Mechanistic Study of Pb(II) Removal by TiO2 and Effect of PO4.

As a commercial adsorbent, TiO2 shows a high adsorption capacity for lead (Pb(II)). However, the molecular structure of Pb(II) adsorption on TiO2 is still unknown. Meanwhile, as a widely used corrosion inhibitor, phosphate (PO4) is usually added into drinking water, and its influential mechanism on Pb(II) removal by TiO2 remains unknown. Here, the mechanisms of Pb(II) adsorption on TiO2 and the effect of PO4 were systematically investigated using a combination of spectroscopic analyses and surface complexation modeling. The adsorption structure of Pb(II) on TiO2 was revealed as a tridentate mononuclear configuration by the extended X-ray absorption fine structure (EXAFS) analysis. In the presence of 0.1-5 mg/L PO4, Pb(II) was removed mainly by adsorption on TiO2 rather than precipitation. Ternary complexation between Pb(II) and PO4 on TiO2 surfaces was found based on EXAFS and in situ Fourier transform infrared characterizations. These complexation structures were used to build a surface complexation model to accurately simulate and predict Pb(II) removal under different conditions. This study provides essential information about the mechanisms of Pb(II) removal by TiO2 and develops a model to predict adsorption behaviors, especially in the presence of PO4.

Identifying the existence and molecular structure of the dissolved HCO3-Ca-As(V) complex in water.

Calcium (Ca2+) and bicarbonate (HCO3-) ions co-exist with arsenic (As) in natural water systems, while Ca-based materials such as lime and cement are widely used to immobilize As(V) in contaminated solids. In this paper, a new dissolved ternary complex, HCO3-Ca-As(V), was discovered and its molecular structure was identified. The results from the batch experiments showed that adding As(V) to the solutions containing Ca2+ and HCO3- increased the dissolved Ca concentration from 4.8 to 73.2 mg/L at pH 11. Both infrared and X-ray absorption spectroscopy indicated the presence of dissolved HCO3-Ca-As(V) complex. Based on the quantitative geometric information obtained from the spectroscopic results, the molecule of (OH)OC-O-(OH2)4Ca-O2-As(OH)2 was identified by the density functional theory (DFT) calculation. Although Ca2+ and As(V) can form complex without HCO3-, the presence of HCO3- further enhanced the stability of the dissolved Ca complex, as evidenced by the lower binding energy (BE) of HCO3-Ca-As(V) (-329.1959 kJ/mol) than Ca-As(V) (4.7171 kJ/mol). The discovery of dissolved HCO3-Ca-As(V) complex is important for understanding the mobility of As(V) in natural water, and the possible release of As(V) in contaminated solids treated with Ca-based materials.

Lead immobilization by phosphate in the presence of iron oxides: Adsorption versus precipitation.

As a commonly used corrosion inhibitor, phosphate (PO4) has a complicated effect on the fate and transport of lead (Pb) in drinking water systems. While the formation of pyromorphite has been recognized to be the major driving force of the Pb immobilization mechanism, the role of adsorption on iron oxides is still not clear. This study aims to clarify the contributions of adsorption and precipitation to Pb removal in a system containing both iron oxides and PO4. A combination of batch experiments, X-ray absorption spectroscopy, infrared spectroscopy, and electron spectroscopy was employed to distinguish the adsorbed and precipitated Pb species. The results indicated that the adsorption of Pb on iron oxides still occurred even when the solution was supersaturated to pyromorphite (i.e., 5 mg/L P with 0.1-30 mg/L Pb in 0.01 M NaCl solution at neutral pH). In the tap water containing 0.92 mg/L P and 1 mg/L Pb, adsorption on iron oxides contributed more (62-67%) than precipitation (33-38%) in terms of Pb removal. Surprisingly, the pre-formed pyromorphite is transformed to adsorbed species after mixing with iron oxides in water for 24 h. The illustration of this transformation is important to understand the immobilization mechanisms and transport behaviors of Pb in drinking water systems after the utilization of PO4.

Leaching behavior of Cr(III) in stabilized/solidified soil.

The leaching behavior of chromium was studied using batch leaching tests, surface complexation modeling and X-ray absorption near edge structure (XANES) spectroscopy. A contaminated soil sample containing 1330 mg-Cr kg(-1) and 25600 mg-Fe kg(-1) of dry soil was stabilized/solidified (S/S) with 10% cement, 25% cement, 10% lime and a mixture of 20% flyash and 5% lime. The XANES analysis showed that Cr(III) was the only Cr species in untreated soil and S/S-treated samples. The leachate Cr concentration determined using the toxicity characteristic leaching procedure (TCLP) was reduced from 5.18 mg l(-1) for untreated soil to 0.84 mg l(-1) for the sample treated with 25% cement. The Cr leachability in untreated and treated soil samples decreased dramatically as the pH increased from 3 to 5, remained at similar levels in the pH range between 5 and 10.5, and further decreased at pH>10.5. Modeling results indicated that the release of Cr(III) was controlled by adsorption on iron oxides at pH<10.5, and by precipitation of Ca(2)Cr(2)O(5).6H(2)O at pH>10.5.

Carbonate effects on hexavalent uranium removal from water by nanocrystalline titanium dioxide.

A novel nanocrystalline titanium dioxide was used to treat depleted uranium (DU)-contaminated water under neutral and alkaline conditions. The novel material had a total surface area of 329 m(2)/g, total surface site density of 11.0 sites/nm(2), total pore volume of 0.415 cm(3)/g and crystallite size of 6.0 nm. It was used in batch tests to remove U(VI) from synthetic solutions and contaminated water. However, the capacity of the nanocrystalline titanium dioxide to remove U(VI) from water decreased in the presence of inorganic carbonate at pH > 6.0. Adsorption isotherms, Fourier transform infrared (FTIR) spectroscopy, and surface charge measurements were used to investigate the causes of the reduced capacity. The surface charge and the FTIR measurements suggested that the adsorbed U(VI) species was not complexed with carbonate at neutral pH values. The decreased capacity of titanium dioxide to remove U(VI) from water in the presence of carbonate at neutral to alkaline pH values was attributed to the aqueous complexation of U(VI) by inorganic carbonate. The nanocrystalline titanium dioxide had four times the capacity of commercially available titanium dixoide (Degussa P-25) to adsorb U(VI) from water at pH 6 and total inorganic carbonate concentration of 0.01 M. Consequently, the novel material was used to treat DU-contaminated water at a Department of Defense (DOD) site.

Removal of depleted uranium from contaminated soils.

Contamination of soil and water with depleted uranium (DU) has increased public health concerns due to the chemical toxicity of DU at elevated dosages. For this reason, there is great interest in developing methods for DU removal from contaminated sources. Two DU laden soils, taken from U.S. Army sites, were characterized for particle size distribution, total uranium concentration and removable uranium. Soil A was found to be a well graded sand containing a total of 3210 mg/kg DU (3.99 x 10(4) Bq/kg, where a Becquerel (Bq) is a unit of radiation). About 83% of the DU in the fines fraction (particle diameter <0.075 mm, total DU 7732 mg/kg (9.61 x 10(4) Bq/kg)) was associated with the carbonate, iron and manganese oxide and organic matter fractions of the material. Soil B was classified as a sandy silt with total DU of 1560 mg/kg (1.94 x 10(4) Bq/kg). The DU content in the fines fraction was 5171 mg/kg (6.43 x 10(4) Bq/kg). Sequential extraction of the Soil B fines fraction indicated that 64% of the DU was present either as soluble U(VI) minerals or as insoluble U(IV). Citric acid, sodium bicarbonate and hydrogen peroxide were used in batch experiments to extract DU from the fines fraction of both soils. Citric acid and sodium bicarbonate were relatively successful for Soil A (50-60% DU removal), but not for Soil B (20-35% DU removal). Hydrogen peroxide was found to significantly increase DU extraction from both soils, attaining removals up to 60-80%.

Chemical reactions between arsenic and zero-valent iron in water.

Batch experiments and X-ray photoelectron spectroscopic (XPS) analyses were performed to study the reactions between arsenate [As(V)], arsenite [As(III)] and zero-valent iron [Fe(0)]. The As(III) removal rate was higher than that for As(V) when iron filings (80-120 mesh) were mixed with arsenic solutions purged with nitrogen gas in the pH range of 4-7. XPS spectra of the reacted iron coupons showed the reduction of As(III) to As(0). Soluble As(III) was formed when As(V) reacted with Fe(0) under anoxic conditions. However, no As(0) was detected on the iron coupons after 5 days of reaction in the As(V)-Fe(0) system. The removal of the arsenic species by Fe(0) was attributed to electrochemical reduction of As(III) to sparsely soluble As(0) and adsorption of As(III) and As(V) to iron hydroxides formed on the Fe(0) surface under anoxic conditions. When the solutions were open to atmospheric air, the removal rates of As(V) and As(III) were much higher than under the anoxic conditions, and As(V) removal was faster than As(III). The rapid removal of As(III) and As(V) was caused by adsorption on ferric hydroxides formed readily through oxidation of Fe(0) by dissolved oxygen.

Adsorption of As(V) and As(III) by nanocrystalline titanium dioxide.

This study evaluated the effectiveness of nanocrystalline titanium dioxide (TiO(2)) in removing arsenate [As(V)] and arsenite [As(III)] and in photocatalytic oxidation of As(III). Batch adsorption and oxidation experiments were conducted with TiO(2) suspensions prepared in a 0.04 M NaCl solution and in a challenge water containing the competing anions phosphate, silicate, and carbonate. The removal of As(V) and As(III) reached equilibrium within 4h and the adsorption kinetics were described by a pseudo-second-order equation. The TiO(2) was effective for As(V) removal at pH<8 and showed a maximum removal for As(III) at pH of about 7.5 in the challenge water. The adsorption capacity of the TiO(2) for As(V) and As(III) was much higher than fumed TiO(2) (Degussa P25) and granular ferric oxide. More than 0.5 mmol/g of As(V) and As(III) was adsorbed by the TiO(2) at an equilibrium arsenic concentration of 0.6mM. The presence of the competing anions had a moderate effect on the adsorption capacities of the TiO(2) for As(III) and As(V) in a neutral pH range. In the presence of sunlight and dissolved oxygen, As(III) (26.7 microM or 2mg/L) was completely converted to As(V) in a 0.2g/L TiO(2) suspension through photocatalytic oxidation within 25 min. The nanocrystalline TiO(2) is an effective adsorbent for As(V) and As(III) and an efficient photocatalyst.

Surface complexation of organic arsenic on nanocrystalline titanium oxide.

The adsorption mechanisms of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) on nanocrystalline titanium oxide (TiO2) were investigated with X-ray absorption spectroscopy (XAS), surface charge and zeta potential measurements, adsorption edge, and surface complexation modeling. XAS data demonstrated that MMA and DMA formed bidentate and monodentate inner sphere complexes with the TiO2 surface, respectively. The charge and zeta potential behaviors of TiO2 as a function of ionic strength suggested that the point of zero charge (PZC) and isoelectric point (IEP) of TiO2 were identical at pH 5.8. Adsorption of MMA and DMA on TiO2 shifted the IEP to pH 4.1 and 4.8, respectively, indicating the formation of negatively charged surface complexes. A satisfactory interpretation of the experimental data was provided by the charge distribution (CD) multi-site complexation (MUSIC) model with the triple plane option under the constraint of the XAS evidence.

Removal of arsenic from water by zero-valent iron.

Batch and column experiments were conducted to investigate the effect of dissolved oxygen (DO) and pH on arsenic removal with zero-valent iron [Fe(0)]. Arsenic removal was dramatically affected by the DO content and the pH of the solution. Under oxic conditions, arsenate [As(V)] removal by Fe(0) filings was faster than arsenite [As(III)]. Greater than 99.8% of the As(V) was removed whereas 82.6% of the As(III) was removed at pH 6 after 9h of mixing. When the solution was purged with nitrogen gas to remove DO, less than 10% of the As(III) and As(V) was removed. High DO content and low solution pH also increased the rate of iron corrosion. The removal of arsenic by Fe(0) was attributed to adsorption by iron hydroxides generated from the oxic corrosion of Fe(0). The column results indicated that a filtration system consisting of an iron column and a sand filter could be used for treatment of arsenic in drinking water.

Development of a gravity-independent wastewater bioprocessor for advanced life support in space.

Operation of aerobic biological reactors in space is controlled by a number of challenging constraints, mainly stemming from mass transfer limitations and phase separation. Immobilized-cell packed-bed bioreactors, specially designed to function in the absence of gravity, offer a viable solution for the treatment of gray water generated in space stations and spacecrafts. A novel gravity-independent wastewater biological processor, capable of carbon oxidation and nitrification of high-strength aqueous waste streams, is presented. The system, consisting of a fully saturated pressurized packed bed and a membrane oxygenation module attached to an external recirculation loop, operated continuously for over one year. The system attained high carbon oxidation efficiencies often exceeding 90% and ammonia oxidation reaching approximately 60%. The oxygen supply module relies on hydrophobic, nonporous, oxygen selective membranes, in a shell and tube configuration, for transferring oxygen to the packed bed, while keeping the gaseous and liquid phases separated. This reactor configuration and operating mode render the system gravity-independent and suitable for space applications.

Combined effects of anions on arsenic removal by iron hydroxides.

Batch experiments were conducted to investigate the combined effects of phosphate, silicate, and bicarbonate on the removal of arsenic from Bangladesh groundwater (BGW) and simulated groundwater by iron hydroxides. The apparent adsorption constants indicated that the affinity of the anions for iron hydroxide sites decreased in the following order arsenate>phosphate>arsenite>silicate>bicarbonate. Phosphate, silicate, and bicarbonate decreased the removal of As(III) even at relatively low concentrations and low surface site coverage. Phosphate (0-0.08 mM), silicate (0-0.8 mM), and bicarbonate (0-14 mM) in separate solutions had none to moderate effects on As(V) removal in a solution containing 6.7 mg/l Fe and 0.3 ppm As(V). In the presence of bicarbonate and silicate the adverse effect of phosphate on As(V) adsorption was magnified. The residual As(V) concentration after iron hydroxide treatment increased from less than 13 microg/l in separate bicarbonate (2.2 mM) and phosphate (0.062 mM) solutions to 110 microg/l in the solution containing both anions. The results suggested the combined effects of phosphate, silicate, and bicarbonate caused the high mobility of arsenic in Bangladesh water.

Removal of selenocyanate from water using elemental iron.

Batch experiments were conducted to investigate the removal of selenocyanate (SeCN-) from oil refinery wastewater and artificial wastewater with elemental iron [Fe(0)]. The chemical forms of selenium in the reacted solids were determined with X-ray photoelectron spectroscopy (XPS) and a sulfite extraction procedure. SeCN- was effectively removed from the wastewater with Fe(0) filings when the water pH was controlled at approximately 6. SeCN- was removed by Fe(0) through the formation of elemental selenium [Se(0)] and ferrous selenide. The possible chemical reactions between SeCN- and Fe(0) included deselenation of SeCN- and electrochemical reduction of Se(0) to selenide. A cost-effective process may be developed for the treatment of SeCN- in wastewater using Fe(0).

Lead leachability in stabilized/solidified soil samples evaluated with different leaching tests.

Leaching tests and model calculations were performed to investigate the immobilization mechanisms of Pb and compare different leaching protocols. Stabilization/solidification (S/S) treatments reduced Pb concentrations in the toxicity characteristic leaching procedure (TCLP) leachate from 5.9 mg/L for untreated soil to less than 0.7 mg/L. The results of eight different leaching protocols show that: (1) the main factor controlling the Pb concentration in the leachate is the final pH; (2) the final pH is a function of the leachant acidity; and (3) for a given final pH, the type of leachant has a relatively minor effect on leachability. The diffuse layer adsorption model, aqueous and precipitation reactions were employed in the MINTEQA2 program to describe the Pb leaching behavior. Both leaching tests and model simulations indicate that the Pb leaching behavior can be divided into three stages based on the leachate pH: a high alkalinity leaching stage at pH > 12, where Pb formed soluble hydroxide anion complexes and leached out; a neutral to alkaline immobilization stage in the pH range of 6-12, which was characterized by low Pb leachability caused by adsorption and precipitation; and an acid leaching stage with pH < 6, where the acid neutralizing capacity (ANC) of the S/S materials was totally consumed and therefore free Pb-ion leached out.

Direct evidence of arsenic(III)-carbonate complexes obtained using electrochemical scanning tunneling microscopy.

Electrochemical scanning tunneling microscopy (ECSTM), ion chromatography (IC), and electrospray ionization-mass spectrometry/mass spectrometry were applied to investigate the interactions between arsenite [As(III)] and carbonate and arsenate [As(V)] and carbonate. The chemical species in the single and binary component solutions of As(III), As(V), and carbonate were attached to a Au(111) surface and then imaged in a 0.1 M NaClO4 solution at the molecular level by ECSTM. The molecules formed highly ordered adlayers on the Au(111) surface. High-resolution STM images revealed the orientation and packing arrangement of the molecular adlayers. Matching the STM images with the molecular models constructed using the Hyperchem software package indicated that As(III) formed two types of complexes with carbonate, including As(OH)2CO3- and As(OH)3(HCO3-)2. No complexes were formed between As(V) and carbonate. IC chromatograms of the solutions revealed the emergence of the new peak only in the aged As(III)-carbonate solution. MS spectra showed the presence of a new peak at m/z 187 in the aged As(III)-carbonate solution. The results obtained with the three independent methods confirmed the formation of As(OH)2CO3-. The results also indicated that As(OH)3 could be associated with HCO3- through a hydrogen bond. The knowledge of the formation of the As(III) and carbonate complexes will improve the understanding of As(III) mobility in the environment and removal of As(III) in water treatment systems.

Adsorption mechanism of arsenic on nanocrystalline titanium dioxide.

Arsenate [As(V)] and arsenite [As(III)] interactions at the solid-water interface of nanocrystalline TiO2 were investigated using electrophoretic mobility (EM) measurements, Fourier transform infrared (FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. The adsorption of As(V) and As(III) decreased the point of zero charge of TiO2 from 5.8 to 5.2, suggesting the formation of negatively charged inner-sphere surface complexes for both arsenic species. The EXAFS analyses indicate that both As(V) and As(III) form bidentate binuclear surface complexes as evidenced by an average Ti-As(V) bond distance of 3.30 A and Ti-As(III) bond distance of 3.35 A. The FTIR bands caused by vibrations of the adsorbed arsenic species remained at the same energy levels at different pH values. Consequently, the surface complexes on TiO2 maintained the same nonprotonated speciation at pH values from 5 to 10, and the dominant surface species were (TiO)2AsO2- and (TiO)2AsO- for As(V) and As(III), respectively. The surface configurations constrained with the spectroscopic results were formulated in the diffuse layer model to describe the adsorption behavior of As in the pH range between 4 and 12. The study suggests that TiO2 is an effective adsorbent for As removal due to its high surface area and the presence of high affinity surface hydroxyl groups.

Immobilization mechanisms of arsenate in iron hydroxide sludge stabilized with cement.

Leaching tests, Fourier transform infrared spectroscopy (FTIR), extended X-ray absorption fine structure (EXAFS) spectroscopy, and thermodynamic modeling were performed to investigate arsenate [As(V)] immobilization mechanisms in iron hydroxide sludge stabilized with cement. The sludge from a groundwater remediation site in Tacoma, WA was mixed and immobilized with premixed cement to reach cement-to-sludge ratios of 2.5, 3.3, 5, 10, and 20 (wt premixed cement/wt dry sludge). The EXAFS analysis determined that As(V) formed bidentate mononuclear complexes on the iron hydroxide surface in the sludge. The adsorbed As(V) had a characteristic FTIR band at 830 cm(-1). Cement treatment converted the adsorbed As(V) to calcium arsenate precipitate with a FTIR peak at 860 cm(-1). The chemical forms of the As(V) were incorporated in an adsorption triple layer model (TLM) to describe the leaching behavior of As(V) in a pH range between 3 and 12. Cement treatment significantly reduced arsenic mobility because of the formation of the sparingly soluble calcium arsenate.

Carbonate effects on hexavalent uranium adsorption by iron oxyhydroxide.

Carbonate dramatically affects the adsorption of uranium (U(VI)) onto iron hydroxides and its mobility in the natural environment. Batch tests, zeta potential measurements, and Fourier transform infrared (FTIR) spectroscopic studies were utilized to characterize the nature of U(VI) adsorption on ferrihydrite. Adsorption isotherms demonstrated that carbonate had a negative effect on U(VI) adsorption on ferrihydrite at pH > 6. Zeta potential measurements indicated that U(VI) was adsorbed as a cationic species (SO-UO2+) in the absence of carbonate and as anionic U(VI) complexes in the presence of carbonate at neutral pH. FTIR spectroscopic measurement of adsorbed U(VI) suggested that it was retained as uranyl carbonate complexes in the presence of carbonate. An increase in carbonate concentration caused a shift in the antisymmetric stretching vibration of the uranyl (UO2(2+)) U-O bond toward lower wavenumbers, which indicated an increasing carbonate effect in the adsorbed uranyl carbonate complexes. The adsorbed U(VI) species were successfully incorporated into a surface complexation model to describe the adsorption of U(VI) by ferrihydrite from artificial solutions and contaminated water.