## 2 Materials and Methods
### Materials and Instrumentation  
The work presented here was carried out entirely in a self-designed PCC column based on the example of [PERSON] et al. [28]. As stationary phase material, MWCNTs (CNT-K, Future Carbon GmbH, 95448 Bayreuth, Germany) with an outer diameter of 20-40 nm and a density of 2.1 g/cm\({}^{3}\) were packed in a curled, strong cation exchange membrane (CMI-7000, Membranes International Inc., Ringwood, NJ 07456, USA). Before the column was packed with the particles, the MWCNTs were treated with 1 M HCI at 80 \({}^{\circ}\)C for more than 4 h to reduce the catalytic residues and other contaminants. Afterwards, they were neutralized in DI-water and stored as a MWCNT-DI-water suspension. As mobile phases, either pure DI-water with a conductivity below 0.02 mS/cm (pH = 6.2) or 10 mM phosphate buffer (pH = 7.2, 1.7 mS/cm) were used. The buffer was prepared with sodium dihydrogen phosphate, (absolute, Ph. Eur. AppliChem GmbH, 64291 Darmstadt, Germany) and di-sodium hydrogen phosphate (\(\geq\)98%, Ph. Eur., water free, Carl Roth GmbH & Co. KG, 76185 Karlsruhe, Germany). When necessary, the pH value was regulated with hydrochloric acid (37%, VWR International GmbH, 64295 Darmstadt, Germany) and sodium hydroxide (\(\geq\)99% p.a., ISO, Carl Roth GmbH & Co. KG, 76185 Karlsruhe, Germany). Maleic acid (absolute, Ph. Eur, AppliChem GmbH, 64291 Darmstadt, Germany) was used as model adsorbate for potential-step experiments; sodium chloride (\(\geq\)98%, Ph. Eur., Carl Roth GmbH & Co. KG, 76185 Karlsruhe, Germany) was used as tracer to quantify the homogenity of the packed bed. All PCC experiments were performed with an AktaTM pure system (AktaTM pure 25 M, GE Healthcare GmbH, 79111 Freiburg, Germany). As analytical tools, UV-Vis spectroscopy was used for molecule detection at 216 nm, and a conductivity sensor and a pH sensor were applied at the column outlet. Furthermore, we used a Gamry G750 potentiostat (Gamry Instruments, Warminster, PA 18974, USA) connected to the column for electrochemical application and evaluation. For static binding experiments, an Agilent HPLC system (Agilent 1100 Series HPLC Value System, Agilent Technologies, Inc., Santa Clara, CA 95051, USA) was employed, including a UV-Vis spectrophotometer at 216 and 280 nm. Adsorbates for the static binding experiments were L-alanine (Ala, Serva Electrophoresis GmbH, 69115 Heidelberg, Germany), L-histidine (His, \(\geq\)99% Cellpure, Carl Roth GmbH & Co. KG, 76185 Karlsruhe, Germany), L-phenylalanine (Phe, Merck KGaA, 64293 Darmstadt, Germany), L-typtophan (Trp, Serva Electrophoresis GmbH, 69115 Heidelberg, Germany), maleic acid (absolute, Ph. Eur, AppliChem GmbH, 64291 Darmstadt, Germany) and fumaric acid (FA, AppliChem GmbH, 64291 Darmstadt, Germany). The specific surface area was determined using the BET method with nitrogen in a Gemini VII system (Micromerities Instrument Corp., Norcross, GA 30093-2901, USA), while the particle agglomerate size distribution was measured in suspension with a Microtrac SYNC (Microtrac Retsch GmbH, 42781 Haan, Germany). Transmission electron spectroscopy (TEM; JEM 100-CX, JEOL GmbH, 85356 Freising, Germany), scanning electron microscopy (SEM; CamScan 4, Oxford Instruments/ JSM 5900 LV, JEOL GmbH, 85356 Freising, Germany) and X-ray photoelectron spectroscopy measurements (XPS; LHS 10 XPS with MgK\(\alpha\) X-ray source (h\(\upsilon\) = 1253.6 eV), Leybold Heraeus GmbH, 50968 Koln, Germany) were carried out additionally.