Tasuku Kamei

Effect of pH on the removal of arsenic and antimony using reverse osmosis membranes

Abstract Increasing attention has been focused on the health effects associated with ingestion of low levels of arsenic and antimony in drinking water. Accordingly, this study was conducted in order to identify the effect of solution pH on the removal efficiency of arsenic and antimony for drinking water using recently developed RO membranes. In this study it was found that the removals of As(V) and Sb(V) are much higher than those of As(III) and Sb(III) over all investigated pH levels (pH 3–10). The removal of arsenic compounds was strongly affected by the solution pH, especially As(III), whereas the removal of antimony compounds shows no dependence on the solution pH since the oxidation state of antimony changes from Sb(III) to Sb(V) within very short periods of time. It was demonstrated that pH control for the membranefeed water is more essential for the successful removal of arsenic compounds than the case of antimony compounds. Consequently, it is assumed that the removal of antimony in drinking water by RO membranes has a higher efficiency than that of arsenic compounds, regardless of pH changes.

Performance of nanofiltration for arsenic removal.

Performance of rapid sand filtration inter-chlorination system was compared with nanofiltration (NF) to reduce the arsenic health risk of drinking water. It was found that rapid sand filtration with inter-chlorination is not effective in removing arsenic. If total arsenic concentration in raw water is below 50 microg/L regardless of the turbidity of raw water, arsenic can be removed below WHO guideline value of 10 microg/L by conventional coagulation (polyaluminum chloride dosage is about 1.5 mg Al/L). However, if the raw water arsenic concentration exceeds 50 microg/L, more coagulant dosage or enhanced coagulation is needed. To adopt optimum coagulant dosage for arsenic removal, it needs to monitor raw water arsenic concentration, but it is difficult because arsenic measurement is time consuming. In addition, if raw water contains As(III), it is difficult for rapid sand filtration inter-chlorination system to meet an arsenic maximum contaminant level of 2 microg/L, which would achieve reduction of cancer risk below 10(-4). On the other hand, the NF membrane (NaCl rejection 99.6%) could remove over 95% of As(V) under relatively low-applied pressure (< 1.1 MPa). Furthermore, more than 75% of As(III) could be removed using this membrane without any chemical additives, while trivalent arsenic could not be removed by rapid sand filtration system without pre-oxidation of As(III) to As(V). Because both As(V) and As(III) removals by NF membranes were not affected by source water composition, it is suggested that NF membrane can be used in any types of waters.

Comparing polyaluminum chloride and ferric chloride for antimony removal

Antimony has been one of the contaminants required to be regulated, however, only limited information has been collected to date regarding antimony removal by polyaluminium chloride (PACl) and ferric chloride (FC). Accordingly, the possible use of coagulation by PACl or FC for antimony removal was investigated. Jar tests were used to determine the effects of solution pH, coagulant dosage, and pre-chlorination on the removal of various antimony species. Although high-efficiency antimony removal by aluminum coagulation has been expected because antimony is similar to arsenic in that both antimony and arsenic are a kind of metalloid in group V of the periodic chart, this study indicated: (1) removal density (arsenic or antimony removed per mg coagulant) for antimony by PACl was about one forty-fifth as low as observed for As(V); (2) although the removal of both Sb(III) and Sb(V) by coagulation with FC was much higher than that of PACl, a high coagulant dose of 10.5mg of FeL(-1) at optimal pH of 5.0 was still not sufficient to meet the standard antimony level of 2 microg as SbL(-1) for drinking water when around 6 microg as SbL(-1) were initially present. Consequently, investigation of a more appropriate treatment process is necessary to develop economical Sb reduction; (3) although previous studies concluded that As(V) is more effectively removed than As(III), this study showed that the removal of Sb(III) by coagulation with FC was much more pronounced than that of Sb(V); (4) oxidation of Sb(III) with chlorine decreased the ability of FC to remove antimony. Accordingly, natural water containing Sb(III) under anoxic condition should be coagulated without pre-oxidation.

Removal efficiency and homologue patterns of dioxins in drinking water treatment.

Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and coplanar polychlorinated biphenyls (Co-PCBs) analyses in raw and treated water throughout Japan were implemented to identify the concentration and homologue patterns of dioxins before and after the water treatment process. In 40 surface water and 5 ground water treatment plants, the removal efficiency of dioxins and the influence of extent chlorination on dioxins increase in drinking water were also studied. Raw water and treated water were sampled twice, summer and winter. The mean concentration in raw water and treated water of dioxins was 56.45 pg/L (0.15 pg WHO-TEQ/L) and 4.24 pg/L (0.019 pg WHO-TEQ/L), respectively. Location of water treatment plants not only significantly influenced the concentration level of dioxins but also resulted in different homologue patterns of dioxins. Levels of dioxins in ground water were much less than that of surface water in both raw and treated water. This study shows most dioxin congeners are well removed (87% removal efficiency) by water treatment. However, in some water treatment plants, the level of TeCDFs (pg WHO-TEQ/L) increased as a result of chlorination.

Rapid and economical indicator for evaluating arsenic removal with minimum aluminum residual during coagulation process

Detection of various types of contaminants in water treatment plant by sophisticated analytical methods such as inductively coupled plasma/mass spectrometry and gas chromatography/mass spectrometry requires hours to days to provide the results. Because naturally occurring ultraviolet (UV) active compounds are commonly present in almost all source waters and can be rapidly monitored by UV absorbance at 260 nm (E260), the extent of correlation between the removal efficiency of E260 and the removal efficiency of As(V) with minimum soluble residual Al by coagulation process was investigated. Percentage removals for E260 were well correlated to those of As(V). When sufficient alum or polyaluminum chloride (PACl) was added for 60-65% removal of E260, 90-95% removal of As(V) was achieved with minimum soluble residual Al regardless of the initial level of turbidity, E260, and As(V). As E260 analysis is precisely available even by an unskilled plant operator in a few minutes, E260 removal efficiency appears to be the promising economical indicator for monitoring the effectiveness of the coagulation process for the removal of contaminants with minimum residual Al.