Influence of natural organic matter on equilibrium adsorption of neutral and charged pharmaceuticals onto activated carbon

Abstract

Natural organic matter (NOM) can influence pharmaceutical adsorption onto granular activated carbon (GAC) by direct adsorption competition and pore blocking. However, in the literature there is limited information on which of these mechanisms is more important and how this is related to NOM and pharmaceutical properties. Adsorption batch experiments were carried out in ultrapure, waste- and surface water and fresh and NOM preloaded GAC was used. Twenty-one pharmaceuticals were selected with varying hydrophobicity and with neutral, negative or positive charge. The influence of NOM competition and pore blocking could not be separated. However, while reduction in surface area was similar for both preloaded GACs, up to 50% lower pharmaceutical removal was observed on wastewater preloaded GAC. This was attributed to higher hydrophobicity of wastewater NOM, indicating that NOM competition may influence pharmaceutical removal more than pore blocking. Preloaded GAC was negatively charged, which influenced removal of charged pharmaceuticals significantly. At a GAC dose of 6.7 mg/L, negatively charged pharmaceuticals were removed for 0-58%, while removal of positively charged pharmaceuticals was between 32-98%. Charge effects were more pronounced in ultrapure water, as it contained no ions to shield the surface charge. Solutes with higher log D could compete better with NOM, resulting in higher removal.

Super-fine powdered activated carbon (SPAC) for efficient removal of micropollutants from wastewater treatment plant effluent

“…This effect may be enhanced for SPAC and due to a greater number of large molecules at the surface of SPAC. Moreover, as observed previously (de Ridder et al, 2011), increased adsorption of negatively charged organic matter on the surface of the carbon particles can lead to a more negatively charged surface. The increased EfOM loading on SPAC could consequently increase electrostatic repulsion of anionic pharmaceuticals such as sulfamethoxazole.…”

“…3), suggesting adverse effects of competing wastewater matrix components on the adsorption of target micropollutant. It is largely acknowledged that background organic matter reduces the number of adsorption sites available for micropollutants, either through direct competition for adsorption sites and/or pore blocking, and consequently decreases the adsorption efficiency of PAC (de Ridder et al, 2011;Delgado et al, 2012;Kovalova et al, 2013a;Mailler et al, 2014;Margot et al, 2013). This effect is well illustrated in Fig.…”

Occurrence and Fate of Pharmaceuticals and Personal Care Products in Wastewater

“…It was argued that the adsorbed EfOM would lower the net surface pH to negative of the otherwise neutral or slightly positively charged PAC [94], thereby attracting the positively charged compounds and repulsing the negatively charged compounds. The compounds with medium affinity to PAC, showing 70-90% removal, were predominantly negatively charged substances with hydrophobic properties (e.g., acidic lipid regulators and analgesic and anti-inflammatories, log D ow 1.7-3.4, Table 2) indicating that the electrostatic repulsion could be offset by hydrophobic partitioning, though other mechanisms also contribute to the attraction [93]. It has been reported that for compounds with log D ow > 3.7, hydrophobic partitioning is the dominating mechanism leading to PAC adsorption [93], which explains high removal (>90%), for instance, for mefenamic acid (log D ow 5.12).…”

“…The compounds with medium affinity to PAC, showing 70-90% removal, were predominantly negatively charged substances with hydrophobic properties (e.g., acidic lipid regulators and analgesic and anti-inflammatories, log D ow 1.7-3.4, Table 2) indicating that the electrostatic repulsion could be offset by hydrophobic partitioning, though other mechanisms also contribute to the attraction [93]. It has been reported that for compounds with log D ow > 3.7, hydrophobic partitioning is the dominating mechanism leading to PAC adsorption [93], which explains high removal (>90%), for instance, for mefenamic acid (log D ow 5.12). The somewhat unexpected poor removal (70-90%) of the 276 Persistent Organic Pollutants positively charged azithromycin (log D ow 2.8) was suggested to be caused by exclusion in the micropores of the PAC due to its large size [82].…”

Impacts of ozonation on the competition between organic micro-pollutants and effluent organic matter in powdered activated carbon adsorption

“…The second aspect was in the focus of this study. Activated carbon adsorption of OMP can be strongly deteriorated by the adsorption of EfOM (Yu et al, 2012;Zietzschmann et al, 2014bZietzschmann et al, , 2014c or natural organic matter (de Ridder et al, 2011;Matsui et al, 2003;Ternes et al, 2002). The competition mechanisms are direct competition for adsorption sites on the activated carbon internal surface, and blockage of activated carbon pores with larger molecules of the EfOM bulk organics .…”