Halides: Water Pollutants Or Not?

Halides are salts of the halogens, which include chlorine, bromine, iodine, and astatine. They are known to have a significant impact on water pollution, particularly in saline waters. Research has shown that halide ions can affect the degradation of organic contaminants in water through processes such as photolysis and advanced oxidation processes (AOPs). The presence of halides can influence the efficiency of contaminant removal, with varying effects depending on the specific halide and the target organic compound. While halides can participate in contaminant destruction, they can also lead to the formation of halogenated byproducts. Additionally, halides have been identified as water pollutants themselves, with reports of organic halides and heavy metals polluting surface waters.

Halide ions can scavenge hydroxyl radicals, which are used to decontaminate water

Halides are known to be water pollutants. For instance, organic halides and heavy metals were found in water samples, waterborne particulates, and sediments of the Rhine river and its tributaries between 1984 and 1987.

Halide ions (Cl-, Br-, and I-) are present in high quantities in seawater (around 0.54 M Cl-, 0.81 mM Br-, and 0.47 μM I-). These ions can be used to produce reactive species through reactions with other chemicals. For example, halide ions can react with peroxymonosulfate (PMS) to form reactive halogen species HOX (HOCl, HOBr, and HOI).

Halide ions are also known to scavenge hydroxyl radicals (HO*). Advanced oxidation processes (AOPs) generate non-selective hydroxyl radicals (HO*) to decontaminate water. However, halide ions can scavenge these radicals during AOP treatment, reducing their effectiveness in destroying contaminants. This is particularly true for contaminants containing electron-poor reaction centers, where the presence of halides can almost halt the destruction process.

Despite this, the scavenging of HO* by halides can have a positive effect. The HO* radicals are converted into reactive halogen species (RHS) that are more selective in reacting with electron-rich organic compounds. This conversion enhances the efficiency of contaminant removal by focusing the oxidizing power of the system on the target contaminant rather than other organic matter present in the water.

Halides can convert hydroxyl radicals to reactive halogen species, which are selective in their contaminant destruction

Halides are a common presence in natural waters. In seawater, the average halide composition is 540 mM chloride, 0.8 mM bromide, and 100–200 nM iodide. In surface freshwater and groundwater, the levels are lower, with up to 21 mM chloride and 0.05 mM bromide.

Halides can convert hydroxyl radicals to reactive halogen species (RHS). This conversion occurs through sensitized photolysis and reactions with secondary photoproducts. The resulting RHS can participate in a variety of reactions with DOM and anthropogenic compounds, including oxidation and incorporation of halogen.

The conversion of hydroxyl radicals to RHS by halides is a selective process. RHS preferentially targets electron-rich chromophores, such as those found in organic compounds. This selectivity enhances the efficiency of electron-rich contaminant removal in saline waters by focusing the oxidizing power of the system on the target contaminant rather than on other substances.

The impact of halides on contaminant destruction is variable. The presence of halides can suppress the destruction of certain contaminants, particularly those with electron-poor reaction centers. However, for single-target organic contaminants, halides can enhance destruction rates. For example, kinetic modeling of phenol destruction demonstrated that RHS contributed significantly to phenol destruction, mitigating the impact of HO* scavenging.

The role of halides in contaminant destruction is an active area of research, with ongoing studies investigating the complex chemistry of halogens in waters and their implications for contaminant fate and water purification technologies.

Halides can impact the efficiency of contaminant degradation by sulphate and hydroxyl radical-based advanced oxidation processes

Halides are indeed water pollutants. Halide ions, in particular, can impact the efficiency of contaminant degradation by sulphate and hydroxyl radical-based advanced oxidation processes (AOPs). AOPs are a broad-spectrum contaminant destruction option for water decontamination.

Halide ions scavenge hydroxyl radicals (HO*) during AOP treatment, which can reduce the treatment efficiency of certain organic compounds. The impact of this suppression varies depending on the target compound, with destruction of contaminants containing electron-poor reaction centres in seawater nearly halted, while 17beta-estradiol removal declined by only 3%.

HO* scavenging by halides converts HO* to radical reactive halogen species (RHS) that participate in contaminant destruction but react more selectively with electron-rich organic compounds. The presence of anions enhances the efficiency of electron-rich contaminant removal by focusing the oxidizing power of the system on the target contaminant.

Research has also evaluated the treatment of mixtures of contaminants with each other and with natural organic matter (NOM). NOM serves as an oxidant scavenger, and the conversion of non-selective HO* to selective radicals due to the presence of anions enhances the efficiency of contaminant removal in saline waters.

Halides can decrease the photolysis rate of organic contaminants in water

Halides are water pollutants, and their presence in water can decrease the photolysis rate of organic contaminants. Halides can be converted into radical and non-radical reactive halogen species (RHS) through sensitized photolysis and reactions with secondary reactive oxygen species (ROS) produced by sunlight-initiated reactions in water.

RHS generated from halides are reactive toward organic compounds, and some reactions result in the incorporation of halogen into byproducts. The presence of halides affects the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species. They also influence the photobleaching of dissolved natural organic matter (DOM) and alter the rates and products of pollutant transformations.

The specific halide present also makes a difference. For example, the impact of Cl-, Br-, and carbonates on the UV/H2O2 treatment of model compounds in saline waters showed that Br- had a greater effect on contaminant destruction rate suppression than Cl-. Similarly, in the case of the female sex hormone 17β-estradiol, increasing halide concentrations decreased the DOM-sensitized photolysis rate, with bromide accounting for a significant portion of the rate decrease.

In conclusion, halides can decrease the photolysis rate of organic contaminants in water by converting non-selective hydroxyl radicals to selective radicals, influencing the reactivity of various species in solution, and through specific effects of certain halides. These effects of halides on the photolysis of organic contaminants are important considerations in water treatment processes.

Halides are water pollutants that can be found in the Rhine River and its tributaries

The presence of halides in the Rhine River has been a significant issue. In the 1980s, research conducted by the International Commission for the Protection of the Rhine (ICPR) found that French potassium mines contributed to one-third of the total chloride pollution affecting water quality in the Netherlands. Germany was also responsible for a significant portion of the pollution. As a result of these findings, the Dutch government recalled its ambassador from France, as they were concerned about the impact of pollution on their country as the downstream community.

The Rhine River Basin encompasses nine countries in Europe, including Austria, Belgium, France, Germany, Italy, Liechtenstein, Luxembourg, the Netherlands, and Switzerland. With such a large and diverse basin, the river has faced challenges in managing water problems, including pollution from various sources. However, international cooperation on the Rhine River Basin has been ongoing since 1815, and efforts to improve water quality have been made.

The "Rhine 2020" programme, adopted in 2001, has achieved success in reducing heavy metals and other pollutants in the river. The load of nitrogen from the Rhine catchment area into the North Sea has been reduced through the expansion of wastewater treatment plants, and inputs of metals have been significantly lowered. The ICPR has also published recommendations for the reduction of micropollutants, with a goal of at least a 30% reduction by 2040.

While progress has been made, there are still unachieved goals within the "Rhine 2020" programme that require further efforts. The ICPR "Rhine 2040" programme was adopted in 2020 to continue these improvements and ensure the cleanliness and health of the Rhine River and its tributaries.

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