +/- 0.2 nm. The surface coverage of the lipid coating increased from approximately 15% to 80% when the concentration of the lipid suspension was increased from 0.014 to 0.15 mM. Although further increases up to 1.5 mM resulted in an incremental increase in surface coverage of only 5%, multilayer structures of 20- to 40-nm height formed on top of the first bilayer. The findings provide a structural explanation for the results of earlier kinetic studies showing that the presence of the lipid decreases the rate of pyrite oxidation in air and in water. Lipid coatings applied to iron-sulfide bearing minerals are a possible approach to preventing oxidation and acidification and thereby mitigating environmental damage that can result from acid mine drainage.     

Heterogeneous oxidation of Fe(II) on ferric oxide at neutral pH and a low partial pressure of O2

The objective of this study was to identify the rate and mechanism of abiotic oxidation of ferrous iron at the water-ferric oxide interface (heterogeneous oxidation) at neutral pH. Oxidation was conducted at a low partial pressure of O2 to slow the reactions and to represent very low dissolved oxygen (DO) conditions that can occur at oxic/anoxic fronts. Hydrous ferric oxide (HFO) was partially converted to goethite after 24 h of anoxic contact with Fe(II), consistent with previous results. This resulted in a significant decrease in sorption of Fe(II). No conversion to goethite was observed after 25 min of anoxic contact between HFO and Fe(II). O2 was then introduced into the chamber and sparged (transfer half-time of 1.6 min) into the previously anoxic suspension, and the rate of oxidation of Fe(II) and the distribution between sorbed and dissolved Fe(II) were measured with time. The concentration of sorbed Fe(II) remained steady during each experiment, despite removal of all measurable dissolved Fe(II) in some experiments. The rate of oxidation of Fe(II) was proportional to the concentration of DO and both sorbed and dissolved Fe(II) up to a surface density of 0.02 mol Fe(II) per mol Fe(III), i.e., approximately 0.2 Fe(II) per nm2 of ferric oxide surface area. This result differs from previous studies of heterogeneous oxidation, which found that the rate was proportional to sorbed Fe(II) and DO but did not find a dependence on dissolved Fe(II). Most previous experiments were autocatalytic; i.e., the initial concentration of ferric oxide was low or none, and sorbed Fe(II) was not measured. The results were consistentwith an anode/cathode mechanism, with O2 reduced at electron-deficient sites with strongly sorbed Fe(II) and Fe(II) oxidized at electron-rich sites without sorbed Fe(II). The pseudo-first-order rate constants for oxidation of dissolved Fe(II) were about 10 times faster than those previously predicted for heterogeneous oxidation of Fe(II).     

Photocatalytic oxidation of arsenite in TiO2 suspension: kinetics and mechanisms

Arsenite [As(III)] and arsenate [As(V)] are highly toxic aquatic contaminants. Since arsenite is more mobile in natural waters and less efficiently removed in adsorption/coagulation processes than arsenate, the oxidation of arsenite to arsenate is desirable in water treatment. We performed the photocatalytic oxidation of arsenite in aqueous TiO2 suspension and investigated the effects of pH, dissolved oxygen, humic acid (HA), and ferric ions on the kinetics and mechanisms of arsenite oxidation. Arsenite oxidation in UV-illuminated TiO2 suspension was highly efficient in the presence of dissolved oxygen. Homogeneous photooxidation of arsenite in the absence of TiO2 was negligibly slow. Since the addition of excess tert-butyl alcohol (OH radical scavenger) did not reduce the rate of arsenite oxidation, the OH radicals should not be responsible for As(III) oxidation. The addition of HA increased both arsenite oxidation and H2O2 production at pH 3 under illumination, which could be ascribed to the enhanced superoxide generation through sensitization. We propose that the superoxide is the main oxidant of arsenite in the TiO2/UV process. The addition of ferric ions also significantly enhanced the arsenite photooxidation. In this case, the addition of tert-butyl alcohol reduced the arsenite oxidation rate, which implied thatthe OH radical-mediated oxidation path was operative in the presence of ferric ions. Since both Fe3+ and HA that were often found with the arsenic in groundwater were beneficial to the photocatalytic oxidation of arsenite, the TiO2/UV process could be a viable pretreatment method. This can be as simple as exposing the arsenic-polluted water in a TiO2-coated trough to sunlight.     

Oxidation of Fe(II) in natural waters at high nutrient concentrations

The Fe(II) oxidation kinetic was studied in seawater enriched with nutrients as a function of pH (7.2-8.2), temperature (5-35 °C), and salinity (10-36.72) and compared with the same parameters in seawater media. The effect of nitrate (0-1.77 × 10(-3) M), phosphate (0-5.80 × 10(-5) M) and silicate (0-2.84 × 10(-4) M) was studied at pH 8.0 and 25 °C. The experimental results demonstrated that Fe(II) oxidation was faster in high nutrient concentrations affecting the lifetime of Fe(II) in nutrient rich waters. Silicate displayed the most significant effects on the Fe(II) oxidation rate with values similar to those determined in seawater enriched with all the nutrients. A kinetic model was applied to the experimental results in order to account for changes in the speciation and to compute the fractional contribution of each Fe(II) species to the total rate constant as a function of pH. FeH(3)SiO(4)(+) played a key role in the Fe(II) speciation, dominating the process at pH over 8.4. At pH 8.0, FeH(3)SiO(4)(+) represented 18% of the total Fe(II) species. Model results show that when the concentration of silicate is 3 × 10(-5) M as in high nutrient low chlorophyll areas, FeH(3)SiO(4)(+) contributed at pH 8.0 by 4% increasing the rate to 11% at 1.4 × 10(-4) M. The effect of nutrients, especially silicate, must be considered in any further study in seawater media cultures and eutrophic oceanic areas.     

The effects of freeze-thaw and sonication on mitochondrial oxygen consumption, electron transport chain-linked metmyoglobin reduction, lipid oxidation, and oxymyoglobin oxidation

Mitochondria potentially influence Mb redox stability in meat by (1) decreasing partial oxygen pressure via oxygen consumption, (2) mitochondrial electron transport chain (ETC)-linked reduction of MetMb, and/or (3) oxidation of mitochondrial membrane lipid. The objective of this study was to investigate the effect of freeze-thaw and sonication treatments on mitochondrial oxygen consumption, ETC-dependent MetMb reducing activity, lipid oxidation, and Mb redox stability. Mitochondria were frozen and thawed (-18°C for 2h and 4°C for 0.5h) for 3 cycles, or sonicated for 30s with a sonic dismembrator. State III oxygen consumption rate (OCR) was decreased by both treatments at pH 7.2, and by sonication only at pH 5.6 (P<0.05). There was no effect on state IV OCR (P>0.05). Respiratory control ratio (RCR) was decreased by freeze-thaw and sonication at pH 7.2 and 5.6 (P<0.05). Sonication increased mitochondrial lipid oxidation and MetMb formation (P<0.05); a similar effect was observed in sonicated samples in the presence of ascorbic acid and ferric chloride (P<0.05). Sonication also decreased mitochondrial ETC-dependent MetMb reduction (P<0.05). These results suggested that sonication treatment had the potential to affect Mb stability via mitochondrial lipid oxidation and/or ETC-mediated MetMb reduction, but the effect on myoglobin stability by freeze-thaw treatment was minimal.     

pH dependence of Fenton reagent generation and As(III) oxidation and removal by corrosion of zero valent iron in aerated water

Corrosion of zerovalent iron (ZVI) in oxygen-containing water produces reactive intermediates that can oxidize various organic and inorganic compounds. We investigated the kinetics and mechanism of Fenton reagent generation and As(III) oxidation and removal by ZVI (0.1m2/g) from pH 3-11 in aerated water. Observed half-lives for the oxidation of initially 500 microg/L As(III) by 150 mg Fe(0)/L were 26-80 min at pH 3-9. At pH 11, no As(III) oxidation was observed during the first two hours. Dissolved Fe(III) reached 325, 140, and 6 microM at pH 3, 5, and 7. H2O2 concentrations peaked within 10 min at 1.2, 0.4, and < 0.1 microM at pH 3, 5, and 7, and then decreased to undetectable levels. Addition of 2,2'-bipyridine (1-3 mM), prevented Fe(II) oxidation by O2 and H2O2 and inhibited As(III)oxidation. 2-propanol (14 mM), scavenging OH-radicals, quenched the As(III) oxidation at pH 3, but had almost no effect at pH 5 and 7. Experimental data and kinetic modeling suggest that As(III) was oxidized mainly in solution by the Fenton reaction and removed by sorption on newly formed hydrous ferric oxides. OH-radials are the main oxidant for As(III) at low pH, whereas a more selective oxidant oxidizes As(III) at circumneutral pH.     

Preparation and evaluation of GAC-based iron-containing adsorbents for arsenic removal

Granular activated carbon-based, iron-containing adsorbents (As-GAC) were developed for effective removal of arsenic from drinking water. Granular activated carbon (GAC) was used primarily as a supporting medium for ferric iron that was impregnated by ferrous chloride (FeCl2) treatment, followed by chemical oxidation. Sodium hypochlorite (NaClO) was the most effective oxidant, and carbons produced from steam activation of lignite were most suitable for iron impregnation and arsenic removal. Two As-GAC materials prepared by FeCl2 treatment (0.025 -0.40 M) of Dacro 20 x 50 and Dacro 20 x 40LI resulted in a maximum impregnated iron of 7.89% for Dacro 20 x 50 and 7.65% for Dacro 20 x 40Ll. Nitrogen adsorption-desorption analyses showed the BET specific surface area, total pore volume, porosity, and average mesoporous diameter all decreased with iron impregnation, indicating that some micropores were blocked. SEM studies with associated EDS indicated that the distribution of iron in the adsorbents was mainly on the edge of As-GAC in the low iron content (approximately 1% Fe) sample but extended to the center at the higher iron content (approximately 6% Fe). When the iron content was > approximately 7%, an iron ring formed at the edge of the GAC particles. No difference in X-ray diffraction patterns was observed between untreated GAC and the one with 4.12% iron, suggesting that the impregnated iron was predominantly in amorphous form. As-GAC could remove arsenic most efficiently when the iron content was approximately 6%; further increases of iron decreased arsenic adsorption. The removal of arsenate occurred in a wide range of pH as examined from 4.4 to 11, but efficiency was decreased when pH was higher than 9.0. The presence of phosphate and silicate could significantly decrease arsenate removal at pH > 8.5, while the effects of sulfate, chloride, and fluoride were minimal. Column studies showed that both As(V) and As(III) could be removed to below 10 microg/L within 6000 empty bed volume when the groundwater containing approximately 50 microg/L of arsenic was treated.     

Advances in seeded ambient temperature ferrite formation for treatment of acid mine drainage

Advances in the seeded ambient temperature ferrite process for treatment of acid mine drainage (AMD) are described. Magnetite formation requires that the oxidation rate of ferrous to ferric does not exceed the rate at which ferrous iron is incorporated into the crystal structure (dehydroxylation-crystallization). If the oxidation rate is too high, then ferric-only oxides form, an effect exacerbated by the presence of calcium. NaOH was used to raise the pH of simulated AMD, which also contained calcium so as to simulate the use of lime (i.e., the dissolved Ca/ Fe2+ concentration was maintained at 1:1 bythe coaddition of CaCl2 because this is the Ca/Fe ratio that occurs when pH is elevated by the dissolution of lime). Raising the pH to 10.5 causes Fe2+ to precipitate as "ferrous intermediate" (FI), which is then partially oxidized to magnetite (Fe2+Fe3+2O4). The inhibitory effect of calcium is overcome by a combination of magnetite seed particles, high FI concentrations, and aging. High FI concentrations are easily obtained, even from AMD low in Fe2+, by a contact stabilization reactor-settler sequence. Results for simultaneous removal of cobalt, a metal found in significant concentrations in South African AMD, are also presented.     

食盐含量对猪肉肌内脂肪水解和氧化的动力学影响

为探究猪肉加工中不同食盐含量对肌内脂肪水解和脂肪氧化的影响,以猪背最长肌为原料,分别加入1％、2％、3％、4％和5％的食盐,经不同温度(15～30℃)处理后,研究肌内甘油三酯和磷脂水解的动力学规律及游离脂肪酸和脂肪氧化的动力学变化.结果表明,磷脂水解、甘油三酯水解和脂肪氧化均符合一级动力学模型,游离脂肪酸变化符合零级动...     

The influence of the anticaking agent potassium ferrocyanide and salt on the oxidative stability of frozen minced pork meat

The anticaking agent potassium ferrocyanide, K(4)Fe(CN)(6), was found to affect lipid oxidation in frozen, minced pork meat both in 'normal' concentrations, when added together with food grade salt to yield 2 % NaCl in the product, and in 'unrealistic' high concentrations added separately or together with analytical grade salt. The level of K(4)Fe(CN)(6) obtained from adding 2 % food grade salt accelerated the development of lipid hydroperoxides, but affected the development of TBARS to a lesser degree. High level of K(4)Fe(CN)(6) seems to protect hydroperoxides from degradation to secondary lipid oxidation products measured as TBARS. The use of salt with the anticaking agent, K(4)Fe(CN)(6), in meat processing should be reconsidered for each product since it may enhance lipid oxidation. A mechanism for mediation by the Fe(CN) (6)(4-)Fe(CN) (6)(3-) redox couple of pigment-catalysed lipid oxidation is suggested, based on an observed correlation between oxymyoglobin oxidation (measured as tristimulus colorimetric parameter, a) and lipid oxidation (measured as TBARS) for the frozen pork patties.     

Lipid Peroxidation by Microsomal Fractions Isolated from Light and Dark Muscles of Herring (Clupea harengus)

Enzymic lipid peroxidation by light and dark muscle microsomes of herring (Clupea harengus) required ATP or ADP, NADH and Fe. NADPH could not effectively replace NADH. Inhibition was observed at high concentrations of ADP and NADH but not Fe. The optimal pH for the reaction of both types of microsomes was between 6 and 7. The average peroxidation rate was 362 and 1143 nmoles MDA per mg protein per hr at 6°C for the light and dark muscle microsomes, respectively. The energy of activation for the light and dark muscle microsomes was similar. The light muscle microsomes lost activity faster than the dark muscle microsomes when exposed to 35°C. Ferrous ion stimulated enzymic lipid peroxidation of light and dark muscle microsomes over that observed with ferric ion.     

Antioxidant effect of fractions from chicken breast and beef loin homogenates in phospholipid liposome systems

The antioxidant effects of meat fractions from chicken breast and beef loin were compared. Five meat fractions – homogenate (H), precipitate (P), supernatant (S), high-molecular-weight (HMW) and low-molecular-weight (LMW) fractions – were prepared from chicken breast or beef loin. Each of the fractions were added to a phospholipid liposome model system containing catalysts (metmyoglobin, ferrous and ferric ion) or iron chelating agents to determine the effects of each fraction on the development of lipid oxidation during incubation at 37 °C for 120 min. All fractions from chicken breast showed stronger antioxidant effects against iron-catalyzed lipid oxidation than those from beef loin. Iron chelating capacity of water-soluble LMW and water-insoluble (P) fractions from both meats were responsible for their high antioxidant capacities. High concentration of myoglobin, which served as a source of various catalysts, was partially responsible for the high susceptibility of beef loin to lipid oxidation. Storage-stable ferric ion reducing capacity (FRC) was detected in all fractions from both meats, and was a rate-limiting factor for lipid oxidation in the presence of free ionic iron. Higher antioxidant capacity and lower myoglobin content in chicken breast were primarily responsible for its higher oxidative stability than beef loin. DTPA-unchelatable compounds, such as ferrylmyoglobin and/or hematin were the major catalysts for lipid oxidation in beef loin, but free ionic iron and storage-stable FRC also played important roles during prolonged storage.     

Effect of Acid Pretreatment on the Stability of Ascorbic Acid Complexes with Various Iron Sources in a Wheat Flake Cereal

The effect of acid incubation of ascorbic acid with each of five iron sources (ferrous sulfate, ferric chloride, ferric orthophosphate, hydrogen and electrolytically reduced iron) on iron solubilization in a wheat flake cereal was evaluated. Incubation produced more soluble iron at pH 2 but not necessarily at the endogenous pH of the cereal nor at pH 6. At pH 2, Fe⁺² rather than Fe ⁺³ was produced, apparently by a reduction of bound Fe ⁺³ and subsequent release of Fe⁺². At pH 6, the soluble iron was mainly in a complexed form. This indicates that acid incubation with ascorbate might facilitate bioavailability of iron if it were incorporated by fortification techniques.     

Photochemical oxidation of As(III) in ferrioxalate solutions

Photochemical reactions involving aqueous Fe(III) complexes are known to generate free radical species such as OH* that are capable of oxidizing numerous inorganic and organic compounds. Recent work has shown that As(III) can be oxidized to As(V) via photochemical reactions in ferric-citrate solutions; however, the mechanisms of As(III) oxidation and the potential importance of photochemical oxidation in natural waters are poorly understood. Consequently, the objectives of this study were to evaluate oxidation rates of As(III) in irradiated ferrioxalate solutions as a function of pH, identify mechanisms of photochemical As(III) oxidation, and evaluate the oxidation of As(III) in a representative natural water containing dissolved organic C (DOC). The oxidation of As(III) was studied in irradiated ferrioxalate solutions as a function of pH (3-7), As(III), Fe(III), and 2-propanol concentration. Rates of As(III) oxidation (0.5-254 microM h(-1)) were first-order in As(III) and Fe(III) concentration and increased with decreasing pH. Experiments conducted at pH 5.0 using 2-propanol as an OH* scavenger in light and dark reactions suggested that OH* is the important free radical responsible for As(III) oxidation. Significant rates of As(III) oxidation (4-6 microM h(-1)) were also observed in a natural water sample containing DOC, indicating that photochemical oxidation of As(III) may contribute to arsenic (As) cycling in natural waters.     

Anaerobic, nitrate-dependent oxidation of pyrite nanoparticles by Thiobacillus denitrificans

Pyrite is a key mineral in the global biogeochemical cycles of sulfur and iron, yet its anaerobic microbial oxidation has eluded geochemists and microbiologists for decades. Recent reports indicated that anaerobic oxidation of pyrite is occurring, but the mechanism remains unclear. Here, we provide evidence for the capability of Thiobacillus denitrificans to anaerobically oxidize a putatively nanosized pyrite particle fraction with nitrate as electron acceptor. Nanosized pyrite was readily oxidized to ferric iron and sulfate with a rate of 10.1 μM h(-1). The mass balance of pyrite oxidation and nitrate reduction revealed a closed recovery of the electrons. This substantiates a further "missing lithotrophy" in the global cycles of sulfur and iron and emphasizes the high reactivity of nanominerals in the environment.     

Effect of remineralization on heavy-metal leaching from cement-stabilized/solidified waste

Crushed samples of stabilized/solidified (s/s) waste were leached at constant leachate pH in the pH range 4-7 with nitric acid solutions to evaluate the influence of remineralization on metal release. The s/s waste consisted of synthetic heavy-metal sludge containing 0.1 mol L(-1) copper nitrate, 0.1 mol L(-1) zinc nitrate, and 0.1 mol L(-1) lead nitrate mixed with ordinary Portland cement. Unleached and leached particles were characterized by scanning electron microscopy and energy-dispersive X-ray spectrometry. Two consecutive leaching fronts advancing from the surface of the particles toward the center were identified: the first front was associated with the dissolution of portlandite and partial reaction of the calcium silicate hydrate gel, while the second front was associated with the dissolution of calcium-aluminum hydroxy sulfates such as ettringite and monosulfate. At pH 4 and 5, a remineralization zone rich in heavy metals formed immediately behind the second leaching front. The shell extending from the remineralization zone to the surface of the particles was depleted in calcium, sulfate, and heavy metals. As a result of remineralization, heavy-metal releases to the leachate were reduced by factors ranging between 3.2 and 6.2 at pH 4 and between 74 and 193 at pH 5. At pH 6 and 7, remineralization of Pb and Zn occurred further behind the second leaching front and closer to the surface of the particles. The amount of heavy-metal release depended on both the leachate pH and the remineralization factor.