Effect of operating conditions in removal of arsenic from water by nanofiltration membrane

The removal of arsenic from synthetic waters and surface water by nanofiltration (NF) membrane was investigated. In synthetic solutions, arsenic rejection experiments included variation of arsenic retentate concentration, transmembrane pressure, crossflow velocity and temperature. Arsenic rejection increased with arsenic retentate concentration. Arsenic was removed 93-99% from synthetic feed waters containing between 100 and 382 μg/L as V, resulting in permeate arsenic concentrations of about 5 μg/L. Under studied conditions, arsenic rejection was independent of transmembrane pressure, crossflow velocity and temperature. In surface water, the mean rejection of As V was 95% while the rejection of sulfate was 97%. The co-occurrence of dissolved inorganics does not significantly influence arsenic rejection. The mean concentration of As in collected permeated was 8 μg/L. The mean rejection of TDS, total hardness and conductivity were 75, 88 and 75% respectively.     

Nanofiltration Membrane Process for the Removal of Arsenic from Drinking Water

The removal of arsenic from drinking water by nanofiltration membranes was investigated. Experiments were conducted with tap water to which arsenate and arsenite were added. Two types of nanofiltration membranes, i.e., NF‐90 and NF‐200, have been tested. The effect of various operating conditions, e.g., applied pressure, feed concentration, pH and temperature, were also investigated. The pH and arsenic concentration in the feed and the operating temperature are found to be decisive factors in determining the arsenic concentration remaining in the permeate. The level of removal of As(V) was higher than 98 % for both membranes, but that of As(III) was much lower. It can be concluded that by controlling the operating parameters, source water containing As(V) may be recovered as drinking water to EPA maximum contaminant level quality standards, but that water containing As(III) must undergo a pre‐oxidation treatment before passing through the nanofiltration membrane in order to maintain drinking water quality.     

Removal of DDT in drinking water using nanofiltration process

The removal of DDT[(1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane)] with synthetic waters was carried out on a nanofiltration (NF) pilot unit. The influence of initial DDT concentration, pH, flux and recovery on the removal of DDT was studied. The presence of humic acid and some inorganic (CaCl₂, NaCl, and CaSO₄) matters was also tested in the experiment. The removal percent and that of their adsorption on the membrane have been calculated. The results reveal that DDT was easy to be adsorbed on the membranes and the higher the applied pressure the more rapidly saturation of the membrane was achieved. At the initial concentration of 77 μg/L, the equilibrium for DDT adsorption can be achieved in 30 min. With the initial DDT concentration from 5 to 20 μg/L, the removal percent was from 95 to 85%. On condition that recovery was not changed, higher flux can lead to low rejection of DDT. On the other hand, low recovery can have a high rejection when the fluxes were the same. Humic acid can hinder DDT from passing through the membrane by adsorption and inorganic matter (NaCl, CaCl₂ and CaSO₄) can improve the removal percent by reducing the pore size of the membrane.     

Removal of natural organic matter by ultrafiltration and nanofiltration for drinking water production

Water from Lake Butoniga near the town of Buzet, Croatia, was used as a source for drinking water production. Since lake water has a high concentration of trihalomethane precursors, a treatment was necessary. A process including ozonation, flocculation and filtration was chosen on the basis of preliminary work in a trial pilot plant with a capacity of 10 m³ h⁻¹. Although the chosen process succeeded in producing water that met the demands for drinking water, the efficiency of the removal of natural organic matter was relatively low. Ultrafiltration (UF) and nanofiltration (NF) processes were investigated as alternatives and possible upgrades of the process. Experiments were conducted at pilot plants with the Mavibran SP 006A and Romicon PM 10, PM 50, GM 80 and PM 500 OF membranes as well as with the Filmtec NF 45 NF membrane. Since most of the organic matter in the lake water was smaller than 6-8 kD, the use of the NF process was proposed. To avoid fouling of the NF membrane, we used flocculated and filtrated water from the trial plant as NF feed water. This combination produced water of high quality while process parameters remained stable over the entire period of investigation.     

Fouling characteristics of an ultrafiltration membrane used in drinking water treatment

The fouling characteristics of ultrafiltration membranes used in drinking water were investigated when used alone and when used in an integrated biofilter-membrane system to treat a humic-acid laden solution. Membrane strands from sacrificial modules operating in parallel with bench-scale modules were analysed from both systems (with and without pretreatment). Chemical and microbiological analyses were performed on these strands together with different process streams along the treatment train. Microscopic observations performed on the sacrificial membrane strands revealed that most of the fouling material was organic in nature with high numbers of viable microorganisms. When comparing their fouling characteristics, a positive effect from the biofilter was observed on the performance of the membrane with pretreatment, decreasing in general the amount of material deposited and reducing the fouling rate. Membranes were tested at two different permeate fluxes; this variable did not have an effect on the overall amount of material deposited, but it significantly impacted the membrane fouling rate.     

Rejection of As(III) and As(V) from arsenic contaminated water via electro-cross-flow negatively charged nanofiltration membrane system

A specially designed electro-cross-flow nanofiltration (NF) membrane system was used for this investigation. To enhance the rejection of arsenic ionic species like H₂AsO₄⁻, a NF membrane having a negative surface charge was fabricated via the interfacial polymerization process. The membrane was characterized by SEM, AFM, surface charge density, molecular weight cut-off (MWCO), total and skin thickness and pure water flux. The parameters that affected the rejections of As(III) and As(V) were studied; they included the initial arsenic concentration, the applied potential, pH of the feed, the cross-flow filtration pressure and the presence of different salts in the feed. Among those parameters, the pH of the feed greatly affected As(V) rejection; As(V) ([As(V)]_o = 1000 ppb) rejection was increased from 72.3 to 98.5% when pH of the feed was changed from 3.0 to 10.0. This might be due to the fact that higher pH enhanced the formation of negative divalent anion like HAsO₄²⁻ which should be rejected more effectively by the negative surface charge of the NF membrane. Beside the effect of the negative surface charge of the membrane, applied potential increased the As(V) rejection by 48.2% when the applied potential was increased from 0 to 2.0 V for a feed containing 1000 ppb initially. For the same change of applied potential rejection of As(III) was increased from 52.3 to 70.4%; this might be the result of the formation of anionic species like H₂AsO₃⁻ from the neutral molecule of H₃AsO₃ by the applied potential.     

Separation performance of a nanofiltration membrane influenced by species and concentration of ions

Nanofiltration (NF), which has been largely developed over the past decade, is a promising technology for the treatment of organic and inorganic pollutants in surface and ground waters. The ESNA 1 membrane from the Nitto Denko Corporation of Japan is made of aromatic polyamide, which provides salt rejection from 50% to 90%. In this paper permeation experiments of aqueous solutions of five chlorides (NH₄Cl, NaCl, KCl, MgCl₂ and CaCl₂), three nitrates (NaNO₃, Mg(NO₃)₂ and Ca(NO₃)₂), and three sulfates (NH₄)₂SO₄, Na₂SO₄ and MgSO₄) were carried out. The effects of species and concentration of salts on the separation performance of the ESNA 1 membrane were investigated. The experimental results showed that the rejection to most salts by the ESNA 1 membrane decreased with the growth of the concentration. Then, the reflection coefficient and solute permeability of ESNA 1 membrane were calculated by the Spiegler-Kedem equation from experimental data. The reflection coefficients of the ESNA 1 membrane to salts are all above 0.95. The salt permeabilities, except for magnesium and calcium salts, increased with the growth of concentration. The sequence of rejection to anions by the ESNA 1 membrane is R(SO²⁻₄) > R(Cl⁻) > R(NO⁻₃) at the same concentration which ranges from 10 mol/m³ to 100 mol/m³. The sequence of rejection to anions by the ESNA 1 membrane can be written as follows: R(Na⁺) > R(K⁺) > R(Mg²⁺) > R(Ca²⁺) at 10 mol/m³ concentration and R(Mg²⁺) > R(Ca²⁺) > R(Na⁺) > R(K⁺) at 100 mol/m³ concentration.     

Pilot study of the removal of THMs, HAAs and DOC from drinking water by GAC adsorption

The objective of this pilot study was to evaluate the performance of a GAC postfilter-adsorber for the removal from the drinking water of Athens, Greece, of the two main groups of chlorination by-products, trihalomethanes (THMs) and haloacetic acids (HAAs), as well as of dissolved organic carbon (DOC). The analyses performed during the whole operation period (638 days) showed that the GAC breakthrough capacity for DOC was much higher than the capacity for total HAAs, which was higher than that for total THMs. The removal of THMs and the most part of the removal of HAAs and DOC should be attributed to adsorption by GAC, while that of a smaller part of HAAs and DOC may be attributed to biodegradation in the adsorber bed, where dechlorination, caused catalytically by the carbon surface, favoured microbial growth. Additionally, the GAC postfilter-adsorber showed a much higher adsorption efficiency than a GAC filter-adsorber, due to the smaller size of the carbon and the lower hydraulic loading rate. Also, observed desorption incidents of THMs (mainly) and HAAs, especially during the postfilteradsorber operation, were favoured by the same factors. Formation of THMs within the GAC bed was also indicated by the mass balance of total THMs during the whole cycle.     

Economical evaluation of the fluoride removal by nanofiltration

Economic evaluation was carried out for a plant of fluoride removal by nanofiltration having a capacity of 2400 m³/d (100 m³/h) corresponding to a water consumption for 50.000 capita following the Moroccan considerations in rural medium. The design of this plant was carried out for the predetermined optimized conditions corresponding to a recovery rate of 84%, a fluoride rejection of 97.8% and a pressure pump of 10 bar. The capital cost was estimated to 748,003 € and the calculated operating cost to 0.212 €/m³. These costs were briefly compared to other ones.     

Enhanced fluoride removal from drinking water by magnesia-amended activated alumina granules

This paper describes the fluoride removal potential of a novel sorbent, magnesia-amended activated alumina (MAAA) from drinking water. MAAA, prepared by calcining magnesium hydroxide impregnated alumina at 450 °C has shown high fluoride sorption potential than activated alumina from drinking water. Batch sorption studies were performed as a function of contact time, pH, initial fluoride concentration, and adsorbent dose. Studies were also performed to understand the effect of various other co-existing ions present in real ground water samples. X-ray powder diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDAX) and a gas adsorption porosimetry analyses were used to characterize the physicochemical properties of MAAA. More than 95% removal of fluoride (10 mg l⁻¹) was achieved within 3 h of contact time at neutral pH. Sorption of fluoride onto MAAA was found to be pH dependant and a decrease in sorption was observed at higher pHs. Among the kinetic models tested, pseudo-second-order model fitted the kinetic data well, suggesting the chemisorption mechanism. Among the various isotherm model tested, Sips model predicted the data well. The maximum sorption capacity of fluoride deduced from Sips equation was 10.12 mg g⁻¹. Most of the co-existing ions studied have negligible effect on fluoride sorption by MAAA. However, higher concentrations of bicarbonate and sulfate have reduced the fluoride sorption capacity.     

Removal of 2,4,6‐Trinitrotoluene from “Pink Water” Using Molecularly‐Imprinted Absorbent

Molecularly‐Imprinted Polymers (MIPs) were designed, prepared, and tested for the removal of TNT from “pink water”, which contains 2,4,6‐trinitrotoluene (TNT) and other nitrobenzene compounds (NBCs) produced in army ammunition plants. We have investigated the interactions between TNT template and different monomers using molecular modeling and spectrometric methods, which enables us to optimize the MIP compositions. A suspension polymerization method was utilized to prepare MIP beads with an average diameter of 25 µm, and an imprinting effect of 4.3 was achieved. Furthermore, we have constructed a model absorbance reactor using a flash chromatography system, which was successfully used to monitor the removal of TNT and other NBCs on‐line, and the MIP absorbent was regenerated in‐situ. Compared with granular activated carbon, the capacity of the MIP absorbent is 65‐fold higher. In addition, the MIP absorbent can be regenerated easily, and the capacity only decreases by 7 % after four rounds of regeneration. Besides “pink water”, the MIP absorbent can also be used to remove NBCs in the waste stream from TNT production.     

Removal of arsenic from drinking water by the electrocoagulation using Fe and Al electrodes

A novel technique of electrocoagulation (EC) was attempted in the present investigation to remove arsenic from drinking waters. Experiments were carried out in a batch electrochemical reactor using Al and Fe electrodes with monopolar parallel electrode connection mode to assess their efficiency. The effects of several operating parameters on arsenic removal such as pH (4–9), current density (2.5–7.5 A m⁻²), initial concentration (75–500 μg L⁻¹) and operating time (0–15 min) were examined. Optimum operating conditions were determined as an operating time of 12.5 min and pH 6.5 for Fe electrode (93.5%) and 15 min and pH 7 for Al electrode (95.7%) at 2.5 A m⁻², respectively. Arsenic removal obtained was highest with Al electrodes. Operating costs at the optimum conditions were calculated as 0.020 € m⁻³ for Fe and 0.017 € m⁻³ for Al electrodes. EC was able to bring down aqueous phase arsenic concentration to less than 10 μg L⁻¹ with Fe and Al electrodes. The adsorption of arsenic over electrochemically produced hydroxides and metal oxide complexes was found to follow pseudo second-order adsorption model. Scanning electron microscopy was also used to analyze surface topography of the solid particles at Fe/Al electrodes during the EC process.     

Synthesis of the cotton cellulose based Fe(III)-loaded adsorbent for arsenic(V) removal from drinking water

A novel macroporous bead adsorbent, Fe(III)-loaded ligand exchange cotton cellulose adsorbent [Fe(III)LECCA], is synthesized for selective adsorption of arsenate anions [As(V)] from drinking water in batch and column systems. As(V) adsorption on Fe(III)LECCA was independent of pH, especially in drinking water pH range. Film diffusive control mechanism will benefit As(V) exchange with Fe(III)LECCA whether in batch or in column experiments. When treating the tap water at 26.0 BV/h, the column still preserves 83% of the original saturation adsorption capacity of the As(V) aqueous solution. These results have indicated that Fe(III)LECCA has the potential to act as an adsorbent for the removal of As(V) from drinking water considering its availability, nontoxicity and cost-effectiveness.     

Biofouling of ultra- and nanofiltration membranes fordrinking water treatment characterized by fluorescence in situ hybridization (FISH)

Fouling layer formation in membrane processes and the role of bacteria in it were investigated using rRNA-targetedfluorescently labelled oligonucleotides. In a laboratory-scale flat-channel test unit, nano- and ultrafiltration of an oligotrophic reservoir water were performed. The bacterial composition of the primary fouling layer showed a dominance of the γ-subclass of proteobacteria. The mature fouling layer was dominated by bacteria of the α- and β-subclass, which was similar to the population structure of the raw water. The results indicate that in particular γ-proteobacteria have attachment mechanisms to form the primary biofilm, which is then colonized by other bacteria. It could be shown that bacterial activity was high enough to obtain good fluorescence signals, though staining of active cells by a tetrazolium salt failed. Inorganic precipitation played a minor role in the fouling layer formation, while natural organic matter made up its main part. Scanning electron microscopy revealed both layer and pore fouling for the ultrafiltration membrane due to organic deposition.     

Softening of Hamma drinking water by nanofiltration and by lime in the presence of heavy metals

The paper presents results of a study investigating softening by nanofiltration of the Hamma underground waterresource. The Hamma wells supply the City of Constantine with hard drinking water containing calcium bicarbonate. The main parameters investigated were pressure, cross-flow velocity and water temperature. Nanofiltration was found to be a very efficient process for partial reduction of the temporary hardness, within the pressure limits considered, the calcium and bicarbonates retentions reaching 34% and 30%, respectively. Experiments were also conducted to modify the furring capacity of the water by doping the Hamma water with metal cations. Addition of Zn²⁺ and Cu²⁺ induced some inhibition, but no effect was observed in the cases of Fe²⁺ and Mn²⁺.     

Salt‐Controlled Selectivity in “on Water” and “in Water” Passerini‐Type Multicomponent Reactions

We have demonstrated that simple sodium salts can completely reverse the product ratios of the Passerini reaction in aqueous media. Furthermore, the use of the “salting‐in” salt and a small excess of the nucleophile gives significantly higher yields than the use of the saturated solution of the nucleophile alone.     

饮用水除砷技术研究新进展

重点介绍了近年来各种除砷新技术的研究进展,内容包括:强化混凝、吸附、离子交换、膜法、预氧化除砷技术。评价了各除砷技术的优缺点与适用范围,并认为饮用水除砷技术的发展将呈现出以下特点:As(Ⅲ)和As(V)同步去除技术的开发;易于取得或制备、生物化学稳定性高、吸附容量大、选择性高、再生能力强的新型除砷吸附剂的开发;多种除砷技术联合,多重去除机理协同的除砷流程的开发;低能耗、低成本除砷技术的开发;生物除砷技术的开发。     

DK2540型工业纳滤膜处理微蚀废液的研究

为了循环利用深圳某电路板厂的微蚀刻废液,首先研究了利用铜粉还原去除废液中的氧化性物质,然后通过用DK2540型工业高脱盐纳滤膜处理,浓缩废液中的铜,回收其中的酸.实验结果为在室温下,桨搅拌速率为300 r/min,投料比n（Cu）∶n（Na2S2O8）=1.2,反应时间70 min时,对过硫酸钠有最高去除率89.37％.在压力4 MPa,进料流量770 L/h时,Cu2＋有最高截留率90.22％,H＋有最高透过率75.68％.对于ρ（铜）=1.55 g/L的废液,可以浓缩ρ（铜）=20 g/L,原液体积减少95％以上.实验表明利用膜法可以实现微蚀废液的循环利用.     

Arsenic removal from drinking water by electrocoagulation using iron electrodes

Arsenic removal from drinking water was investigated using electrocoagulation (EC) followed by filtration. A sand filter was used to remove flocs generated in the EC process. Experiments were performed in a batch electrochemical reactor using iron electrodes with monopolar parallel electrode connection mode to assess their efficiency. The effects of several operating parameters on arsenic removal such as current density (1.5–9.0 mA cm^?2), initial arsenic concentration (50–500 μg L^?1), operating time (0–15 min), electrode surface area (266–665 cm²), and sodium chloride concentrations (0.01 and 0.02M) were examined. The EC process was able to decrease the residual arsenic concentration to less than 10 μg L^?1. Optimum operating conditions were determined as an operating time of 5 min and current density of 4.5 mA cm^?2 at pH of 7. The optimum electrode surface area for arsenic removal was found to be 266 cm² taking into consideration cost effectiveness. The residual iron concentration increased with increasing residence time, and maximum residual iron value was measured as 287 μg L^?1 for electrode surface area of 266 cm². The addition of sodium chloride had no significant effect on residual arsenic concentration, but an increase in current density was observed.