Desalination 250 (2010) 553–556

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Desalination

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Removal of DDT in drinking water using nanofiltration process☆

Weihai Pang, Naiyun Gao, Shengji Xia ⁎State Key Laboratory of Pollution Control and Resource Reuse, Tongii University, Shanghai, 200092, China

☆ Presented at the Conference on Membranes in DProduction, 20–24 October 2008, Toulouse, France⁎ Corresponding author. Tel.: +86 21 65982691.

E-mail address: xiashengji@hotmail.com (S. Xia).

0011-9164/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.desal.2009.09.022

a b s t r a c t

a r t i c l e i n f o

Available online 14 October 2009

Keywords:Drinking waterNanofiltrationPersistent organic pollutants

The removal of DDT[(1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane)] with synthetic waters was carried outon a nanofiltration (NF) pilot unit. The influence of initial DDT concentration, pH, flux and recovery on theremoval 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 themembrane have been calculated. The results reveal that DDT was easy to be adsorbed on the membranes andthe higher the applied pressure the more rapidly saturation of the membrane was achieved. At the initialconcentration of 77 μg/L, the equilibrium for DDT adsorption can be achieved in 30min. With the initial DDTconcentration from 5 to 20 μg/L, the removal percent was from 95 to 85%. On condition that recovery was notchanged, higher flux can lead to low rejection of DDT. On the other hand, low recovery can have a highrejection when the fluxes were the same. Humic acid can hinder DDT from passing through the membraneby adsorption and inorganic matter (NaCl, CaCl2 and CaSO4) can improve the removal percent by reducingthe pore size of the membrane.

rinking and Industrial Water

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

DDT[(1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane)], known asan endocrine disruptor, has been extensively used as an organicchlorine pesticide and was designated by World Health Organization(WHO) as one of the twelve persistent organic pollutants urgentlyneeded to be eliminated. Although the use of some chloroorganicinsecticides in agriculture was banned in most countries in the last20 years of the last century, thousands of tons of obsolete pesticidedeposits in farms and other places have been jeopardizing the envi-ronment [1–3]. High DDT concentration was observed in the air overTaihu Lake, a lake near Shanghai, China, which is the water supplysource for several nearby cities [4]. The level of DDT is also regarded asone of the non-regular indices in the newly issued drinking waterstandard in China. The chemical structure of DDT is shown in Fig. 1.

Studies of the removal of DDT during water treatment have beenlimited due to the low concentration of it in water sources and theassociated difficulties in analysis. Nanofiltration (NF), as a promisingmembrane technology, could be an alternative method for removinglowmolecular weight organic micropollutants [5–7]. Nanofiltration isa relatively recent membrane process used most often with low TDSwaters such as surfacewater and fresh groundwater, with the purposeof softening (polyvalent cation removal) and removal of disinfectionby-product precursors such as natural organic matter and synthetic

organic matter. Nanofiltration features a fractionation capacity fordifferent organic components in aqueous solutions up to a range of300 kg/kmol molecular weight. The aim of this study is to test theefficiency of NF membranes to remove DDT and to investigate theinfluence of the organic matter (humic acid) and the inorganic matter(CaCl2, NaCl, and CaSO4) on the DDT removed and adsorbed by themembranes.

2. Materials and methods

The NF pilot (Fig. 2) used in this study is composed of a plastic feedtank, a separation module and a pump. The tested NF membraneswere from Toray, China. The effective area of the NF membrane is0.5 m2 and the material of it is PVDF.

All the solutions were prepared from ultra pure water produced byreverse osmosis equipment in our laboratory. All the chemicals wereat least the ACS grade, and purchased from Sinopharm ChemicalReagent Co., Ltd. DDT during the experiment was measured by a GasChromatographer (Shimadzu GC-2010) with a HP-5 fused silicacapillary column (30 m×0.32 mm I.D., film thickness of 0.25 μm). Thedetection limit was 0.5 μg/L and the repeatability relative standarddeviation was less than 9%. Residual pollutants in water were ex-tracted with n-hexane and one microlitre of extraction was analyzed.The analyseswere carried out under the following conditions: injectortemperature 250 °C; detector temperature 300 °C; the temperature ofGC oven was 240°C, N2 as carrier gas at a flow rate of 3.0 mL/min.

To study the influence of inorganic matter on the removal ofpesticides, NaCl, CaCI2 and CaSO4 were added to the solutions to reachthe concentrations of 300 and 600 mg/L. To investigate the effect of

Fig. 1. Chemical structure of DDT.

Fig. 3. The equilibrium for DDT adsorption (initial concentration=77.4 μg/L).

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the presence of NOM, DDT solutions containing 5, 10, 15, 20 mg/L ofNOMwere prepared and left to stand for 24 h in order to allow contactbetween the NOM and DDT before filtration. Flux and recoveryinvestigated in the bench scale study for the membrane are presentedin Fig. 6. Flux and recovery were controlled by adjusting valves onpermeate and concentrate flow streams.

3. Results and discussion

3.1. The adsorption of DDT by nanofiltration membrane

DDT solution (6L) was prepared in the plastic tank to test theadsorption of the NFmembrane. At the initial concentration of 77 μg/L,the equilibrium for DDT adsorption can be achieved in 30min as Fig. 3.On condition that no permeate flow was out in the process, it can beseen that DDT concentration decreased significantly, from 77.4 μg/L atthe beginning of filtration to 52.2 μg/L, while the cumulated quantityadsorbed reached the value of 506 μg/m2. The results reveal that DDTwas easy to be adsorbed on the membranes.

The adsorption of pollutant onto the membrane can be physical orchemical in nature or both [5]. The former is a completely reversibleprocess, while the latter can be irreversible for strong chemical bondssuch as polymerization or reversible for weak secondary chemicalbonds such as hydrogen bonding and complexation. During theprocess of membrane filtration for DDT separation, it is possible thatboth chemical (hydrogen bonding) and physical (hydrophobicinteractions) adsorption occurs. When, in a second step, the DDTsolution in the cell is replaced by pure water, at least 2 μg/L DDT canbe observed in permeate water in the following 30min, whichsuggests that DDT–polyamide bonds are not very strong and DDTcould be released from the membrane structure into the permeate.

3.2. Effect of initial DDT concentration

Different concentration of DDT solutions were prepared forfiltration and the concentration was defined as the initial concentra-tion. Before filtration, the membrane had absorbed DDT in solution toequilibrium. During the process of experiment, the feed flowwas keptat 35 L/h and the pressure of it was kept at 0.24 MPa. With the initialDDT concentration ranging from 5 to 20 μg/L, the rejection was from

Fig. 2. Schematic representation of the nanofiltration pilot (1 feed tank, 2 pump,3 separations module, 4 feed, 5 permeate, 6 retenate).

95 to 85% (Fig. 4). The increase of initial DDT concentration hadnegative effect on DDT removal percent.

3.3. Effect of pH

At the initial concentration of 20 μg/L, the rejection of DDT is shownin Fig. 5. It can be seen that there was no much difference betweenthe rejections at different pHs. At highor lowpHvalue there lots of ions(H+ or OH−) adsorbed on the membrane charge. Polar componentshave a lower rejection when the membrane charge increases, becausethey are dipoles which can have a preferential orientation towardsthemembrane. Usually the side of the dipolewith a charge opposite tothe membrane charge is the closest to the membrane. In this way, thepreferential orientation results in an increased attraction, an increasedpermeation and thus a lower rejection [8]. With low molecularpolarity, DDT is hardly ionized and the pKa value of DDT is very high,which lead to the influence of pH on the rejection not obvious.

3.4. Effect of flux

For drinking water production with NF, it is important to achieve ahigh flux as well as high quality water. Flux experiments wereconducted using purified water at different transmembrane pres-sures. Both the rejection of DDT and the recovery of flow are plotted asfunctions of flux for NF membrane in Fig. 6. The higher the flux, thelower the DDT rejection was observed. From Fig. 6, it may be con-cluded that it is difficult to simultaneously achieve a high permeateflux and high DDT rejection. Such phenomenon might be explainedthat DDT molecules had been entrapped in the pores of membrane,

Fig. 4. DDT rejection with different feeding concentrations.

Fig. 5. Effect of pH on the rejection of DDT.

Fig. 6. Effect of flux on the rejection of DDT.

Fig. 8. Effect of inorganic matters on the rejection of DDT.

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higher flux lead to stronger hydrodynamic shear which causeentrapped DDT molecules release from the membrane.

3.5. Effect of organic matters

Before filtration with NF membrane module, DDT solutionscontaining 5, 10, 15, and 20 mg/L of NOM were prepared and left tostand for 24 h. The DDT concentration in the feed water was 33.5 μg/Land the feed flow was set at 45.3 L/h and the permeate flow at 1 L/h.

Fig. 7. Effect of humic acid on the rejection of DDT.

This solution was let to rest for 24 h in order to allow the contactbetween humic acid and DDT. The obtained results are presented inFig. 7. From Fig. 7, it can be seen that the rejection of DDT is becominghigher with more humic acids added in solution. The reason is thathumic acids are hydrophobic substances with a highmolecular weightand are poorly soluble in water. DDT adsorbs easily on such moleculesjust as much by physiosorption (weak links) as by chemisorption(ionic links) and form macromolecules [9].

3.6. Effect of inorganic matters

To investigate the influence of inorganic matter on the removal ofDDT, 500 and 750 ppm of CaCI2 or CaSO4 and 30 μg/L of DDT wereadded to the water. Before filtration, the pHs of the solutions wereadjusted to 7.5. As shown in Fig. 8, there is a slight increase in therejection of the DDT for the higher dose of NaCl, CaCl2 and CaSO4. Theimprovement in the removal percent of the DDT by NF membranemay be caused by the blocking effect of the pores by the ions athigh concentrations [10]. CaCl2 and CaSO4, owing to their divalentnature, are easy to be rejected by NF membrane, which helps theNF membrane to achieve higher rejection when compared with NaCl[11].

4. Conclusion

In this work, NFmembranewas used to remove DDT from drinkingwater. The results indicate that DDT can be effectively removed by NFmembrane from water. The main mechanisms which govern theprocess of elimination of DDT by NF are adsorption on the membraneand repulsion (steric and electrostatic) by themembrane. The increaseof initial DDT concentration had negative effect on DDT removalpercent and the influence of pH on the rejection is not obvious.

In addition, with higher flux the rejection of DDTwill be lower. It isdifficult to simultaneously achieve a high permeate flux and high DDTrejection.

DDT is easily absorbed on humic acid and can be removedtogether. With the blocking effect of the pores by the ions, thepresence of organic matter (humic acid) and inorganic matter (NaCl,CaCl2 and CaSO4) can improve the elimination of DDT.

Acknowledgements

This work was kindly supported by the Foundation of NationalNatural Science Foundation of China, Project 50808134, and byChinese National Eleven Five-Year Scientific and Technical SupportPlans (No. 2006BAJ08B02 and 2006BAJ08B06) and also by The State

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Key Laboratory of Pollution Control and Resource Reuse, China(No. PCRRY08002).

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