The intensive use of pesticides on large areas of agricultural soil has given rise to concern about their fate in the environment. For many years, their presence as well as their environmental impact has attracted much attention due to the wide use of such compounds in agriculture and household applications (Beck et al., 1993; Serrano et al., 2005; Mansour, 1993). Less than 1% of total applied pesticides reach the target pests, the vast majority being dispersed in the different environmental compartments (water, air and soil) but mainly in aquatic environment via agricultural runoff or leaching (Pimentel, 1995). The application of herbicides on agricultural soils is a well-established and effective practice to control weed growth. They represent about 50% of the demand for agricultural chemicals; their prolonged use involves the risk of their retention in crops and soils. Soil is an ultimate reservoir for these products, whether applied directly or received indirectly from spray drift and residues of treated sites (Barbash & Resek, 1996). These products can be the subject of various transformations and they can react with different fractions of soil minerals or organic. Mineral soil colloids play an important role in adsorption of polar organic compounds, mainly due to the high surface areas associated to their small particle size and, in the case of Smectites, because they have internal (expandable) surfaces accessible to water and polar organic molecules such as pesticides (Lagaly, 1994; Cox et al., 1995; Lee & Kim, 2002). These clay minerals possess layered structures and the presence of interlayer space thus gives a sterically constrained reaction environment for pesticide molecules when intercalated (Caine et al., 1999). Iron is one of the most abundant transition metals in soil and is considered to play a large role in photoinduced redox reactions generating active oxygen species such as .OH (Sherman, 1989). The clay surface is also covered with humic substances that represent 60%−70% of soil organic matter, probably via formation of clay–metal– organic complexes. These complexes are characterized by the presence of stable radical species detected by ESR (Senesi & Schnitzer, 1977). Regarding the presence of these substances in soil, they may either contribute to the degradation of the pesticide through the formation of reactive species (Albanis et al., 2002) or be involved as inhibitors by a screen effect phenomenon (Hebert & Miller, 1990).

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Adsorption and Photochemical Behaviour of the Herbicide Monuron on Clay Surfaces

Combined effects of some cations such as calcium (Ca2+), iron (Fe2+), manganese (Mn2+), and magnesium (Mg2+) and operating time on the removal of arsenic by air-injected electrocoagulation (EC) reactor with aluminum (Al) ball electrodes were investigated. The operating conditions were optimized with the Box–Behnken design of response surface methodology (RSM). The response variables were selected from the program as removal efficiency, residual arsenic concentration, energy consumption and operating cost (OC) in the EC process. A total of 46 experimental run was performed. The removal efficiency of arsenic increased with an increase in iron concentration (0.5–4.5 mg/L). The rest of the cations showed no noticeable effect on arsenic removal efficiency. The maximum arsenic removal efficiency and minimum OC at the optimum operating conditions (CCa: 305 mg/L, CMg: 42 mg/L, CFe: 3.3 mg/L, CMn: 2.34 mg/L, initial pH of 7.5 applied current of 0.15 A, Al ball size of 7.5 mm, 5.0 cm of Al ball anodes height in the EC reactor, air-fed rate of 6.0 L/min and tEC: 16.83 min) in the EC process were 99.9% and 0.0332 $/m3 for initial arsenic concentration of 200 μg/L, respectively. The removal mechanism of As(III) by EC seems to be oxidation of As(III) to As(V) and subsequent removal by adsorption/complexation with aluminum hydroxides generated in the process. The results showed that the air-injected EC reactor can be used effectively for arsenic and hardness removal simultaneously from real groundwater sources.

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Combined influence of some cations on arsenic removal by an air-injection EC reactor using aluminium ball electrodes

Water Resources Planning and Management: Groundwater contamination in Bangladesh

Arsenic is a crystalline metalloid present in the earth’s crust in different minerals. Various natural processes and human interventions have mobilized As into the groundwater system over the years. The dissolved As presence in drinking water is now a significant health concern in India and many other countries. As is a proven carcinogen, and long-term ingestion of low concentrations of As can lead to serious health issues. As in drinking water was first regulated by the United States Public Health Service (USPHS), which set a maximum permissible concentration (MPC) of 50 μg/L. Later, based on lung and bladder cancer risk, the United States Environmental Protection Agency (USEPA) promulgated a maximum contaminant level (MCL) of 10 μg/L. Indian standard for drinking water also recommends a total As concentration of 10 μg/L as the acceptable limit. However, some of the developed countries have As standards lower than 10 μg/L as well. The removal of As in drinking water is inevitable, considering its widespread presence in groundwater, potential human toxicity, and stringent regulation in drinking water. Several point-of-use (PoU) and community-based treatment technologies have been developed to provide As-free water to the affected population. The processes adopted in these technologies include coagulation and filtration, sorptive filtration, ion-exchange, electrocoagulation, and membrane filtration. The present chapter reviews these technologies critically and their suitability in the Indian context. The chapter also provides insight into the As occurrence in the environment, its speciation, toxicity, and health effects.

T.S. Singh K.K. Pant

Papers

A comparative study for the removal of As(III) and As(V) by activated alumina