Research articlePharmaceutical emerging pollutants removal from water using powdered activated carbon: Study of kinetics and adsorption equilibrium
Graphical abstract
Introduction
The growth of the global economy, despite its periodical crises, has promoted the development of new chemical products required for agriculture, industry, and human and veterinary health. The disposal of these compounds, those newly developed and those that have existed for years, are having an impact in waters, air and soil. More than two million tons of wastewater a day is released into the global environment. As a consequence, water pollution has become a serious problem in several countries, where there is a lack of an efficient water treatment plants (Geissen et al., 2015).
In the last years, there is a new generation of emerging pollutants, being of particular interest the pharmaceutical products. Several of these compounds remains in the environment, even after the water treatment processes (Katsigiannis et al., 2015).
The new analytical tools available in the laboratories allow us to detect these kinds of pollutants in wastewater, suspended solid matter and even in underground water. More than 3000 biological active compounds can be detected in very low concentrations (ng.L−1) in very complex matrices (Richardson, 2010). Different anti-inflammatory, beta-blockers, antidepressants and antiepileptic compounds have been detected in superficial waters. Their concentrations in the environment are strongly dependent on the consumption patterns of the population, regions and countries. Their chemical stability and solubility in waters are factors that can introduce fluctuations in the measured concentrations (Ma et al., 2017).
A lack of regulations has allowed the release of large amounts of these pharmaceuticals. There are no reference limits for most of these compounds (Deblonde et al., 2011; Ma et al., 2017). Their presence in the environment is a consequence of the disposal of wastewater without previous treatment. However, previous studies have shown that not all conventional wastewater treatment plants are designed to eliminate these types of pollutants (Arditsoglou and Voutsa, 2010; Fent et al., 2006; Kolpin et al., 2002; Stasinakis et al., 2008).
The effective elimination of these compounds from wastewater before discharge, including the development of specific tertiary treatments, is an emerging issue in environmental science and engineering (Rivera-Utrilla et al., 2013). There are several compounds which can be removed either by biodegradation or by sorption on suspended solids. However, carbamazepine and sildenafil citrate are among those compounds which are recalcitrant in conventional treatment plants. Additional processes, such as adsorption by activated carbon can be used, and, as far as it is known, this adsorbent does not generate toxins or pharmacologically active products (Katsigiannis et al., 2015).
Dyes, heavy metals and some organic compounds can be removed using activated carbon (Chang et al., 2015b; Mailler et al., 2016). Moreover, granulate activated carbon is normally employed in several treatment plants in America and Canada, as a consequence of its adsorption capacity and its possibility to be recovered (Snyder et al., 2007). In addition, powdered activated carbon has been applied for removing many problematic organic pollutants, herbicides, and flavor and malodorous compounds, since it has a greater capacity of adsorption than the GAC, however, the CAP is a difficult material to regenerate (Westerhoff et al., 2005).
Thus, different studies have evaluated the efficiency of activated carbon to remove emerging pollutants from water samples. However, few experimental data regarding their behavior and mathematical modeling are available for these compounds (Katsigiannis et al., 2015; Snyder et al., 2007).
In the present study, the efficiency of removing carbamazepine and sildenafil citrate from waters using powdered activated carbon as a function of concentration as well as the physicochemical characteristic of the water employed in the experiments were evaluated.
The adsorption equilibrium was analyzed using Langmuir, Freundlich, combined Langmuir-Freundlich as well as Redlich-Peterson adsorption isotherms. The adsorption kinetic was discussed in order to model the powdered activated carbon capacity in the treatment of waters containing emerging pollutants.
Section snippets
Adsorbents and adsorbates
Powered activated carbon of vegetable origin (Biopack, CAS [7440-44-0], 99% purity) was employed in the experiments. Particle size ranged between 1 and 150 μm, with an apparent density of 250 kg/m3, surface area of around 1328.3 m2/g, and average pore and volume size values of 37 Å and 1.06 cm3/g, respectively, as measured from Brunauer-Emmett-Teller (BET) experiments. The pH of the carbon dispersed in water ranged between 2 and 4.
The commercial granular activated carbon mesh-29 used had a
Solubility
As far as the measured pharmaceutical solubility in methanol at 25 °C is of the order 1 × 104 mg/L for Cbz and 5 × 103 mg/L for Sil, stock solutions were prepared in methanol containing the largest amount of the compounds with the least potential interference of methanol at the moment of the preparation of the aqueous solutions where the initial concentration of the pharmaceuticals is larger than 18 mg/L (see Table 1).
Experiments in solution mixtures of water-methanol were carried out in order
Conclusions
The experimental results presented in this paper showed the adsorption capacity of powdered activated carbon to remove Cbz and Sil from aqueous solutions. The difference with granular activated carbon, the solution matrix on the rate of adsorption and equilibrium of adsorption were also analyzed. Two and three parameter models of adsorption isotherms were applied in the analysis of the experimental results, as well as the pseudo-first, pseudo-second order, non-linear equation rate and
Acknowledgement
The authors wish to thank the Argentina National Research Council (CONICET, PIP 2010-2012-0723), and the National Agency of Science and Technology (ANCyT, PICT 2014-0919).
Ing. Nasly Delgado also thanks CONICET for the fellowship that was granted to fund this research.
This study is dedicated to the memory of Dr. Alicia E. Ronco (1945-2016) for her dedicated support of the present study as well as her large contribution in environmental sciences research in Argentina.
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