Chi-Wang Li

Monitoring the properties of natural organic matter through UV spectroscopy: A consistent theory

UV spectra of natural organic matter (NOM) can be represented by three bands, each a Gaussian function of energy. The bands, referred to as the local-excitation (LE), benzenoid (Bz) and electron-transfer (ET) bands, can be ascribed to three types of electronic transitions typical in aromatic compounds. At wavelengths from 200 to 400 nm, an adequate fit to the spectrum can be obtained by consideration of only the Bz and ET bands. The parameters of these two bands are sensitive to the composition of NOM and to alterations in NOM caused by coagulation and chlorination. Removal of NOM fractions with higher-than-average concentrations of carboxyl-, hydroxyl- and ester-substituted aromatic rings or oxidation of those fractions by chlorine decreases the intensity and width of the ET and Bz bands. The ratio of absorbances at the band maxima is an indicator of the average degree of activation of the aromatic rings and permits prediction of the reactivity of the aromatic moiety in chlorination reactions. In addition, measurements of absorbance changes induced by chlorination permit one to monitor certain aspects of chlorination reactions in situ and to evaluate the formation of disinfection by-products.

The decrease of UV absorbance as an indicator of TOX formation

Abstract The decrease in UV absorbance (ΔUV) caused by chlorination of NOM correlates linearly with the amount of TOX formed, for a remarkably wide range of water quality conditions and reaction times. THM formation also correlates with ΔUV, but the relationship is more sensitive to solution pH and is probably not linear over the entire range of conditions/times relevant for water treatment. It appears that all Cl-DOC reactions that destroy UV absorbance generate TOX, but substantial amounts of THMs are formed only after a significant amount of reaction has occurred.

Concentration and purification of chromate from electroplating wastewater by two-stage electrodialysis processes

A designed two-stage electrodialysis system is proposed to concentrate and purify chromate from a low pH electroplating wastewater using monovalent selective electrodialysis membranes. With low pH of the raw water (pH 2.2) in the first stage, chromate was presented as HCrO(4)(-) and monovalent ions (HCrO(4)(-), NH(2)SO(3)(-), Na(+) and Cl(-)) were able to pass through the membrane thus chromate was concentrated up to 191%. Higher current density, flowrate and more membrane area all increased the chromium recovery. When pH was adjusted to 8.5 before entering the second stage, the chromate species was presented as divalent CrO(4)(2-) and retained in the concentrated stream, and the rest monovalent ions (NH(2)SO(3)(-), Na(+) and Cl(-)) were separated by passing through the membrane. For example, 45% of the chlorides were separated in this study. The separation efficiencies in the second stage were also increased when the current density, flowrate and membrane area were increased. Electron Spectroscopy for Chemical Analysis was used to examine the surface chromate species for stage 1, and anion exchange membrane showed more chromate fouling comparing to cation exchange membrane due to more adsorption and concentration polarization effects for the anion exchange membrane.

Polyelectrolyte enhanced ultrafiltration (PEUF) for the removal of Cd(II): effects of organic ligands and solution pH.

This study focused on the effects of pH and organic ligands, namely ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and citric acids, on the removal and recovery of Cd(II) in polyelectrolyte enhanced ultrafiltration (PEUF). Polyethylenimine (PEI), which can bind with both positively charged metal ions by coordination bonding and negatively charged ligand-metal complexes by charge attraction, was employed as a chelating polymer. The removal and recovery of Cd species was greatly dependent on the chemistry of organic ligands according to solution pH, particularly being related to the distribution of Cd-ligand complexes at different pH levels. In the presence of EDTA, the dominant Cd species are negatively charged Cd(EDTA)(2-) and CdH(EDTA)(-) over the range of pH levels investigated, interacting with PEI via electrostatic attraction and being less pH dependent. On the other hand, the pH effects of both NTA and citric acid systems are similar to that for the system without organic ligands. This was associated with the fact that free Cd ions were predominant at the acidic pH range in both NTA and citric acid systems.

A comprehensive review: electrospinning technique for fabrication and surface modification of membranes for water treatment application

In this world of nanotechnology, nanofibrous structures offer specialized features, such as mechanical strength and a large surface area, which makes them attractive for many applications. Their large surface area to volume ratio also makes them highly efficient. Among all the techniques for generating nanofibers, electrospinning is an emerging and efficient process. Additionally, the electrospinning technique allows a uniform pore size, which is considered to be one of the important characteristics of membranes. Therefore, electrospun nanofibrous membranes have been used in water purification applications. Furthermore, the technique is widely utilized for generating membranes for membrane distillation and nanofiltration processes, for the removal of contaminants. However, in this review paper, more emphasis is given to the optimization of specific parameters and the preparation of polymeric solutions for fabricating specialized nanofibrous non-woven membranes, and surface modification for application in water treatment technology. Other issues, such as technology limitations, research challenges, and future perspectives, are also discussed.

The relationship between TOX formation and spectral changes accompanying chlorination of pre-concentrated or fractionated NOM

The change in the absorbance upon chlorination (the differential absorbance, AA) of natural organic matter (NOM) that has been concentrated, isolated, and/or fractionated from five sources was explored as a possible indicator of the formation of total organic halogen (TOX) in the samples. The results demonstrate that concentration and isolation of NOM using techniques that are currently in widespread use does not significantly alter the TOX-deltaA272 relationship that applies to the unprocessed NOM. However, when such samples are fractionated, the TOX-deltaA272 relationships for the different fractions are not identical. In particular, when the hydrophobic and hydrophilic neutral fractions of NOM are chlorinated, the amount of TOX formed per unit of A272 destroyed is significantly larger than the corresponding value for other NOM fractions. This observation might reflect the relatively high content of proteins and other amino acid structures that can be disinfection by-product precursors but that absorb little or no light at 272 nm.

Color, Dye and DOC Removal, and Acid Generation During Fenton Oxidation of Dyes

The removal of color, dye and dissolved organic carbon by Fenton discoloration was investigated using the synthetic dye wastewaters containing various dyes (reactive blue 19, Eriochrome Black T or Fast Green FCF). The results indicated that discoloration of dyes was very rapid but mineralization of dyes was insignificant based on the removal of dissolved organic carbon. The rates of color, dye and dissolved organic carbon removal were in the order of reactive blue 19>Fast Green FCF>Eriochrome Black T. The generation of SO2− 4 and NO- 3 increased with the progress of the Fenton reaction. The concentrations of SO2− 4 and NO− 3 generated are in the order of reactive blue 19>Fast Green FCF> Eriochrome Black T. A mathematic model was proposed to formulate the formation of SO2− 4 and NO− 3 during dye degradation. Results indicated that one S-containing and two N-containing functional groups are involved in the oxidation reaction, and that S-containing groups are involved in the oxidation reaction earlier than N-containing functional groups.

Studies of metal-binding sites in natural organic matter and their role in the generation of disinfection by-products using lanthanide ion probes

In this study, the complexation of Tb3+ with natural organic matter (NOM) was studied by the method of time-resolved fluorescence spectroscopy. In the presence of NOM, the excitation of Tb3+ was observed in a wide range of wavelengths, for which virtually no excitation of free Tb3+ took place. The pseudo-quantum yield spectra (excitation intensity normalized by corresponding light absorbance values) had a maximum at 282 nm. This indicated that the excitation of NOM-bound Tb3+ proceeds through energy transfer from aromatic groups in NOM. The concentration of the metal-binding sites (C(L)) was determined by titration with Tb3+ and was found to range from 0.21% to 0.83% of total moles of organic carbon. The actual number of the carbon atoms that comprise these functionalities was hypothesized to be at least seven times higher. The C(L) values were well correlated with the reactivity of NOM with chlorine quantified by total organic halogen formation potential and with the contribution of polyhydroxyaromatic moieties determined by pyrolysis-GC/MS method. The correlation of C(L) with the contributions of aromatic and carboxylic moieties in NOM determined by 13C NMR was poor. Based on the data, it was concluded that the metal binding functionalities in NOM are closely associated with halogen attack sites.

Ni removal from aqueous solutions by chemical reduction: Impact of pH and pe in the presence of citrate

The chemical precipitation of Ni ions from industrial wastewater at alkaline pH values creates waste chemical sludge (e.g., Ni(OH)2). We herein focused on Ni removal via chemical reduction using dithionite, by converting Ni(II) to its elemental or other valuable forms. Without the presence of a chelator (e.g., citrate), the nickel reduction efficiency increased with increasing dithionite:Ni molar ratio, reaching ∼99% at ratios above 3:1. The effect of pH on Ni reduction was in agreement with the standard redox potentials (pe0) of dithionite, which became more negative with an increase in pH leading to greater Ni reduction efficiencies. With the formation of Ni-citrate chelates, however, the Ni reduction deteriorated. Elevated pH and temperature improved nickel reduction, due to the greater reducing power of dithionite. The optimal pH value for Ni(II) reduction was found to be ∼8. Injecting Cu seed particles enhanced the rate and amount of Ni reduced. NiS and Ni3S2 were identified in the crystal of the resulting solids by X-ray crystallography, and the presence of elemental Ni was explained by X-ray photoelectron spectroscopy. The chemical reduction of actual printed circuit board wastewater with the dithionite:Ni(II) molar ratio dose of 12:1 retrieved ∼99% nickel after 30-min reaction at 40°C.

Application of forward osmosis (FO) under ultrasonication on sludge thickening of waste activated sludge

Forward osmosis (FO) is an emerging process for dewatering solid-liquid stream which has the potential to be innovative and sustainable. However, the applications have still been hindered by low water flux and membrane fouling when activated sludge is used as the feed solution due to bound water from microbial cells. Hence, a novel strategy was designed to increase sludge thickening and reduce membrane fouling in the FO process under ultrasonic condition. The results from the ultrasound/FO hybrid system showed that the sludge concentration reached up to 20,400 and 28,400 mg/L from initial sludge concentrations of 3000 and 8000 mg/L with frequency of 40 kHz after 22 hours, while the system without ultrasound had to spend 26 hours to achieve the same sludge concentration. This identifies that the presence of ultrasound strongly affected sludge structure as well as sludge thickening of the FO process. Furthermore, the ultrasound/FO hybrid system could achieve NH4+-N removal efficiency of 96%, PO4(3-)-P of 98% and dissolved organic carbon (DOC) of 99%. The overall performance demonstrates that the proposed ultrasound/FO system using seawater as a draw solution is promising for sludge thickening application.