Phosphate recovery from anaerobic digester effluents using CaMg(OH)4.

Dolomite lime (DL) (CaMg(OH)4) was used as an economical source of Mg(2+) for the removal and recovery of phosphate from an anaerobic digester effluent of a municipal wastewater treatment plant (MWWTP) wastewater. Batch precipitation results determined that phosphate was effectively reduced from 87 to less than 4mg-P/L when the effluent water was mixed with 0.3g/L of DL. The competitive precipitation mechanisms of different solids in the treatment system consisting of Ca(2+)-Mg(2+)-NH4(+)-PO4(3-)CO3(2-) were determined by comparing model predictions with experimental results. Thermodynamic model calculations indicated that hydroxyapatite (Ca10(PO4)6(OH)2), Ca4H(PO4)3∙3H2O, Ca3(PO4)2(beta), and Ca3(PO4)2(am2) were more stable than struvite (MgNH4PO3∙6H2O) and calcite (CaCO3). However, X-ray diffraction (XRD) analysis determined the formation of struvite and calcite minerals in the treated effluent. Kinetic experimental results showed that most of the phosphate was removed from synthetic effluent containing NH4(+) within 2hr, while only 20% of the PO4(3-) was removed in the absence of NH4(+) after 24hr of treatment. The formation of struvite in the DL-treated effluent was due to the rapid precipitation rate of the mineral. The final pH of the DL-treated effluent significantly influenced the mass ratio of struvite to calcite in the precipitates. Because more calcite was formed when the pH increased from 8.4 to 9.6, a pH range of 8.0-8.5 should be used to produce solid with high PO4(3-) content. This study demonstrated that DL could be used for effective removal of phosphate from the effluent and that resultant precipitates contained high content of phosphate and ammonium.

Spectrophotometric analyses of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) in water.

A simple and accurate spectrophotometric method for on-site analysis of royal demolition explosive (RDX) in water samples was developed based on the Berthelot reaction. The sensitivity and accuracy of an existing spectrophotometric method was improved by: replacing toxic chemicals with more stable and safer reagents; optimizing the reagent dose and reaction time; improving color stability; and eliminating the interference from inorganic nitrogen compounds in water samples. Cation and anion exchange resin cartridges were developed and used for sample pretreatment to eliminate the effect of ammonia and nitrate on RDX analyses. The detection limit of the method was determined to be 100 µg/L. The method was used successfully for analysis of RDX in untreated industrial wastewater samples. It can be used for on-site monitoring of RDX in wastewater for early detection of chemical spills and failure of wastewater treatment systems.

Surface-enhanced Raman scattering spectroscopy of explosive 2,4-dinitroanisole using modified silver nanoparticles.

2,4-Dinitroanisole (DNAN) is being used as a replacement for 2,4,6-trinitrotoluene (TNT) as a less-sensitive melt-cast medium explosive than TNT. In this paper, we studied the surface-enhanced Raman spectroscopy (SERS) analysis of DNAN using Ag nanoparticles (AgNPs) modified by L-cysteine methyl ester hydrochloride. Due to the formation of a Meisenheimer complex between DNAN and the modifier, the modified AgNPs can detect 20 µg/L (0.2 ng) and 0.1 mg/L (1 ng) DNAN in deionized water and aged tap water, respectively. Three other chemicals (L-cysteine, N-acetyl-L-cysteine, and L-cysteine ethyl ester hydrochloride) were used as AgNPs modifiers to study the mechanism of the SERS of DNAN. It was confirmed that the amino group of L-cysteine methyl ester hydrochloride was the active group and that the methyl ester group significantly contributed to the high SERS sensitivity of DNAN. In order to further test the mechanism of Meisenheimer complex formation, the effect of anions and cations present in natural water on the SERS of DNAN was studied. It was found that CO(3)(2-), Cl(-), and K(+) at 100 mg/L did not negatively affect the SERS of 10 mg/L DNAN, while SO(4)(2-), Na(+), Mg(2+), and Ca(2+) at 100 mg/L significantly quenched the SERS of 10 mg/L DNAN. The negative effect of the bivalent cations could be offset by SO(4)(2-).

Fabrication and evolution of multilayer silver nanofilms for surface-enhanced Raman scattering sensing of arsenate.

Surface-enhanced Raman scattering (SERS) has recently been investigated extensively for chemical and biomolecular sensing. Multilayer silver (Ag) nanofilms deposited on glass slides by a simple electroless deposition process have been fabricated as active substrates (Ag/GL substrates) for arsenate SERS sensing. The nanostructures and layer characteristics of the multilayer Ag films could be tuned by varying the concentrations of reactants (AgNO3/BuNH2) and reaction time. A Ag nanoparticles (AgNPs) double-layer was formed by directly reducing Ag+ ions on the glass surfaces, while a top layer (3rd-layer) of Ag dendrites was deposited on the double-layer by self-assembling AgNPs or AgNPs aggregates which had already formed in the suspension. The SERS spectra of arsenate showed that characteristic SERS bands of arsenate appear at approximately 780 and 420 cm-1, and the former possesses higher SERS intensity. By comparing the peak heights of the approximately 780 cm-1 band of the SERS spectra, the optimal Ag/GL substrate has been obtained for the most sensitive SERS sensing of arsenate. Using this optimal substrate, the limit of detection (LOD) of arsenate was determined to be approximately 5 µg·l-1.

Surface-enhanced Raman scattering for arsenate detection on multilayer silver nanofilms.

Surface-enhanced Raman scattering (SERS) has recently emerged as a promising method for chemical and biomolecular sensing. SERS quantification analysis of arsenate (As(V)) was investigated using multilayer Ag nanofilms deposited on glass slides as SERS-active substrates (Ag/GL substrates) by an electroless deposition process. The As(V) limit of detection (LOD) was determined to be ∼5 µg L(-1) or lower with or without coexisting multiple background electrolytes (Na(+), K(+), Ca(2+), Mg(2+), Cl(-), NO(3)(-), SO(4)(2-) and H(2)PO(4)(-)). The presence of the background electrolytes at low concentrations was observed to enhance the SERS sensitivity of the substrate towards As(V) more than twofold. Standard calibration curves were prepared in the absence and presence of the background electrolytes. Excellent linear relationships between the peak heights of the As(V) SERS band at ∼780 cm(-1) and the As(V) concentrations were obtained in a concentration range of 0-250 µg L(-1). The selectivity of the Ag nanofilm towards oxyanions was examined to be in the order of As(V)≫phosphate≫nitrate, sulphate. A low sample-to-sample relative standard deviation (RSD) of 5.2% was also determined, suggesting the Ag/GL substrate was uniform and highly reproducible. Experimental results indicated that the SERS method could be used for quantitative analysis of As(V) in groundwater samples.

Surface-enhanced Raman spectroscopy of arsenate and arsenite using Ag nanofilm prepared by modified mirror reaction.

A modified mirror reaction was developed to prepare a sensitive and reproducible Ag nanofilm substrate for the surface-enhanced Raman scattering (SERS) analysis of arsenate (As(V)) and arsenite (As(III)). A good linear relationship between the SERS intensity of As(V) and As(III) and their concentrations in the range from 10 to 500 microg-As/L was achieved using the SERS substrate. As(V) and As(III) appear to be adsorbed on the Ag nanofilm through formation of surface complexes with Ag, based on comparisons of the Raman spectra of the arsenic species in solutions, on the SERS substrate, and in silver arsenate and arsenite solids. As(V) and As(III) species on the SERS substrate and in the solids had the same Raman band positions at 780 and 721 cm(-1), respectively. The effect of eight ions in natural waters on the SERS analysis of As(V) was studied. K(+), Na(+), SO(4)(2-), CO(3)(2-), and NO(3)(-) in the range of 0.1-100 mg/L did not interfere with the SERS detection of As(V) for a As(V) concentration greater than 100 microg-As/L. While Cl(-) (50 mg/L), Mg(2+) (10 mg/L), and Ca(2+) (1 mg/L) were found to quench the SERS intensity of 100 microg/L As(V). Cl(-) (at concentrations >10 mg/L) formed silver chloride with the adsorbed Ag(+) and decreased the SERS detection limits for arsenic species. The mechanism of the Ca(2+) effect on the SERS analysis of As(V) was through the formation of surface complexes with As(V) in competition with Ag. When the Ca(2+) concentration increased from 0 to 100 mg/L, the amount of As(V) adsorbed in Ag nanoparticles was reduced from 38.9 to 11.0 microg/mg-Ag. When the Ca(2+) concentration increased to values higher than 1 mg/L in the As(V) solution, the As(V) peak height was decreased in the corresponding SERS spectra and the peak position shifted from 780 to 800 cm(-1). The fundamental findings obtained in this research are especially valuable for the development of sensitive and reliable SERS methods for rapid analysis of arsenic in contaminated water.

Size effects of nanocrystalline TiO2 on As(V) and As(III) adsorption and As(III) photooxidation.

The physicochemical properties of TiO(2) particles in the diameter range between 6.6 and 30.1 nm and the effect of the crystalline size on arsenic adsorption and photocatalytical oxidation were investigated. TiO(2) nanoparticles of different sizes were single-phase anatase. The adsorption capacity of the TiO(2) for As(III) and As(V) increased linearly with the N(2) Brunauer-Emmett-Teller surface area (S(BET)) of the particles. There was not much difference in the rate of As(III) photooxidation when the diameter of the TiO(2) nanoparticles was between 6.6 and 14.8 nm. However, the As(III) photooxidation rate clearly decreased when the particle size increased to 30.1 nm. Arsenite photooxidation data could be fitted with a first-order kinetics equation.

Mechanisms of photocatalytical degradation of monomethylarsonic and dimethylarsinic acids using nanocrystalline titanium dioxide.

Photodegradation mechanisms of monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) with nanocrystalline titanium dioxide under UV irradiation were investigated. In the presence of UV irradiation and 0.02 g/L TiO2, 93% MMA (initial concentration is 10 mg-As/L) was transformed into inorganic arsenate, [As(V)], after 72 h of a batch reaction. The mineralization of DMA to As(V) occurred in two steps with MMA as an intermediate product. The photodegradation rate of MMA and DMA could be described using first-order kinetics, where the apparent rate constant is 0.033/h and 0.013/h for MMA and DMA, respectively. Radical scavengers, including superoxide dimutase (SOD), sodium bicarbonate, tert-butanol, and sodium azide, were used to study the photodegradation mechanisms of MMA and DMA. The results showed that hydroxyl radicals (HO*) was the primary reactive oxygen species for the photodegradation of MMA and DMA. The methyl groups in MMA and DMAweretransformed into organic carbon, including formic acid and possibly methanol, also through photochemical reactions. The results showed that nanocrystalline TiO2 can be used for the photocatalytical degradation of MMA and DMA and subsequent removal of the converted As(V), since the high adsorption capacity of the material for inorganic arsenic species has been demonstrated in previous studies.

Competitive sorption behavior of copper(II) and herbicide propisochlor on humic acids.

The competitive sorption behavior of Cu2+ and propisochlor on humic acids (HA) was investigated using a batch method. The sorption equilibrium time of propisochlor on HA was 12 h when its initial concentration was 4 or 10 mg L(-1). The results showed that the Langmuir model can best describe the sorption behavior of propisochlor on HA. The added Cu2+ reduced the solid-phase concentration of propisochlor on HA, and the Langmuir model can also describe the sorption behavior of propisochlor on HA when the Cu2+ concentration is 100 and 200 mg L(-1). The sorption of propisochlor on HA did not remarkably affect the sorption of Cu2+ on HA when the concentration of propisochlor was below 20 mg L(-1). Cu2+ may compete with propisochlor for the sorption sites of HA, such as carboxylic and phenolic groups. It can be concluded that Cu2+ fast adsorbed to the HA matrix, altered its physical and chemical properties, and thus decreased the solid-phase concentrations of propisochlor on HA. In natural water, Cu2+ may promote the release of propisochlor from HA, and thus affect its transport, transformation, and fate in the environment.

Adsorption behavior of herbicide butachlor on typical soils in China and humic acids from the soil samples.

Three kinds of soils in China, krasnozem, fluvo-aquic soil, and phaeozem, as well as the humic acids (HAs) isolated from them, were used to adsorb the herbicide butachlor from water. Under the experimental conditions, the adsorption amount of butachlor on soils was positively correlated with the content of soil organic matter. HAs extracted from different kinds of soils had different adsorption capacity for the tested herbicide, which was positively correlated with their content of carbonyls. The adsorption mechanism was studied using Fourier transform infrared spectroscopy and cross-polarization with magic angle spinning 13C nuclear magnetic resonance (CP-MAS 13C NMR) techniques. It was showed that the adsorption mainly took place on the C=O, phenolic and alcoholic O-H groups of HAs. It was also confirmed that the adsorption mechanism was hydrogen bonds formation between the above groups of HAs and butachlor molecules.