(493a) Effectiveness of Activated Carbon Adsorption for the Removal of Endocrine Disrupting Chemicals In Drinking Water

The US Environmental Protection Agency (USEPA) defines an EDC as  an exogenous agent that interferes with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, reproduction, development, and/or behavior. Such chemicals as those above have been found existing in wastewater, surface waters,  sediments, groundwater, and even drinking water.  As they  exist in extremely low concentration (μg/L or ng/L levels), and so highly sensitive measurement techniques are employed to detect them. The EDCs have been attributed as a cause of reproductive disturbance in humans and wildlife even at low concentrations.   Human exposure to these chemicals in the environment is a critical concern with unknown long-term impacts. Natural and synthetic EDCs are released into the environment by humans, animals and industry, mainly through sewage treatment systems before reaching the receiving  waters bodies including soil, surface water, sediment and ground water.  

The EDC removal methods fall into three categories, namely,  physical removal,  biodegradation and chemical advanced oxidation. There is much research data on membrane processes for the effective removal of organic micro-pollutants; from pesticides to pharmaceuticals to personal care chemicals. In recent years, research on EDC removals by the membrane processes have greatly increased.  Studies have discovered that the rejection efficiency of EDCs by membranes strongly depended on their physico-chemical properties, such as molecule weight, hydrophobicity  (octanol-water partition coefficient or log  K_ow), aqueous solubility, electrostatic properties,  and other characteristics.  Nevertheless, membrane technologies are very expensive as compared to other technologies such as advanced oxidation and  activated carbon adsorption processes.   Advanced oxidation technologies such as various combinations of ozone, hydrogen peroxide and  UV radiation have been found effective in the destruction of most EDCs to meet the treatment goals.  However, residuals have of these chemicals have been found after extensive treatment, and in some cases complete oxidation of EDCs had not been achieved besides yielding disinfection byproducts including aldehydes, ketones and haloacetic acids. 

Under the circumstances activated carbon adsorption  is a well-known, efficient and cost-effective technology for removing various organic contaminants including the EDCs.  Ggranular activated carbon (GAC) is commonly employed in packed bed adsorbers for  the removal of trace organic pollutants from water, although powder activated carbon (PAC) has also been used in some cases..  During the last few years, several researchers have demonstrated that activated carbon has a strong capability of removing a broad range of representative EDCs for artificial and real waste water in the laboratory and pilot-scale and  full-scale operations.  Studies on EDCs removal by GAC mainly focus on the removal efficiencies for EDCs in different water systems and the influence of factors such as the physicochemical properties of EDCs and the types of GAC produced from different materials.  The effects of some fundamental environmental parameters including adsorbent concentration, pH, salinity and the presence of humic acid and surfactant on adsorption.  A major concern in the application of fixed-bed GAC adsorbers is that that adsorption capacity of the activated carbon  was greatly impaired by the presence of natural organic matter (NOM) such as humic and fulvic substances present in most nature water sources, and the problem is exacerbated in the case of EDCs  regarding their removal at low concentrations (micro-grams or  nano-grams per liter level).  Furthermore, water quality parameters including  alkalinity, hardness and presence of NOM influence the effectiveness of activated carbon adsorption.   The presence of divalent cations in natural waters such as calcium influence the properties of NOM such as those of the macromolecules of humic substances.  The mechanisms of complexation of humic substances with divalenet calcium ions are important within the context of EDC adsorption  at trace levels.  Humic substances have a strong tendency to form complexes with metal ions and EDCs, thereby altering their transport and adsorption characteristics.  The paper will address the adsorption of model EDCs including alachlor, aldicarb, dieldrin, heptachlor, heptachlor epoxide,  toxaphene, and polychlorinated biphenyls (congeners Arochlor 1016 and 1254) at low concentrations from water, in the presence and absence of humic substances.  Particular emphasis will placed on the mechanisms involved in the transport and adsorption phenomena including those of competitive adsorption among humic acid, humic acid-EDC complexes, and the free EDC molecules.  The mechanisms of compaction of humic molecules in the presence of calcium ions and the influence of this phenomenon with respect to competitive adsorption will be addressed.  Additionally, the effectiveness of activated carbon technology  will be discussed with special emphasis on meeting the treatment objectives for waters containing NOM such as humic substances.  The presentation will address the relevant complexation, transport and competitive sorption mechanisms, and the means for improving the efficiency of the process for removal of the EDCs of interest.