activated carbon adsorption
2. Materials and Methods
2.1. Chemicals and analysis
All chemicals used were purchased at Sigma-Aldrich (Belgium), and were of a purity of 98% or above. All glassware used was oven baked at 500 °C for 8 h to remove any carbon traces. Trace organic contaminants were prepared in separate stock solutions in milli-Q water. The ozonation and catalytic reduction experiments were performed on 5 TrOCs (atrazine, carbamazepine, bromoxynil, diatrizoic acid and dinoseb). Chemical structures are given in Appendix A, and physico-chemical parameters are listed in Table 2.1. These were selected based on expected diverse reactivity towards catalytic reduction and ozonation, structural and physico-chemical variation, occurrence in the environment and toxicity: carbamazepine and diatrizoic acid are pharmaceuticals which are widely detected in water streams [235–239]; bromoxynil, atrazine and dinoseb are toxic pesticides of which the use has been regulated, but which are still being detected in high concentrations in soil and water bodies [240–247].
Table 2.1: Trace organic contaminants, with their main chemical properties. Molecular weight, pKa, log Kow, and log D were calculated using Marvin 6.3 (ChemAxon, Hungary). Log Kow, and log D are the mean values (± standard deviation) of the
Viswanadhan [248], Klopman [249] and PhysProp database [250] methods. 𝑘𝑂3 values were obtained from literature.
TrOC formula molecular weight
(g/mol) pKa log Kow
log D at pH 7 𝒌𝑶𝟑 (𝑴−𝟏𝒔−𝟏) pesticides atrazine C8H14ClN5 215.7 3.20 2.20 ± 0.18 2.20 ± 0.18 6 [84] bromoxynil C7H3Br2NO 276.9 5.11 3.06 ± 0.12 1.51 ± 0.15 6 * 102 [96] dinoseb C10H12N2O5 240.2 4.57 3.24 ± 0.02 1.51 ± 0.39 1.5 * 105 [84] pharmaceuticals carbamazepine C15H12N2O 236.3 / 2.77 ± 0.48 2.77 ± 0.48 3 * 105 [84] diatrizoic acid C11H12I3N3O4 613.9 2.18, 11.84 2.89 ± 0.60 -0.64 ± 0.51 0.05 [99]
Trace organic contaminants, and their oxidation and reduction products, were analysed using Ultra High Performance Liquid Chromatography hyphenated to High-Resolution Orbitrap Mass Spectrometry (UHPLC-HR-Orbitrap-MS, benchtop ExactiveTM Orbitrap, Thermo-Fisher
Scientific, USA). The details about this analysis can be found in Addendum B.1 in Appendix B. Oxidation by-products and catalytic reduction by-products were quantified using the above UHPLC-HRMS method, when analytical standards were available. This was the case for Br-HBN
(3-bromo-4-hydroxybenzonitrile), HBN (4-hydroxybenzonitrile), DH-carb (10,11-
dihydrocarbamazepine) and DABA (3,5-diacetamidobenzoic acid). Where no analytical standards were available, an attempt was made to identify oxidation or reduction by-products, by looking up the expected reaction products by means of the accurate mass (m/z) of their precursor ions ([M+H]+, [M-H]- or [M+NH
4]+). For these products, identification was confirmed by (1) a
calculated mass deviation between the theoretical and measured m/z values of the nominal precursor ions lower than 5 ppm; and (2) by comparing the theoretical and measured m/z abundance distributions of the nominal and common isotopes (e.g. 13C, 37Cl or 81Br).
Total organic carbon (TOC) analysis was used to quantify the adsorbed fraction of trace organics and their oxidation or reduction products on activated carbon. 30 mL samples were filtered by using 0.2 µm PES (polyethersulfon) Macherey-Nagel Chromafil syringe filters, acidified with 1.6 mL 2 M HCl, and analyzed with a TOC-5000 analyzer (Shimadzu).
Separate solutions of TrOCs were prepared at concentrations of 5 mg C/L, and these were used during oxidation and reduction experiments. Although this concentration is considerably higher than could be expected in environmentally relevant situations, this high concentration was required in order to enable quantification of AC adsorption of unknown by-products by means of TOC
measurements. Since many by-products are formed during ozonation and catalytic reduction pre- treatments, and many of these by-products are still unknown, conventional TrOC analysis methods (e.g. LC-MS) could not be applied here. TOC analysis on the other hand comprises all oxidation and reduction products, and therefore provides a full overview of the complete oxidation or reduction products mixture, including unknown transformation products.
2.2. Oxidation experiments
Ozone was produced with an Ozomat COM-AD-02 ozone generator (Anseros). Pure oxygen at a flow of 300 mL/min was converted to ozone, which was subsequently bubbled through Milli-Q water using a gas-washing bottle, which was cooled in an ice bath. The O3 saturated solution was
used for the experiments, after determining the actual O3 concentration spectrophotometrically
using an UV-1601 spectrophotometer (Shimadzu), according to the indigo method from Bader and Hoigné (1981) [251]. From this O3 saturated solution, ozone was dosed to separate TrOC
solutions, each one containing one of the five TrOCs, in two steps (atrazine exempted). In a first step at the start of the ozonation experiments, 15 mg O3/L was dosed for bromoxynil, dinoseb,
carbamazepine and diatrizoic acid, and in a second step at 12.5 minutes, an additional 3.75 mg O3/L was dosed. For atrazine, because of its expected recalcitrance towards oxidation [252, 253],
a higher dose of 30 mg O3/L (at the start of the experiment) was used. Even though these are high
ozone doses (typically applied ozone doses are 0.5 – 1.5 mg O3/mg dissolved organic carbon [204,
254]), this regime was chosen in order to ensure an as high as possible oxidation conversion of TrOCs into oxidation products and at the same time as limited mineralization as possible. Samples were taken at time intervals 0, 1, 3, 5, 12.5 and 20 minutes for TrOC and oxidation product analysis using UHPLC-HRMS. These samples were instantaneously spiked with NaNO2 in a 10/1 ratio of
NaNO2/O3 (initial), to quench any residual ozone in the sample which did not undergo reaction with
the TrOCs [255].
2.3. Reduction experiments
Catalytic reduction on separate TrOC solutions, each one containing one of the five TrOCs at 5 mg C/L, was performed using biogenic (using Shewanella oneidensis MR1) bimetallic Pd/Au nanoparticles (bio-Pd/Au). Bio-Pd/Au was produced as described by De Corte et al. (2011) [144]. The Pd and Au concentrations of the prepared bimetallic catalyst were 100 and 2 mg/L respectively (noted as 100/2 mg bio-Pd/Au/L), and TrOC stock solutions were prepared at 10 mg C/L. Subsequently, the catalyst suspension and TrOC solution were added together so that the final concentrations were 50/1 mg bio-Pd/Au/L (corresponding with the highest catalytic activity as found by De Corte et al. (2012) [137]) and 5 mg C/L TrOC. The headspace of the bottles
containing the catalyst and TrOC was filled with hydrogen gas (H2). Blanks were included using
nitrogen gas (N2) in the headspace. To ensure complete reduction of TrOCs, the catalytic reduction
experiments were run for 24h. The pH was kept constant at 7 using HNO3 or NaOH by opening
the bottles, and the headspace in the bottles was renewed after taking a sample. 10 mL samples were taken through a gas-tight septum at time intervals 0, 0.5, 1, 2, 4, 8 and 24 hours, filtered by using 0.2 µm PES Macherey-Nagel Chromafil syringe filters, and analyzed for TrOCs and reduction products using UHPLC-HRMS. At the end of the reduction experiments, the catalyst was removed from the suspension by centrifugation with a Sorval RC 5 Plus centrifuge at 5000 rpm for 7 minutes. The supernatant was then subsequently filtered over a 0.2 µm PES Corning bottle-top filter. This supernatant was used for the adsorption experiments.
2.4. Adsorption experiments
Activated carbon adsorption isotherms were performed on the supernatants after the oxidation and reduction runs. These supernatants contained the reduced or oxidized by-products of the single TrOCs, respectively. Since no mineralization occurred (see Table B.1 in Appendix B), the concentrations of the by-products were similar in terms of molar concentrations as the original TrOCs. By performing adsorption isotherm experiments, the influence of oxidation and reduction on adsorption efficiency could be determined. In addition, isotherms were carried out with the parent compounds at similar concentrations (5 mg C/L). The isotherms were constructed using powdered activated carbon (PAC) type UltraCarb® 830 (Siemens Water Technologies), by adding
0 – 18 mg PAC to 80 mL of the solutions. After equilibrium (isotherms were run continuously on a shaker for 3 days at 25°C [256]), samples were taken, filtered over 0.2 µm PES Macherey-Nagel Chromafil syringe filters, acidified with 1.6 mL 2M HCl and measured using a TOC-5000 analyzer (Shimadzu). TOC was used to measure the total adsorption, to also account for adsorption of the unknown by-products. Freundlich adsorption isotherms were fitted to the carbon loadings (Qe –
in mg TOCadsorbed/mg AC) and measured equilibrium concentrations (Ce – in mg TOC/L) [257].