Retention of pesticide Endosulfan by nanofiltration: Influence of organic matter–pesticide complexation and solute–membrane interactions

Abstract

Nanofiltration (NF) is a well-established process used in drinking water production to effectively remove Natural Organic Matter (NOM) and organic micropollutants. The presence of NOM has been shown to have contrasting results on micropollutant retention by NF membranes and removal mechanisms are to date poorly understood. The permeate water quality can therefore vary during operation and its decrease would be an undesired outcome for potable water treatment. It is hence important to establish the mechanisms involved in the removal of organic micropollutants by NF membranes in the presence of NOM. In this study, the retention mechanisms of pesticide Endosulfan (ES) in the presence of humic acids (HA) by two NF membranes, TFC-SR2 and TFC-SR3, a "loose" and a "tight" membrane, respectively, were elucidated. The results showed that two mechanisms were involved: (1) the formation of ES-HA complexes (solute-solute interactions), determined from solid-phase micro-extraction (SPME), increased ES retention, and (2) the interactions between HA and the membrane (solute-membrane interactions) increased membrane molecular weight cut-off (MWCO) and decreased ES retention. HA concentration, pH, and the ratio between micropollutant molecular weight (MW) and membrane MWCO were shown to influence ES retention mechanisms. In the absence of HA-membrane interactions at pH 4, an increase of HA concentration increased ES retention from 60% to 80% for the TFC-SR2 and from 80% to 95% for the TFC-SR3 due to ES-HA complex formation. At pH 8, interactions between HA and the loose TFC-SR2 increased the membrane MWCO from 460 to 496 g/mol and ES retention decreased from 55% to 30%, as HA-membrane interactions were the dominant mechanism for ES retention. In contrast, for the "tight" TFC-SR3 membrane the increase in the MWCO (from 165 to 179 g/mol), was not sufficient to decrease ES retention which was dominated by ES-HA interactions. Quantification of the contribution of both solute-solute interactions and solute-membrane interactions is hence fundamental in understanding the removal mechanisms of micropollutant by NF membranes in the presence of NOM in order to optimize the treatment process.

IRON AND MANGANESE REMOVAL BY NANOFILTRATION AND ULTRAFILTRATION MEMBRANES: INFLUENCE OF pH ADJUSTMENT

“…3(a) and 3(b), respectively. Similar findings for Fe removal has previously been reported by Jusoh et al [7] and De Munari et al [19]. Regarding to Mn 2+ ions, De Munari and Schafer [14] reported that PA-NF membrane has achieved more than 95% of rejection at pH 7.…”

“…Lower rejection of Mn could be attributed solely to the electrostactic effect interaction between membrane material and Mn 2+ ions [18]. The isoelectric point (IEP) of PA-NF is in the range of 6 to 8 as reported by De Munari et al [19]. This membrane is characterized as an amphoteric membrane whereby it is positively charged in solution less than pH 6 and negatively charged in solution more than pH 8.…”

Triclosan removal by NF from a real drinking water source – Effect of natural organic matter

“…When HA is added, retention of TCS is influenced by its association with HA retained on the surface of membrane and this increases retention [15]. De Munaria et al [12] indicated that the formation of solute-HA complexes and an increase in MWCO of NF membrane are the mechanisms involved. The formation of solute-HA complexes cause an increase in solute retention, while the interactions between HA and the membrane causes a decrease in solute retention [12].…”

“…Nanofiltration (NF) has been considered as an effective and feasible technique for the production of high quality water from various water sources containing micropollutants [5]. However, there are contrasting results regarding the removals of micropollutants by NF [6][7][8][9][10][11][12]. Some researchers have attributed the contradictory results to the different types of micropollutants, membranes and organic matter used and to the complexity of the retention mechanisms [8,[13][14][15].…”

“…Variable effects of NOM on organic micropollutant removal have been reported [7]. Recently, De Munaria et al [12] have contributed the contrasting results to both solute-solute and solute-membrane interactions, and hence indicated that these interactions are fundamental in understanding the removal mechanisms of micropollutants by NF in the presence of NOM. Sanches et al [15] have indicated that the rejection of micropollutants may be influenced by electrostatic and hydrophobic interactions, which may also lead to solute sorption onto the surface of membrane.…”