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Significance Statement
Current pursuit of more efficient water treatment technologies, addressing the threat of water contamination with emergent pollutants, such as pharmaceuticals, is an important research field, under the watchful eyes of the scientific community and governmental bodies, in view of the belief that this type of contamination may require legislative intervention in the near future. Pharmaceutical compounds are already included in an European Union Watch List in 2013/39/EU directive and Decision 2015/495/EU and activated carbons, which are non-specific adsorbents, are considered as the best available decontamination technology for the removal of the pollutants that have a recalcitrant behaviour in conventional water treatment plants.
This manuscript reports the effect of solution pH and water hardness in the adsorption of clofibric acid, an acidic recalcitrant pharmaceutical compound, onto two activated carbons commercialized for water treatment purposes. The distinct porous networks of the activated carbons assayed allowed a deeper insight into the adsorption mechanism of clofibric acid at pH 3 and 8 under increasing water hardness degrees. Higher volumes of larger micropores (supermicropores) allow both higher removal efficiencies and adsorption capacities under similar experimental conditions. The lower removal efficiencies at pH 8 being justified by the increase in the compound solubility. Both theoretical calculations and conductivity measurements support the hypothesis that the complexation of clofibrate anions (pH 8) with calcium cations accounts for the enhanced adsorption capacity of the activated carbons for the target compound.
This work represents a step forward in the comprehension of the adsorption mechanism of clofibric acid onto activated carbons in aqueous media and this physicochemical approach provides new insights to the paramount importance of water hardness in the removal of this recalcitrant and acidic pharmaceutical compound. The results obtained and the proposed adsorption mechanism for clofibric acid adsorption in hard water at pH 8 afford an explanation for literature data regarding the removal efficiency of several pharmaceutical compounds in different water matrixes and the increased efficiency observed for the acidic pharmaceuticals (i.e. clofibric acid and ibuprofen) in the presence of high Ca2+ contents. This hypothesis of Ca2+ coordination should also be considered for other acidic organic compounds, since it may play a fundamental role in the improvement of water treatment processes employing activated carbons.
Figure Legend: Higher removal of clofibric acid by activated carbons in hard water due to clofibric acid complexation with Ca2+
Journal Reference
Chemical Engineering Journal, Volume 286, 2016, Pages 538–548.
Ana S. Mestre1, André Nabiço1,2, Patrick L. Figueiredo1,2, Moisés L. Pinto3, M. Soledade C.S. Santos1, Isabel M. Fonseca2
[expand title=”Show Affiliations”]- Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal
- Departamento de Química, REQUIMTE, LAQV, CQFB, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Caparica, Portugal
- CERENA, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal
Abstract
Clofibric acid is the metabolite and active principle of blood lipid regulators, it represents the class of acidic pharmaceuticals, and is one of the most persistent drug residues detected in the aquatic environment worldwide. This interdisciplinary work evaluates the effect of solution pH and water hardness in clofibric acid adsorption onto commercial activated carbons. Kinetic and equilibrium assays revealed that the highest clofibric acid removal efficiencies (>70%) were attained at pH 3, and that at pH 8 water hardness degree plays a fundamental role in the adsorption process.
In hard water at pH 8 the removal efficiency values increased by 22 or 46% points depending on the carbon sample. Adsorbents’ textural properties also affect the adsorption process since for the microporous sample (CP) the increase of water hardness has a great influence in kinetic and equilibrium data, while for the micro + mesoporous carbon (VP) the variation of the water hardness promoted less significant changes. At pH 3 the increase of water hardness leads to changes in the adsorption mechanism of clofibric acid onto CP carbon signaled by a transition from an S-type to an L-type curve. At pH 8 the change from deionized water to hard water doubles the maximum adsorption capacity of sample CP (101.7 mg g−1 vs 211.9 mg g−1, respectively).
The adsorption enhancement, with water hardness under alkaline conditions, was reasoned in terms of calcium complexation with clofibrate anion exposed by molecular modeling and conductivity studies. Ca2+ complexation by other acidic organic compounds may also occur, and should be considered, since it can play a fundamental role in improved design of water treatment processes employing activated carbons.
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