Ch. Rav-Acha

Binding of organic solutes to dissolved humic substances and its effects on adsorption and transport in the aquatic environment

Abstract Humic substances constitute a major fraction of dissolved organic matter in natural water and effluents. Their effect on the adsorption of organic contaminants to aquifer material was elucidated, and a model was proposed for the adsorption of organic solutes to aquifer solids in the presence of dissolved humic substances. The model is based on the assumption that organic solute binds to dissolved humic substances in a reversible manner to form a solute-humate complex. Following binding, both free and bound fractions of the organic solute are independently adsorbed onto the solid phase. In order to evaluate the validity of the model, the following parameters were determined: (1) the adsorption coefficient of the organic solute to clay; (2) the binding constant of the solute-humate complex; and (3) the adsorption of humic acid (HA) to clay, assuming that the solute-humate complex is adsorbed similarly to humic acid itself. Using these parameters in the model enabled the effect of dissolved humic substances on adsorption to be evaluated. Experimental results obtained for the adsorption of fluoranthene (a model compound of the PAH group) to clay in the presence of dissolved HA were compared with calculated values derived from the model described above. The sensitivity of the model to various parameters was evaluated and a prediction was made with respect to the effect of dissolved humic substances on the adsorption of a variety of organic solutes. It appears that dissolved humic substances solubilize organic solutes which have higher adsorption coefficients to clay than humic substances, but increase the adsorption of solutes having lower adsorption coefficients relative to humic substances.

Trihalomethane formation in chlorinated drinking water : a kinetic model

Abstract Aquatic humic and fulvic acids are the main trihalomethane (THM) precursors formed during surface water chlorination. The plot of THM concentration against humic substance levels is a “convex”-shape, suggesting that a multi-step reaction occurs. In the first step, organochlorine intermediates are produced, and, in the second step, these are converted to THM. A mechanistic model, based on the reaction of chlorine with model compounds, is used to derive kinetic equations for THM formation as a function of the precursor and chlorine concentrations under controlled pH and temperature conditions. This paper describes the application of this model and the correlation between calculated THM levels and experimentally measured values.

Sorption of organics on clay and synthetic humic-clay complexes simulating aquifer processes

Abstract Sorption/partition of several organic solute (contaminants) of a wide range of hydrophobicities was studied on clay and on clay-humic complexes representing aquifer-soil systems. The role of the mineral and of the organic (humic) fractions was elucidated and a model considering both fractions in the sorption process was proposed. The adsorption constants on humic (organic fraction), K∝, were 8–20 times higher than on “pure” clay, Km. But with soils with low to medium organic fractions ( ƒ ∝ ) the contribution of the clay mineral to adsorption was quite significant, in spite of the fact that half of the sorption sites on the mineral surface were blocked by the humic. In the range of very low organic content in aquifer soil a non-linear pattern going through a minimum is observed between the overall partition coefficient and soil-organic fraction, transforming to the familiar linear relationship at higher ƒ ∝ s . Both the K∝ and Km followed the linear-free energy relationship to the octanol-water partition coefficient Kow.

The reactions of chlorine dioxide with aquatic organic materials and their health effects

Abstract This review describes the main reactions that have been studied in water between chlorine dioxide and various groups of organic compounds. Although the reactions of ClO2 with some model organic compounds have also been summarised, special emphasis has been given to the reaction products that were found in actual water plants treated with ClO2 or under conditions closely related to these. Some conclusions were drawn with respect to the mechanism by which ClO2 reacts with aquatic organic materials. As a result of the proposed reaction mechanisms a prediction was made as to which main organic products will be formed and which health effects may be expected. The last section of the review deals with the impact of ClO2 on some kinds of organic micropollutants, such as pesticides and polycyclic aromatic hydrocarbons (PAH) which are often found in surface water sources and thus may also be found in drinking water.

The effect of chemical oxidants on effluent constituents for drip irrigation

Abstract One of the main problems concerning drip irrigation by effluents is the clogging of drippers and emitters, which, due to their small diameters, are extremely vulnerable to clogging by effluent constituents. Earlier observations revealed that this phenomenon can be reduced if effluents are treated with oxidants. In order to realize the role of oxidants in these processes, and to obtain meaningful and reproducible results, synthetic effluents rather than authentic effluents were used. However, the synthetic effluents were composed so as to provide the best possible resemblance to natural effluents, and the effects of Cl2 and ClO2 on their constituents were studied. The demand of these effluents for Cl2 was 5–8 mg l−1 and for ClO2 3–4 mg l−1. Two mg l−1 of either oxidant caused a very fast bacteria inactivation that reached four orders of magnitude after 1 min. However, with respect to algae, concentrations up to 20 mg l−1 of either oxidant did not affect the number of algae cells, although they caused a remarkable decrease in algal viability as expressed by its chlorophyll content and replication ability. Both oxidants have demonstrated a notable aggregation effect on the effluents. The conclusions of the results described above were examined in a pilot system. Continued chlorination by 5–10 mg l−1 Cl2 applied directly to the drippers was not very effective. The reason for this was the presence of clogging agents, “immune” to low Cl2 concentrations, produced as early as in the reservoir, and carried down to the drippers by the effluent stream. Batch treatment combined with settling was much more efficient, and reduced the clogging significantly, because in this case the Cl2 reacted not only as a disinfectant, but also as a coagulant due to the oxidation of humic constituents.

The action of chlorine dioxide on aquatic organic materials during the disinfection of drinking water

Abstract Measurements of the Total Organic Halogens (TOX) produced in the National Water Carrier (NWC) of Israel after disinfection with chlorine dioxide revealed that compared to chlorine, ClO2 forms only a very small amount of chlororganic compounds. Mixtures of Cl2 and ClO2 that have been reported to reduce the concentrations of both the chlorinated organics as well as the chlorite Ions produced by ClO2, were found not to be very useful because of the fact that high bromide concentrations are present In the NWC. Therefore high ratios of ClO2/Cl2 are needed to reach a significant reduction in Trihalomethanes (THMs). The amount of THMs can however be satisfactorily reduced when ClO2 is introduced a few hours before chlorine. A comparison between Cl2 and ClO2 with regard to their reactions with a series of polyeyelic aromatic hydrocarbons (PAHs) showed that the ClO2 reactions are more unique. While the highly carcinogenic compounds, benz(a)pyrene and 1,2‐benzanthracene react with ClO2 much more rapidly...

The catalytic effect of catiohic amino micelles on the hydrolysis of substituted phenyl esters

Abstract The catalytic effects of two aminocationic micelles on the hydrolysis of substituted phenyldecanoate esters and a positively charched benzoate ester (CPNBA) were determined. The micellaric catalysts were of the general structure [CH3(CH2)3N(CH3)2(CH2)nNH2]Br where n=2 (micelle 1); n=3 (micelle 2). The kinetics followed the expression: kobs =ko+kcat x Ka/(Ka+H+)+koOH[OH-]. From the comparison of the kc OH rates with specific base catalysis rates deduced from reactions in non catalytic micelles, it was concluded that the kc OH term, is compatible mainly with an aminolysis reaction catalyzed by hydorxide ion. The Hammett and Bronsted correlations (p=2.8; β=1.0), in addition to the very small deuterium isotope effect, suggested that kcat corresponded with a nucleophilic mechanism. The Bronsted plot of log kcat vs pKa of the phenolate leaving groups in micelles 1 and 2 showed a biphasic behaviour. The break in the curve occured at pKo=5.89 and pKo=6.78 respectively. The partition ratio k±/k-a of the zwiterionic tetrahedral intermediate was derived from the experimental data and produced the following correlation: log k±/k-a=-0.92pKo+0.43pKN+2.466. The ester CPNBA exhibited a deuterium isotope effect of 2.1. From product analysis it was concluded that the reaction proceeds via a general base catalysis of aminolysis.

The rôle of formation and reduction of THM and chlorite concentrations in the disinfection of water with Cl2 and ClO2

Abstract Water disinfected by chlorine produces trihalomethanes (THM), while chlorine dioxide, an alternative to chlorine, is reduced mainly to undesirable chlorite. Mixtures of ClO 2 and Cl 2 may successfully reduce the formation of both THM and chlorite in the absence of bromide. When Lake Kinneret water in Israel was studied, however, it was found that such mixtures caused only a minor reduction in the formation of THM compared with chlorine alone. This is attributed to the effect of bromide present in the water of Lake Kinneret. The formation of THM and chlorite could, however, be satisfactorily reduced (by about 60% and 90%, respectively), if chlorine dioxide were allowed to react in water with the THM precursors before chlorine was introduced.

Determination of oxidants formed upon the disinfection of drinking water with chlorine dioxide

Abstract The disinfection policy of the National Water Carrier (NWC) of Israel has recently been changed from chlorine to chlorine dioxide. One of the objectives was therefore to determine precisely all the inorganic products of such disinfection. It was found that none of the known analytical methods could satisfactorily solve this problem, determining all the inorganic species which were formed. The determination of all the oxidants became possible, however, when a few analytical methods or their combinations were applied simultaneously. The validity of the methods described in this study was further confirmed by the good balance that was obtained between the ClO2 applied and the total oxidants determined after various time intervals. The chlorite was consistently found to be about 65% of the ClO2 consumed. This was explained based on the polyphenolic nature of the hutnic substances that possess the majority of the organic content in the water of the NWC. The small amounts of chlorine which were determi...

Chlorine-Dioxide as an Electron Transfer Oxidant of Olefins

Kinetic and product studies of chlorine-dioxide oxidation of olefins, ranging in their ionization potentials from 9.64±0.3 to 8.0 eV have shown that at the rate-determing stage, the reaction involved the abstraction of an electron from the olefin to yield the anion ClO2 ∸ with generation of the corresponding cation radical ****. The products emerging from the oxidation in H2O were suggested to arise from an hydrolysis reaction of a postulated cyclic-intermediate.