Zwitterionic Biochar Boosts Antibiotic Removal in Municipal Wastewater
Antibiotic contamination in discharged water keeps pushing membrane science toward materials that can do more than simply filter particles out of a stream. This study centers on a zwitterionic biochar additive embedded in a polyethersulfone and sulfonated poly(arylene ether sulfone) composite membrane, with one clear goal: improve removal of tetracycline from contaminated municipal wastewater without sacrificing permeability or stability.
The membrane was built through a phase-inversion process, a familiar fabrication route in separation science but one that can deliver very different outcomes depending on how pore formation, surface chemistry, and polymer compatibility are tuned. Here, the biochar was not used as a passive filler. Its functionalized surface altered the matrix in ways that changed morphology, pore structure, hydrophilicity, surface charge, and the membrane’s thermal and mechanical behavior. Those shifts matter because tetracycline is not a simple neutral solute; its interaction with a membrane varies with charge state and feed chemistry, which means surface design can determine whether the molecule is repelled, adsorbed, or carried through the pores.
The best-performing composite achieved 99.1% tetracycline removal at a permeation flux of 64.1 L m−2 h−1. In membrane terms, that combination is the central balancing act: high selectivity without collapsing throughput. A membrane that rejects nearly all target antibiotic while still moving water at a practical rate is closer to deployable treatment logic than a material that performs well only under ideal laboratory conditions.
Performance Held Up in Mixed Antibiotic Streams and Real Effluent
The strongest signal in the study is not the single-number removal result under standard conditions. It is the way performance persisted when the feed became more complicated. In mixed-antibiotic solution, removal remained at 98.5%, and in antibiotic-loaded real municipal wastewater it reached 94.8%. Real wastewater introduces dissolved organics, salts, and a much less predictable chemical environment than pure test solutions. Many membrane materials look excellent in clean feeds and lose much of that advantage once fouling compounds and competing solutes enter the system.
The membrane also showed strong antifouling behavior, long-term operation, and high rejection of natural organic matter. In practical water treatment, those traits often determine whether a membrane is useful over weeks and months rather than only over a short bench-scale run. Fouling resistance reduces the rise in transmembrane resistance, slows flux decline, and cuts the maintenance burden tied to cleaning cycles. In other words, the membrane’s surface chemistry appears to have done more than improve one analyte’s rejection; it also helped preserve the operating window needed for continuous treatment.
One sentence captures the operational importance: the membrane kept working when the feed stopped being simple.
pH Shifted the Separation Pathway
Feed pH had a pronounced effect on tetracycline separation. Under acidic and alkaline conditions, removal rose to 99.9%, accompanied by a flux of 71.8 L m−2 h−1 at pH 3. That pH dependence fits the chemistry of tetracycline and the membrane surface. As pH changes, tetracycline changes its ionization state, and the membrane’s surface charge environment also shifts how strongly the molecule is attracted or repelled. In a composite with zwitterionic and sulfonated features, electrostatic interactions can be layered on top of hydrogen bonding and pore-sieving effects, creating a separation pathway that is sensitive to the solution environment rather than fixed by size exclusion alone.
This kind of behavior is useful because it reveals the separation mechanism, not just the end result. If the membrane gains efficiency when the feed chemistry pushes the target compound into a more interaction-prone state, then designers can think more explicitly about tuning pH, surface charge density, and polymer composition for specific wastewater scenarios.
DFT Analysis Supported Stronger Membrane–Antibiotic Binding
Density functional theory calculations reinforced the experimental data by showing enhanced tetracycline–membrane interaction in the PES/SPAES/ZBC system. The calculations pointed to a reduced energy gap, greater electron-transfer capacity, and stable tetracycline-bound configurations. In separation materials, that type of computational support is valuable because it helps connect a macroscopic performance result to molecular behavior at the membrane interface.
The computational findings suggest that the nanofiller did not merely roughen the membrane or alter pore architecture. It appears to have created a surface environment where tetracycline could interact more strongly and more persistently with the membrane material itself. That insight may help explain why the composite retained strong rejection in mixed solutions and real municipal wastewater, where competition from other dissolved species would otherwise weaken selectivity.
What This Composite Adds to Wastewater Treatment Design
The membrane platform matters because antibiotic pollution is not a theoretical environmental concern. When antibiotic residues persist in discharged water, they raise pressure on downstream ecosystems and complicate contamination control strategies. Materials that can remove these compounds efficiently while remaining mechanically and chemically robust are directly relevant to municipal treatment systems and advanced polishing steps.
This study positions PES/SPAES blended with functionalized zwitterionic biochar as a promising membrane architecture for antibiotic removal, especially where fouling resistance and sustained flux are essential. The most useful takeaway is not that a single laboratory membrane reached a high rejection value. It is that the material held its performance across a mixed-antibiotic solution, real wastewater, and changing pH conditions, which is the kind of variability that exposes weak designs quickly.
The remaining question is not whether the membrane can reject tetracycline under controlled conditions. The larger issue is how this architecture will scale under prolonged field operation, with variable wastewater composition, cleaning cycles, and the full mechanical demands of plant-level deployment. Those data are not here yet, and they will determine whether this material stays a laboratory success or becomes a practical treatment option.