Micropollutants (MPs) are synthetically produced microcontaminants found in minute concentrations of billionths (nano) to millionths (micro) grams per liter (ppt and ppb, respectively) in wastewater as a result of human activity. These pollutants originate from various sources, including industrial processes, agriculture, household cleaning, personal care products and pharmaceuticals. MPs in wastewater pose a growing concern for their adverse effects on the ecosystem as well as on human health through subsequent bioaccumulation. The EU Urban Wastewater Treatment Directive (adopted by the European Council in early 2024) aims to remove MPs from wastewater and increase the efficiency of wastewater treatment plants in a cost-effective manner. The EU Council agreed that all wastewater treatment plants serving >200,000 people must be equipped with a fourth treatment/purification stage by the end of 2040, intended specifically to remove micropollutants present in very low concentrations.
A broad spectrum of advanced treatment processes, such as ozonation, activated carbon (AC) or their combination, can significantly reduce the MP effluents in water. In this study, a fundamental understanding of the physiochemical aspects of MPs is applied as well as rigorous AC design to quantify the reduction of MPs achieved by implementing AC treatment solutions using water sourced from a wastewater treatment plant in North West Germany.
When identifying the most effective ACs for MP removal, traditional AC properties, such as Iodine Number (measuring mg of Iodine adsorbed from solution per gram of AC in a static test) used in water treatment cannot be applied reliably to predict MPs removal efficacy. Therefore, custom designing ACs and modifying their properties based on fundamental characterization of MPs is key in ensuring effective molecular transport and adsorptive sequestration of the MPs.
MP characterization and implications for AC properties
The removal efficacy of MP contaminants from wastewater is highly dependent on specific properties related to both the contaminant and the AC. For MPs, the contaminant molecular size, solubility, surface charge, polarity and mass diffusion kinetics can affect removal performance. Likewise, the design of the AC to have the appropriate distribution of transport and sequestration pore volume as well as surface characteristics to selectively attract or repel contaminants and other interfering water components are equally important.
A breakdown of common MP properties and associated AC features is shown in Table 1. Molecular diameters of targeted contaminants give a first indication of the necessary carbon pore sizes for transport and sequestration. Sequestration and transport pore sizes are defined to be 1–2 times and up to 10 times the constituent’s molecular diameter, respectively. Irreversible sequestration of the contaminant is enhanced when the contaminant is touching at least one pore wall, hence defining the 1–2 times size factor. Many of the water contaminants are moderate-to-large in size thus in those adsorptive treatment systems the limiting mechanism is in many cases mass diffusion kinetics.
Charge under relevant pH conditions was estimated from the constituent’s dissociation constant (pKa), with lower numbers indicating more likely dissociation of the MPs acid group. The hydrophilicity/hydrophobicity of the MP constituent is approximated by the octanol-water distribution coefficient (log KOW at pH 7), with values greater than 0 indicating higher hydrophobicity and values less than 0 indicating hydrophilicity. Log KOW is an important parameter for predicting the distribution of a substance in various environmental compartments (water, soil, air, etc.). Substances with high log KOW values tend to adsorb more readily to organic matter in soils or sediments because of their low affinity for water. N-H groups and amines in general have a tendency of protonating under acidic environments, forming an ionic complex allowing good capturing conditions using designed powdered activated carbons (PAC).
Methods
The adsorption performance of two lignite derived PACs manufactured by Arq and an industry standard bituminous PAC benchmark were evaluated at the University of Duisburg/IWW Water Center in Essen, Germany using liquid chromatography. The wastewater reference sample was supplied by a wastewater treatment plant located in North Western Germany. The PACs were added as a suspension to the water sample at 3 different concentrations (5mg/L, 10mg/L and 15mg/L) with a contact time of 72 hours (over the weekend period). Table 2 displays relevant properties of the PACs tested.
Results and conclusions
Collective MP removal is shown below in Figure 1. The regulatory criterion requires a minimum of 80wt% average removal of 13 MPs from the wastewater sample. The data demonstrates that CarbPure T removes greater than 80wt% MPs at concentrations beginning at 10mg/L PAC dosage. Again, the AC structure-property-function relationship was intentionally designed to allow the various sized MPs to be effectively captured through a variety of pore size distributions made available (the required transportation pores and micropores). Additionally, likely favorable non-covalent interactions (Van der Waal forces and induced dipole interactions) between the molecular structures of the MPs and the AC pore surfaces further enabled efficient sequestration.
Breaking down the individual removals for each of the MPs (Figure 2), we see a similar removal trend for CarbPure T, whereby > 80wt% MP elimination at 10mg/L PAC concentrations was observed for the majority of the tested MPs.
It is well known that high specific surface area and favorable pore structure of PACs contribute to effective contaminant sequestration. Equally, surface chemistry of the PACs and MPs also plays key roles in adsorption mechanisms and behaviors – each will be explored with examples below.
a) A correct balance of mesopores and micropores are required to transport and capture the MPs in discussion which have a diameter range 5.8Å – 11.7Å. The micropores adsorb and sequester the contaminants on the carbon surface whereas the mesopores promote the appropriate diffusion of the contaminants within the AC material. Given the higher number of transport pores (small mesopores) present within CarbPure T in comparison to CarbPure M (Table 2); it is evident why the former PAC demonstrates a superior capture and removal of the MPs.
b) Both the molecular weight and solubility of the contaminants play a pivotal role in their removal. For example, high molecular weight compounds with low/poor water solubilities are more effectively adsorbed by the respective PACs. Comparing the performances of CarbPure T with CarbPure M, we see three instances where the CarbPure M underperforms, namely for candesartan, hydrochlorothiazide and venlafaxine removal. All three MPs have a good level of solubility in water in addition to their relatively small molecular sizes; making them less effectively adsorbed by CarbPure M. Contrary to this phenomenon, CarbPure T outperforms in the removal of these three MPs for the reasons discussed in d) below.
c) pKa of the contaminant; the adsorbent surface charge plays a role in affecting the adsorption mechanism. With the exception of diclofenac and Irbesartan (weak acids), the remaining MPs have a higher pka – indicative of very weak disassociation in solution. Therefore, higher pKa MP contaminants are harder to adsorb onto the PACs through coulombic interactions when just considering the pka parameter. This can explain why CarbPure M removed both diclofenac and Irbesartan more effectively than candesartan, hydrochlorothiazide and venlafaxine.
d) The association/disassociation of surface functional groups would determine the density of surface charge for electrostatic interactions and reactive sites for chemical interactions (eg ligand exchange) between the PAC surface and the specific MP contaminants. Considering the surface chemistries of the MPs; one notable commonality we see amongst the majority is the presence of amine functional groups (Table 1). Under slight acidic conditions, the nitrogen atom can easily protonate resulting in a positively charged ammonium moiety (cationic formation). It is well known that the adsorption of the cationic organics depends on the acid oxygen containing surface functional groups, whereas, adsorption of non-ionic or anionic organics is closely correlated with surface basicity and oxygen free Lewis basic sites.
CarbPure T has a lower pH (8.7) than CarbPure M (11.3) due to controlled acid-washed components present in the former product (Table 2); yielding acid oxygen-containing functional groups on the carbon surface. This enables CarbPure T to exhibit more surface acidity (carboxylic, phenolic, lactonic groups) but less surface basicity than CarbPure M - in turn, promoting superior capture with the cationic amines via electrostatic interactions. We believe this is the primary reason why Arq CarbPure T is so effective in removing the majority of the MPs (examples include candesartan, hydrochlorothiazide and venlafaxine).
e) The surface chemistries along with the molecular structural orientation within the contaminants influence the contact and capture with the respective PAC. The presence of unhindered polar heteroatoms (O, Cl, F) have a higher tendency to attract indiscriminately to the PAC surfaces through favorable non-covalent interactions as seen for citalopram, carbamazepine and metoprolol (Table 1). The key factor is steric hindrance which can aid or hinder effective binding as seen in the case for hydrochlorothiazide removal. The chlorine atom is sufficiently hindered by the adjacent amino sulfonate group preventing effective bonding to CarbPure M, resulting in overall poor removal. In contrast, diclofenac (having a similar size and pKa) has two unhindered chlorine atoms allowing improved capture and greater removal by the same CarbPure M.
Furthermore, unhindered heterocycles and longer chain aliphatic groups promote hydrogen bonding with the PAC as seen for the likes of clarithromycin and amilsulpride. It is well known that aromatic structures offer rigidity and at times prevent the effective bonding of adjacent heteroatoms to the PAC.
Summary
This study demonstrates the effective removal of MPs using Arq’s PACs in line with the EU’s 80wt% elimination requirement. Employing a rigorous characterization of the physiochemical properties and solution behavior of the MP contaminants whilst applying this fundamental understanding to design the PAC’s porosity and surface to enhance their affinity to the MPs are keys to enhancing removal efficacy. In the current European wastewater treatment market where fourth stage treatments will be mandatory in the years to come, designing highly effective ACs specifically to supplement these treatments and meet these stringent regulatory requirements will be required.
Appendix A: MP elimination
PACs (CarbPure T) has been designed with appropriate mesopores, micropores and additional formulation components which possess specific characteristics of surface chemistry (eg surface acidity/basicity and functional groups), which make them better-suited for this particular class of MP contaminants (eg cationic, anionic, or molecular forms).
About the author
This article was written by Rumman Ahmed, Micala Mitchek, Ariel Li, Robert Huston and Joe Wong of Arq Inc, and Richard Arndt, Robert Brzosa, Carsten Schledorn of LSR/UNICARB.
A shorter version of this article first appeared in the February 2025 issue of Filtration+Separation magazine. To read the full issue, click here
