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Simulating effluent organic matter competition in micro-pollutant adsorption onto activated carbon using a surrogate competitor Stefan Dittmar, Frederik Zietzschmann, Maike Mai, Eckhard Worch, Martin Jekel, and Aki Sebastian Ruhl Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01503 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018
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Environmental Science & Technology
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Simulating effluent organic matter competition in
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micro-pollutant adsorption onto activated carbon
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using a surrogate competitor
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Stefan Dittmar a, Frederik Zietzschmann a, Maike Mai b, Eckhard Worch c, Martin Jekel a,
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Aki S. Ruhl a,*
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a
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Straße des 17. Juni 135, 10623 Berlin, Germany
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b
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Straße des 17. Juni 135, 10623 Berlin, Germany
Technische Universität Berlin, Chair of Water Quality Control, Sekr. KF4,
Technische Universität Berlin, Chair of Soil Science, Sekr. BH 10-1,
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c
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ABSTRACT
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Adsorption onto activated carbon is a promising option for removing organic micro-pollutants
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(OMPs) from wastewater treatment plant (WWTP) effluents. The heterogeneity of activated
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carbons and adsorption competition between OMPs and adsorbable compounds of the effluent
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organic matter (EfOM) complicate the prediction of OMP removals. Thus, reliable and simple
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test systems are desirable. For this study, batch experiments with powdered activated carbon
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(PAC) were carried out to examine methyl orange (MO) as a selected surrogate competitor to
Technische Universität Dresden, Institute of Water Chemistry, 01062 Dresden, Germany
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simulate EfOM adsorption competition. MO solutions were spiked with carbamazepine (CBZ) as
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indicator substance for well-adsorbing OMPs. On the basis of CBZ adsorption isotherms in
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WWTP effluents, MO concentrations for batch test solutions with identical adsorption
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competition towards CBZ were calculated. The calculations were performed according to an
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empirical model of CBZ adsorption in presence of MO, since predictions employing the ideal
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adsorbed solution theory (IAST) proved to be inaccurate. Comparative batch tests with five
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different PAC were conducted with WWTP effluent and respective MO batch test solutions.
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Except for one PAC, the achieved CBZ removals were very similar in WWTP effluent and the
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test solution. Additionally, a universal correlation between MO and CBZ removals was found for
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four PAC.
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INTRODUCTION
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Wastewater treatment plants (WWTPs) are considered a main source of organic micro-
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pollutant (OMP) emissions to the aquatic environment.1 Consequently, several advanced
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wastewater treatments are currently being tested and already have been implemented in large
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scale to meet the insufficient eliminations of numerous OMPs in conventional WWTPs.2 Besides
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ozonation, activated carbon for adsorbing and thus removing target compounds from WWTP
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effluent is one of the most promising options.3-5
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Activated carbons are either used as granules (GAC) in fixed-bed adsorbers or as powdered
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activated carbon (PAC) in slurry reactors. They are derived from different organic precursors
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(e.g. bituminous coal, lignite, charcoal, wood, organic waste) by physical or chemical
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activation.6 As a result, the properties of commercial carbons can vary greatly and so do their
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adsorption capacities regarding different OMPs.7 Characteristic properties that are commonly
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used to describe adsorbents (e.g. BET surface, nitrobenzene number) allow only partially
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successful predictions.8 Additionally, OMP removals also depend on the composition of the
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respective aqueous phase. The composition of WWTP effluent fluctuates both temporally and
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regionally.9-10 WWTP effluents contain a large variety of organic compounds, usually comprised
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as effluent organic matter (EfOM), which compete for adsorption sites and thus impair OMP
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removals. OMPs are present at individual concentrations around ng/L or µg/L whereas EfOM
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amounts to much higher concentrations. EfOM is usually quantified using sum parameters such
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as dissolved organic carbon (DOC). The DOC concentrations of typical WWTP effluents range
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from 4 to 15 mg/L.3, 10
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Considering the time- and location-dependent heterogeneity of WWTP effluents, tests with
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different WWTP samples are inherently lacking reproducibility. More standardized tests, e.g. by
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using test solutions of known composition would clearly be beneficial in order to obtain time-
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and location-independent, yet comparable results for different PAC products or PAC product
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batches.11 Most importantly, such a test system should allow for inferring OMP removals
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achievable with a PAC in real water (EfOM-containing, unknown composition) from OMP
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removals in the test solution of known composition. Additionally, the test system should be as
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simple and fast as possible, applicable for on-site quality controls by producers as well as
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costumers or for estimating and adjusting PAC dosage. One approach to such a test system is the
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use of defined solutions of surrogate competitors spiked with OMPs. Variable adsorption
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competition on OMPs caused by EfOM of different WWTP effluents should be replicated by the
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surrogate, e.g. by varying its initial concentration. In terms of a simple and fast test system, it
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would as well be beneficial if the adsorptive removal of the competitor substance itself is easily
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measurable and can be used to estimate corresponding OMP removals.
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In this study, carbamazepine (CBZ) was chosen as an indicator substance for well-adsorbing
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OMPs as it was proposed by Jekel et al.12 Swiss regulations also include CBZ as indicator
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OMP.13 Batch experiments with PAC were then conducted to test the applicability of methyl
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orange (MO) as a surrogate competitor. The azo dye MO is a common pH indicator with a pKa
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of 3.47 in water14 and an aromatic compound of size similar to many OMPs. Aromatic and low
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molecular weight compounds of the EfOM were pointed out as the main adsorption competitors
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of OMPs.15 They cause direct competition for adsorption sites, whereas larger model molecules
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would mainly induce pore blockage.16, 17
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MATERIALS AND METHODS
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Powdered activated carbons and water samples. Characteristics of the five utilized
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commercial PACs are summarized in Table 1. Information provided by the respective suppliers
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is denoted. pH of the point of zero charge (pHpzc) was determined according to Rivera-Utrilla et
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al.18 except with contact times of 24h instead of 3h. The methods employed for determining
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B.E.T. surface area, average pore diameter and particle size (d50, median diameter according to
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cumulative particle volume) are described in section 2 of the supporting information along with
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more detailed results.
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Table 1. Powdered activated carbons and their characteristics
Abbr.
Product name
SAE
Norit SAE Super
HK
HK 950
CCP AP MP
Carbopal CCP 90D Carbopal AP AquaSorb MP 23
Manufacturer Cabotcorp, USA CSC, Germany Donau Carbon, Germany Donau Carbon, Germany Jacobi Carbons, Germany
B.E.T. Total pore Avg. pore Particle Point of Ash Source surface volume diameter size (d50) zero charge content* material* [m2/g] [cm3/g] [nm] [µm] (pHpzc) [%] Mixture
1114
0.78
2.81
26.6
11.37
-
Charcoal
1911
1.48
3.10
17.6
3.29
