Xiao-hong Guan

Fluoride adsorption onto granular ferric hydroxide: effects of ionic strength, pH, surface loading, and major co-existing anions.

Fluoride adsorption onto granular ferric hydroxide (GFH) was investigated using batch methods, under various ionic strength, pH, surface loading, and major co-existing anion conditions. Adsorption of fluoride on GFH included an initial fast adsorption phase followed by a slow adsorption phase. Within the pH range of 2-11, fluoride adsorption equilibrium was not affected by ionic strength, but was significantly affected by pH. Maximum adsorption was achieved in the pH range of 3-6.5. Under the same pH condition, fluoride adsorption followed the Freundlich isotherm, indicating that the GFH surface was heterogeneous. X-ray photoelectron spectroscopy (XPS) and attenuated total reflection-infrared (ATR-IR) spectroscopy data showed evidence for fluoride sorption on the GFH surface via inner-sphere complexation accompanying increased hydrogen bonding and surface hydroxylation. Major anions, including phosphate, bicarbonate, sulfate, and chloride, reduced fluoride adsorption in the following order: H(2)PO(4)(-)>HCO(3)(-)>SO(4)(2-)>Cl(-).

ATR-FTIR and XPS study on the structure of complexes formed upon the adsorption of simple organic acids on aluminum hydroxide

Information on the binding of organic ligands to metal (hydr)oxide surfaces is useful for understanding the adsorption behaviour of natural organic matter on metal (hydr)oxide. In this study, benzoate and salicylate were employed as the model organic ligands and aluminum hydroxide as the metal hydroxide. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra revealed that the ligands benzoate and salicylate do coordinate directly with the surface of hydrous aluminum hydroxide, thereby forming inner-sphere surface complexes. It is concluded that when the initial pH is acidic or neutral, monodentate and bridging complexes are to be formed between benzoate and aluminum hydroxide while bridging complexes predominate when the initial pH is alkalic. Monodentate and bridging complexes can be formed at pH 5 while precipitate and bridging complexes are formed at pH 7 when salicylate anions are adsorbed on aluminum hydroxide. The X-ray photoelectron (XP) spectra demonstrated the variation of C 1s binding energy in the salicyate and phenolic groups before and after adsorption. It implied that the benzoate ligands are adsorbed through the complexation between carboxylate moieties and the aluminum hydroxide surface, while both carboxylate group and phenolic group are involved in the complexation reaction when salicylate is adsorbed onto aluminum hydroxide. The information offered by the XPS confirmed the findings obtained with ATR-FTIR.

Adsorption Characteristics of As(V), Se(IV), and V(V) Onto Activated Alumina: Effects of PH, Surface Loading, and Ionic Strength

Arsenic, selenium, and vanadium are major anionic elements of concern in drinking water. This research investigated the adsorption characteristics of As(V), Se(IV), and V(V) onto a commercial activated alumina (AA) under different pH, surface loading, and ionic strength conditions using batch systems. The results indicated that the adsorption of these elements was significantly affected by pH and the surface loading. However, ionic strength generally did not impact their adsorption, indicating that the electrostatic effect on the adsorption of these elements was relatively not important compared to surface chemical reactions. A speciation-based adsorption model was used to simulate the adsorption of As(V), Se(IV), and V(V) by activated alumina and to determine the adsorption constants of different element species. This model can satisfactorily predict the adsorption of these elements in a broad pH range from 1.5 to 12 and a wide surface loading range from 1.0 to 50 mg/g activated alumina for different sorbent concentrations, using the same set of adsorption constants.

Adsorption behavior of condensed phosphate on aluminum hydroxide

Sodium pyrophosphate (pyro-P, Na4P2O7), sodium tripolyphosphate (tripoly-P, Na5P3O10), and sodium hexametaphosphate (meta-P, (NaPO3)6) were selected as the model compounds of condensed phosphate to investigate the adsorption behavior of condensed phosphate on aluminum hydroxide. The adsorption was found to be endothermic and divisible into two stages: (1) fast adsorption within 1 h; and (2) slow adsorption between 1 and 24 h. The modified Freundlich model simulated the fast adsorption stage well; the slow adsorption stage was described well by the first-order kinetics. The activation energies of pyro-P, tripoly-P, and meta-P adsorption on aluminum hydroxide were determined to be 20.2, 22.8 and 10.9 kJ/mol P adsorbed, respectively, in the fast adsorption stage and to be 66.3, 53.5 and 72.5 kJ/mol P adsorbed, respectively, in the slow adsorption stage. The adsorption increased the negative charge of the aluminum hydroxide surface. Transmission electron microscopy and energy dispersive X-ray analysis analyses provided evidence that the adsorption was not uniform on the surface and that the small crystals contributed more to the fast adsorption than the normal sites did. The results from X-ray fluorescence spectrometry and X-ray photoelectron spectroscopy tests also revealed the uneven adsorption of condensed phosphate as a function of the penetration depth. More condensed phosphates were adsorbed on the outer surface of aluminum hydroxide than in its inner parts.

Influence of humic acids of different origins on oxidation of phenol and chlorophenols by permanganate.

The influences of humic acids (HAs) of different origins, including two commercial HAs, three soil HAs and one aquatic HA, on phenols oxidation by permanganate were studied. The apparent second-order rate constants of 2-chlorophenol (2-CP)/phenol oxidation by permanganate in the presence of HAs at pH 7 followed the order of commercial HA (Shanghai)>soil HAs>commercial HA (Fluka)>aquatic HA. Moreover, the commercial HA (Shanghai) could accelerate the oxidation of different chlorophenols (CP) significantly under neutral condition. The FTIR analysis demonstrated greater content of CC moieties and less amount of carboxylate, aliphatic groups and polysaccharide-like substances in soil HAs than in aqueous HA, suggesting that the increase of aromaticity in HA was beneficial to the oxidation of phenols by permanganate. The apparent second-order rate constants of 2-CP/phenol oxidation by permanganate in the presence of HAs correlated well with specific visible absorption (SVA) at 665 nm of HAs. High positive correlation coefficients (R(2)>0.75) implied that pi-electrons of HA strongly influenced the reactivity of 2-CP/phenol towards permanganate oxidation, which agreed well with positive correlation between Fluorescence Regional Integration (FRI) and the apparent second-order rate constants. The pi-pi interaction between HAs and phenols, the steric hindrance effect and the dissociation of phenols may affect the oxidation of phenols by permanganate in the presence of HA at pH=7.0.

Quantifying effects of pH and surface loading on arsenic adsorption on NanoActive alumina using a speciation-based model.

Arsenic (As) poses a significant water quality problem and challenge for the environmental engineers and scientists throughout the world. Batch tests were carried out in this study to investigate the adsorption of As(V) on NanoActive alumina. The arsenate adsorption envelopes on NanoActive alumina exhibited broad adsorption maxima when the initial As(V) loading was less than a 50 mg g(-1) sorbent. As the initial As(V) loading increased to 50 mg g(-1) sorbent, a distinct adsorption maximum was observed at pH 3.2-4.6. FTIR spectra revealed that only monodentate complexes were formed upon the adsorption of arsenate on NanoActive alumina over the entire pH range and arsenic loading conditions examined in this study. A speciation-based adsorption model was developed to describe arsenate adsorption on NanoActive alumina and it could simulate arsenate adsorption very well in a broad pH range of 1-10, and a wide arsenic loading range of 0.5-50 mg g(-1) adsorbent. Only four adjustable parameters, including three adsorption constants, were included in this model. This model offers a substantial improvement over existing models in accuracy and simplification in quantifying pH and surface loading effects on arsenic adsorption.

Simultaneous removal of chromium and arsenate from contaminated groundwater by ferrous sulfate: batch uptake behavior.

Chromium and/or arsenate removal by Fe(II) as a function of pH, Fe(II) dosage and initial Cr(VI)/As(V) ratio were examined in batch tests. The presence of arsenate reduced the removal efficiency of chromium by Fe(II), while the presence of chromate significantly increased the removal efficiency of arsenate by Fe(II) at pH 6-8. In the absence of arsenate, chromium removal by Fe(II) increased to a maximum with increasing pH from 4 to 7 and then decreased with a further increase in pH. The increment in Fe(II) dosage resulted in an improvement in chromium removal and the improvement was more remarkable under alkaline conditions than that under acidic conditions. Chromium removal by Fe(II) was reduced to a larger extent under neutral and alkaline conditions than that under acidic conditions due to the presence of 10 micromol/L arsenate. The presence of 20 micromol/L arsenate slightly improved chromium removal by Fe(II) at pH 3.9-5.8, but had detrimental effects at pH 6.7-9.8. Arsenate removal was improved significantly at pH 4-9 due to the presence of 10 micromol/L chromate at Fe(II) dosages of 20-60 micromol/L. Elevating the chromate concentration from 10 to 20 micromol/L resulted in a further improvement in arsenate removal at pH 4.0-4.6 when Fe(II) was dosed at 30-60 micromol/L.

Predicting competitive adsorption behavior of major toxic anionic elements onto activated alumina: A speciation-based approach

Toxic anionic elements such as arsenic, selenium, and vanadium often co-exist in groundwater. These elements may impact each other when adsorption methods are used to remove them. In this study, we investigated the competitive adsorption behavior of As(V), Se(IV), and V(V) onto activated alumina under different pH and surface loading conditions. Results indicated that these anionic elements interfered with each other during adsorption. A speciation-based model was developed to quantify the competitive adsorption behavior of these elements. This model could predict the adsorption data well over the pH range of 1.5-12 for various surface loading conditions, using the same set of adsorption constants obtained from single-sorbate systems. This model has great implications in accurately predicting the field capacity of activated alumina under various local water quality conditions when multiple competitive anionic elements are present.

Competitive adsorption between orthophosphate and other phosphates on aluminum hydroxide

The competitive adsorption between potassium dihydrogen phosphate (ortho-P) and myo-inositol hexaphosphate (IP6) or tripolyphosphate (tripoly-P) and the adsorption isotherms of different phosphates on amorphous aluminum hydroxide were investigated in this study. The adsorption isotherms of ortho-P, IP6, and tripoly-P fitted well with the Freundlich isotherm model, and IP6 had greater affinity for the aluminum hydroxide surface than ortho-P and tripoly-P. The addition sequence had a pronounced influence on the adsorption of the phosphates on aluminum hydroxide over the pH range examined in this study. Adding ortho-P after IP6 had little effect on the quantity of IP6 adsorbed on the aluminum hydroxide at pH 4 to 10; however, the adsorption of IP6 decreased by about half when ortho-P was added simultaneously with or before IP6 addition. The adsorption of tripoly-P was significantly reduced due to the presence of ortho-P over the entire pH range, and the maximum reduction in tripoly-P adsorption occurred when ortho-P was introduced before tripoly-P. On the other hand, ortho-P adsorption was mostly unaffected by the introduction of IP6 or tripoly-P when the pH was more than 7, but it exhibited competitive inhibition at more acidic pH levels. Under a specific total phosphorus concentration with various ortho-P/IP6 or ortho-P/tripoly-P ratios, the amount of total phosphorus adsorbed on the aluminum hydroxide depended on the fraction as well as the intrinsic binding affinity of each phosphate species for the surface sites of the adsorbent. The surface charge of the aluminum hydroxide depends more on the other two phosphates than on ortho-P when they coexist.