Separation and Purification Technology (v.50, #3)

Editorial Board (pp. co2).

Adsorption of phosphate from aqueous solution onto modified palygorskites by Hengpeng Ye; Fanzhong Chen; Yanqing Sheng; Guoying Sheng; Jiamo Fu (pp. 283-290).

In this study, the natural and modified palygorskites were tested to remove phosphate ions from aqueous solution. The modified palygorskites were prepared by being activated with hydrochloric acid and/or thermal treatments. The surface structure of the materials was investigated by means of X-ray diffraction (XRD), a N₂ adsorption–desorption technique and Fourier transform-infrared (FT-IR) to understand the effect of surface properties on the adsorption behavior of phosphate. In the adsorption test, the adsorption isotherms, kinetics, pH effect, desorbability and selectivity were examined. The results showed that the modified palygorskites had faster kinetics and higher adsorption capacities than the natural palygorskite, which can be attributed to the surface structural changes of the materials, and the adsorbed amounts of phosphate tend to decrease with the increase of pH for all samples. The desorbability of P is about 10–13%, and it is relatively difficult for the adsorbed PO₄³⁻ to be desorbed. Palygorskite could selectively adsorbed phosphate in complex solutions, and the selectivity of phosphate adsorption onto palygorskites was 1000–3000 times that of chloride. The relatively low cost and high capabilities of the natural and modified palygorskites make them potentially attractive adsorbents for the removal of phosphate from aqueous solution.

Keywords: Palygorskite; Surface structure; Phosphate adsorption; Adsorption capacity

Treatment of tanning effluent using nanofiltration followed by reverse osmosis by Chandan Das; Piyush Patel; Sirshendu De; Sunando DasGupta (pp. 291-299).

An investigation on the recovery of chromium from the effluent of a chrome-tanning bath has been performed using nanofiltration (NF) followed by reverse osmosis (RO). The experiments are conducted using a rectangular cross flow cell under laminar and turbulent regimes. Significant flux enhancement is achieved using thin wires as turbulent promoters. The performance criteria are evaluated in terms of the concentration of chromium, COD, BOD, TDS, TS, pH, and conductivity of the permeate. The effects of different operating parameters on permeate flux and observed retention of chromium are evaluated experimentally. The retention of chromium is found to be 91–98% for NF and 98.8–99.7% for RO for the experimental conditions of this study. Concentrations of chromium and COD of the final permeate are well within the permissible limits.

Keywords: Chromium recovery; Nanofiltration; Reverse osmosis; Retention; Turbulent promoter

Characterization of composite nanofiltration membrane using two-parameters model of Extended Nernst–Planck Equation by A.L. Ahmad; B.S. Ooi (pp. 300-309).

The characteristics of polyamide membranes with respect to interfacial polymerization of diamine mixtures with trimesoyl chloride (TMC) are studied using two-parameter model of Extended Nernst–Planck Equation. The investigation provided the information about the effect of TMC content and reaction time on the diffusive and convective flow of ions through the membrane. These indirectly reflected structural properties such as effective skin thickness, pore size and structural integrity of membrane. Membrane flux and rejection are related to the TMC content and reaction time, when NaCl and CuSO₄ are used as testing solutes. The diffusive transport,−f′1ΔC1 and convective transport,JvC1,0(1−R′1) contributions are successfully determined by fitting the model to the experimental data to getf′1 andR′1 parameters. It was found that at high TMC content the contribution of convective transport over diffusion transport is increased due to the increase of effective thickness. However, for smaller size and higher diffusive solute like Na⁺, the ratio of diffusive flow over convective flow is increased at high TMC and high reaction time. This indicated that numbers of tightened pores membrane are increased. An optimum membrane with high flux and high copper ion rejection could be obtained by incorporating 0.1% (w/v) of TMC in the polymerization reaction mixture under reaction time period of 5s.

Keywords: Extended Nernst–Planck; Diffusive transport; Convective transport; Nanofiltration; Structural parameters

Removal of fluoride from drinking water by adsorption onto alum-impregnated activated alumina by Sushree Swarupa Tripathy; Jean-Luc Bersillon; Krishna Gopal (pp. 310-317).

The ability of the alum-impregnated activated alumina (AIAA) for removal of fluoride from water through adsorption has been investigated in the present study. All the experiments are carried out by batch mode. The effect of various parameters viz. contact time, pH effect (pH 2–8), adsorbent dose (0.5–16g/l), initial fluoride concentration (1–35mg/l) has been investigated to determine the adsorption capacity of AIAA. The adsorbent dose and isotherm data are correlated to the Bradley equation. The efficacy of AIAA to remove fluoride from water is found to be 99% at pH 6.5, contact time for 3h, dose of 8g/l, when 20mg/l of fluoride is present in 50ml of water. Energy-dispersive analysis of X-ray shows that the uptake of fluoride at the AIAA/water interface is due to only surface precipitation. The desorption study reveals that this adsorbent can be regenerated following a simple base–acid rinsing procedure, however, again impregnation of the regenerated adsorbent (rinsed residue) is needed for further defluoridation process.

Keywords: Fluoride; Impregnation; Activated alumina; Adsorption; EDAX; Desorption

Ultrasonic defouling of reverse osmosis membranes used to treat wastewater effluents by D. Feng; J.S.J. van Deventer; C. Aldrich (pp. 318-323).

On-line ultrasonic cleaning was used to remove fouling from a commercially important polyamide based reverse osmosis membrane during cross-flow filtration of CaSO₄, Fe³⁺ and carboxyl cellulose solutions. In each case, the permeate flux of the membrane increased significantly, with virtually no decrease in rejection in the presence of ultrasonication. Membrane surface characterization via scanning electron microscopy (SEM) confirmed the beneficial effect of ultrasonication on the membrane permeate flux. These studies suggest that ultrasonic defouling may be a very useful approach for the future development of reverse membranes, especially as far as fouling with organic materials is concerned.

Keywords: Defouling; Reverse osmosis membrane; Flux enhancement; Ultrasound

Carbon dioxide removal from air by microalgae cultured in a membrane-photobioreactor by Lihua Cheng; Lin Zhang; Huanlin Chen; Congjie Gao (pp. 324-329).

Elevated CO₂ levels in a closed space or room are of big concerns in many situations. Controlling the CO₂ level within a certain range is one of the most important tasks in a life support system. In this paper, a 10l photobioreactor integrated with a hollow fiber membrane module was constructed to remove CO₂ from air by using the photosynthetic microalga, Chlorella vulgaris. The effects of the inlet CO₂ concentration and the introduction of the membrane module on microalgal CO₂ fixation were investigated. The results showed that the proper inlet CO₂ concentration was determined to be 1.0%, based on the description of the growth characteristics of the microalga, and the gas exchange efficiency was improved greatly when the membrane module was adopted. Compared with an ordinary photobioreactor, not only the retention time of the smaller and more uniform gas bubbles in the new membrane-photobioreactor increased from 2s to more than 20s, but also the dissolved oxygen (DO) dropped by a factor of 30, resulting in the enhancement of the CO₂ fixation rate from 80 to 260mgl⁻¹h⁻¹. When the operating conditions were controlled at cell density of 2.0×10⁷cellsml⁻¹, inlet gas flow rate of 3lmin⁻¹, and light intensity of 157.6 μEm⁻²s⁻¹ at 25–30°C, the 1.0% (v/v) CO₂ in the input aeration gas could be reduced to 0.3% in the discharged gas. Using normal room air (0.04% CO₂) as feed, the CO₂ concentration in the discharged gas could be decreased to the boundary value of 0.015%, indicating that the novel membrane-photobioreactor by intensifying the process of CO₂ conversion and fixation during the microalgal photosynthesis may be a promising solution to CO₂ removal in a closed space or room.

Keywords: CO; ₂; removal; Membrane; Photobioreactor; Microalgae; Chlorella vulgaris

Particle separation performance by use of electrical hydro-cyclone by Hideto Yoshida; Kunihiro Fukui; Wongsarivej Pratarn; Wiwut Tanthapanichakoon (pp. 330-335).

A special electrical hydro-cyclone is developed and tested. In the underflow collection box of the hydro-cyclone, it has a central metal rod electrode and a cylindrical metal wall between which the desired DC electrical potential or no potential is applied. Effect of central rod diameter and length on separation cut size was examined. The aqueous suspensions of silica particles with a median diameter of 754nm were tested using a 20mm-diameter hydro-cyclone without underflow.It was found that the zeta potential of particles increased proportionally with the value of pH. The electrical potential exhibits a stronger effect when the suspension indicates high pH value. The cut size decreases with the increase of initial pH values. This result is due to the increased negative zeta potential under high pH condition and negatively charged particles are easily collected by electrostatic force.The cut size decreases with the increase of electrode diameter. The cut size becomes smallest under high pH, large electrode diameter and long electrode length conditions. For the negatively charged particles, the center electrode should be negative polarity and outer cylindrical wall should be positive. By use of the electrostatic force, the cut size decreases about 9.2% smaller compared to the standard case without electrostatic force.

Keywords: pH; Silica; Electro-cyclone; Cut size; Zeta-potential

Adsorption study of hexavalent chromium using tamarind hull-based adsorbents by A. Verma; S. Chakraborty; J.K. Basu (pp. 336-341).

The adsorption characteristics of hexavalent chromium was studied with an adsorbent developed from waste tamarind hull. Experiments were conducted in batch mode to observe the influence of different parameters such as initial concentration of metal ions, adsorbent dosage, adsorbent particle size, stirrer speed, temperature and pH of the solution. Acidic pH strongly favored the adsorption. With decreasing the pH of the solution from 5.0 to 1.0, the removal of chromium was enhanced from 33% to 99%. The adsorption process was found to follow a pseudo-first-order rate mechanism and the rate constant was evaluated at 30°C. The Freundlich, Redlich–Peterson and the Fritz–Schlunder isotherm fit the equilibrium data satisfactorily. Adsorption of chromium was found to increase with increase in the process temperature. Using an adsorbent dosage of 1.0g/L and an acidic pH (2.0), the equilibrium adsorption capacity of the prepared adsorbent was found to be about 70mg/g at 30°C, which increased to about 81mg/g at 50°C. The entropy change, free energy change and heats of adsorption were determined for the process.

Keywords: Hexavalent chromium; Tamarind hull; Adsorption isotherm; Thermodynamic parameters; pH effect; Rate constant

Separation and purification of two isomorphic steroids by a novel extractive drowning out crystallization by Qiang Nie; Jingkang Wang; Qiuxiang Yin (pp. 342-346).

A novel extraction drowning out crystallization technology was brought forward for separation and purification of two isomorphic steroid pharmaceutical intermediates. In this process, extraction and crystallization are conducted in the same crystallizer simultaneously. The two immiscible (partial miscible) liquid phases were achieved by adding water into the toluene–DMF system. The role played by water is both as an antisolvent for a crystallization and a trigger to generate two immiscible phases for extraction. The advantage of this method is that two relative pure product can be achieved in a crystallization process. The experimental result shows a good prospect of this method for industrial separation of the studied two isomorphic steroids.

Keywords: Extractive drowning out crystallization; Isomorphic compound; Toluene–DMF–water system; Separation; purification

Sorption kinetics of ammonia and ammonium ions on gel and macroporous sulphonic acid cation exchangers by D. Kaušpėdienė; J. Snukiškis (pp. 347-353).

Diffusion of ammonia and ammonium ions in sulphonic acid cation exchangers (gel Purolite SGC 100×10 MBH and macroporous Purolite C 160 MBH) from the solutions, representing the composition of “caustic condensate�? (waste of nitrogen fertilizers production) is affected by pH of initial solution and structure of the matrix of cation exchanger. In gel matrix the effective intraparticle diffusivity ( D_ef) depends greatly on the solution pH because of shrinkage in alkaline and swelling in acidic medium: on decreasing the initial concentration of ammonia from 0.214 to 0.003 and increasing that of ammonium nitrate from 0 to 0.214moll⁻¹ instead, the effect of ion exchange leads to a decrease in pH, resulting in swelling and increase in D_ef from 0.1 to 0.34×10⁻¹⁰ for gel Purolite SGC 100×10 MBH and variation of 0.18–0.11×10⁻¹⁰m²s⁻¹ for macroporous Purolite C 160 MBH (resistant to shrinkage and swelling).In Purolite C 160 MBH both macropore diffusivity (0.07–0.29×10⁻¹⁰m²s⁻¹) and gel (solid phase) diffusivity (0.06–0.19×10⁻¹⁰m²s⁻¹) are higher than micropore diffusivity (0.28–0.56×10⁻¹⁸m²s⁻¹).With respect to the effective intraparticle diffusivity, resistance to nitric acid, used for the regeneration, and high concentration of ammonium nitrate in eluate (up to 110gl⁻¹), Purolite C 160 MBH has been installed for the conversion of ammonia and ammonium ions to ammonium nitrate reusable in the fertilizers production. This allows minimizing the economic loss and preventing the environmental contamination.

Keywords: Cation exchangers; Ammonium recovery; Caustic condensate; Fertilizers production

Application of microfiltration and ultrafiltration processes to cork processing wastewaters and assessment of the membrane fouling by F. Javier Benítez; Juan L. Acero; Ana I. Leal (pp. 354-364).

The filtration of wastewaters generated in the cork industrial process is investigated by using three membranes in tangential filtration laboratory equipment. The three membranes used were two microfiltration membranes with pores sizes of 0.65 and 0.1μm (DUR-0.65 and DUR-0.1 membranes), and a ultrafiltration membrane with a molecular weight cut-off of 300kDa (BIO-300K membrane). The water hydraulic permeability was determined for each membrane (values of 860, 248 and 769Lh⁻¹m⁻²bar⁻¹ were found), and the influence on the permeate flux of the main operating variables, such as transmembrane pressure, feed flow rate, temperature and nature of the membranes, was established. The effectiveness of the different membranes and operating conditions was evaluated by determining the removal obtained for several parameters which measure the global pollutant content of the effluent: COD, absorbance at 254nm, tannic content, color and ellagic acid, which is selected as a major model pollutant among the different organic compounds present in this wastewater. The values of the corresponding retention coefficients depended on the operating conditions, but in all cases were in the sequence: ellagic acid and color>absorbance at 254nm>tannic content>COD. Globally, the higher removals were obtained for the BIO-300K membrane at 20°C, with Q_F=5.3Lh⁻¹ and TMP=1.8bar. Finally, the fouling of the membranes was assessed, and the corresponding mechanism for each membrane was established by fitting the experimental data to various filtration fouling models reported in the literature.

Keywords: Cork processing wastewater; Microfiltration and ultrafiltration membranes; Permeate flux; Retention coefficients; Fouling mechanisms

Electrochemically enhanced adsorption of aniline on activated carbon fibers by Yanhe Han; Xie Quan; Shuo Chen; Huimin Zhao; Chunyue Cui; Yazhi Zhao (pp. 365-372).

For adsorptive separation processes, the adsorption rate and capacity are two important factors affecting the costs. This study describes the anodic polarization of activated carbon fibers (ACFs), which can enhance the adsorption rate and capacity of aniline. The electrosorption kinetics and the affecting factors (bias potential, electrolyte, and pH) of isotherms for aniline on ACFs were investigated. The adsorption/electrosorption of aniline on ACFs follow pseudo-first-order adsorption kinetics, and the adsorption rate improves with increasing bias potential. The electrosorption isotherms, which exhibit a variety of responses depending on bias potential, electrolyte and pH, follow the two classical models of Langmuir and Freundlich. With electrosorption of aniline from aqueous solution, a two-fold enhancement of adsorption capacity is achievable. The initial and saturated ACFs were characterized using scanning electron micrograph (SEM) and Fourier transform infrared spectroscopy (FT-IR). The SEM micrographs show that the surface of ACFs is not oxidized, which is also verified by cyclic voltammetry results. The FT-IR spectroscopy suggests that the interaction between aniline and ACFs is main weak physisorption instead of chemisorption. These experimental results suggest that the electrochemical polarization of ACFs can effectively improve the adsorption rate and capacity of aniline, which may be due to the enhanced affinity between aniline and ACFs instead of the oxidation on the surface of ACFs or in the solution.

Keywords: Activated carbon fibers; Aniline; Electrosorption; Adsorption kinetics; Adsorption isotherms

Reduction of nitrite by sulfamic acid and sodium azide from aqueous solutions treated by gliding arc discharge by Kheira Marouf-Khelifa; Fatiha Abdelmalek; Amine Khelifa; Mohamed Belhadj; Ahmed Addou; Jean-Louis Brisset (pp. 373-379).

Nitrous and nitric acids form in aqueous solutions exposed to a gliding arc discharge burning in humid air. The anions interfere when the concentration of particular solutes such as pollutants must be determined. In particular they falsify the COD measurements and spectral investigations and thus the efficiency of the plasma treatment in pollutant abatement. The nitrite anions must be thus removed, which require specific reagents. The influence of parameters such as solution pH and [reducers]/[NO₂⁻] ratio on the reduction reaction was evaluated. The reduction of nitrite into N₂ either by sulfamic acid or sodium azide is a first-order pH-dependant reaction with regard to nitrite and reducers ( k₁=2.93×10⁻¹m³kmol⁻¹s⁻¹; k₂=6.21×10⁻¹m³kmol⁻¹s⁻¹, respectively). Sodium azide is thus more reactive than sulfamic acid.

Keywords: Nitrite; Kinetics of reduction; Sulfamic acid; Sodium azide; Gliding arc discharge

Feasibility study for reclamation of a secondary treated sewage effluent mainly from industrial sources using a dual membrane process by Jian-Jun Qin; Maung Nyunt Wai; Maung Htun Oo; Kiran Arun Kekre; Harry Seah (pp. 380-387).

A feasibility study for reclamation of a secondary treated sewage effluent mainly from industrial sources (60%) in Singapore has been conducted using a dual membrane UF-RO process. The pilot system had a treatment capacity of 2m³/h. The UF unit and RO unit were operated at 70–80% and 40% water recovery, respectively. Six-month run for the pilot was carried out to study the stability and fouling tendency of membranes.The characteristics of the raw feed indicated that ammonia-N was consistently high at 30–50mg/L. Very high fluctuations in iron (0.3–3.7mg/L), turbidity (1–27.1NTU) and TOC (3.2–56.7mg/L) were observed. Nitrate was low at <0.2mg/L. The results of the study showed that dosage of alum in the UF process significantly reduced organic foulants and phosphate scalants. The polymeric RO membrane could tolerate organics from industrial wastewater and performed >96% salt rejection at the end of the study after 6 months. The study concluded the dual membrane process was capable of reclaiming the sewage effluent mainly from industrial sources for industrial use.

Keywords: Industrial wastewater; Reclamation; Sewage effluent; Ultrafiltration; Reverse osmosis

Kaolinite, montmorillonite, and their modified derivatives as adsorbents for removal of Cu(II) from aqueous solution by Krishna G. Bhattacharyya; Susmita Sen Gupta (pp. 388-397).

Adsorption of metals by clay minerals is a complex process controlled by a number of environmental variables. The present work investigates the removal of Cu(II) ions from an aqueous solution by kaolinite, montmorillonite, and their poly(oxo zirconium) and tetrabutylammonium derivatives. The entry of ZrO and TBA into the layers of both kaolinite and montmorillonite was confirmed by XRD measurement. The specific surface areas of kaolinite, ZrO-kaolinite, TBA-kaolinite, montmorillonite, ZrO-montmorillonite, TBA-montmorillonite were 3.8, 13.4, 14.0, 19.8, 35.8 and 42.2m²/g, respectively. The cation exchange capacity (CEC) was measured as 11.3, 10.2, 3.9, 153.0, 73.2 and 47.6meq/100g for kaolinite, ZrO-kaolinite, TBA-kaolinite, montmorillonite, ZrO-montmorillonite, TBA-montmorillonite, respectively. Adsorption increased with pH till Cu(II) ions became insoluble in alkaline medium. The kinetics of the interactions suggests that the interactions could be best represented by a mechanism based on second order kinetics ( k₂=7.7×10⁻² to 15.4×10⁻²gmg⁻¹min⁻¹). The adsorption followed Langmuir isotherm model with monolayer adsorption capacity of 3.0–28.8mgg⁻¹. The process was endothermic with Δ H in the range 29.2–50.7kJmol⁻¹ accompanied by increase in entropy and decrease in Gibbs energy. The results have shown that kaolinite, montmorillonite and their poly(oxo zirconium) and tetrabutyl-ammonium derivatives could be used as adsorbents for separation of Cu(II) from aqueous solution.

Keywords: Adsorption isotherm; Kinetics; Cu(II); Kaolinite; Montmorillonite; Poly(oxo zirconium) clay; Tetrabutyl-ammonium clay

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Separation and Purification Technology (v.50, #3)