Ultrasonic desorption of mercury from contaminated sediments: influence of the chelating agent DMPS

13 Introduction Mercury is considered a priority pollutant because of its toxicity, mobility and widespread presence in the environment. Mercury bioaccumulates and biomagnifies within the food chain, reaching its highest concentrations in large fish and apex mammalian predators. Adverse effects of mercury on humans concern the central nervous system and pulmonary and kidney functions. Mercury compounds also damage chromosomes, resulting in birth defects, including a variety of neurological impairments, from severe mental retardation to cerebral palsy [145]. Global anthropogenic emissions of mercury into the atmosphere registered a large increase in the last decades, jumping from an estimated 1,900 tons in 1995 [146] to an estimated 2,190 tons in 2000 [147]. As a result of anthropogenic emissions, the mercury background concentration in the atmosphere has been rising since the industrial revolution. According to the Expert Panel on Mercury Atmospheric Processes, pre-industrial atmospheric concentrations constitute approximately one-third of the current atmospheric concentrations and it has been estimated that anthropogenic emissions may currently account for 50-75 percent of the total annual input to the global atmosphere [15]. Mercury compounds are ubiquitous in the environment; they are found in the atmosphere, water bodies, soils, sediments and living beings. In the past, sediments were often considered as a burial sink for contaminants. In fact, it has been demonstrated that, in some conditions, more than 99% of the heavy metals entering a river are stored in river sediments in various forms [148]. Thus, sediments were not regarded as a potential source of pollution. However, due to human activities or natural processes (such as hydrodynamic flows, bioturbation, molecular diffusion and chemical transformation), which provoke variation in the physical–chemical characteristics of water bodies, buried mercury may be remobilized into the overlying water. In this way, contaminated sediments threaten human health directly, through skin contact, or through indirect pollution of the overlying water: human beings may drink contaminated water or, more often, eat organisms contaminated through bioaccumulation of mercury compounds. Because of these reasons, contaminated sediments’ contribution to mercury pollution has recently been reassessed. It is generally accepted that heavy metal contaminated sediments require remediation. Unlike many organic pollutants that may be eliminated or reduced through chemical oxidation techniques or microbial activity, heavy metals will not degrade. Thus, two remediation strategies have been historically adopted for remedying sediments contaminated by heavy metals. The first is an in situ strategy which aims at increasing the stability of metals on sediment particles in situ (through immobilization, containment,