The influence of co-existing dyes on the adsorption of polystyrene nanoplastics by activated biochar and Fe/Mg-layered double oxide composite.

The coexistence of nanoplastics (NPs) and dyes in industrial wastewater creates complex interactions that affect NP adsorption onto adsorbents, yet the mechanisms remain unclear. Batch adsorption experiments used polystyrene nanoplastics (PSNPs) with cationic crystal violet (CV) or anionic methyl orange (MO) on high-surface-area activated biochar (BC) and metal-oxide-functionalized Fe/Mg layered double oxide composite (LDO@BC). For MO, PSNP adsorption on BC was completely suppressed, whereas the Langmuir-fitted maximum PSNP adsorption capacity on LDO@BC increased from 52.53 to 71.94 mg g-1 (+37.0%). On LDO@BC, the effect of MO shifted from significant inhibition at ≤50 mg L-1 PSNPs to significant promotion at ≥75 mg L-1, indicating a competitive-to-synergistic transition between 50 and 75 mg L-1; this transition was not observed on BC. XPS analysis indicates that MO complexation with metal-oxygen bonds may activate new adsorption sites on LDO@BC. In isotherm experiments, the Langmuir-fitted PSNP adsorption capacity on BC increased from 7.47 to 22.62 mg g-1 with CV (+202.8%), while that on LDO@BC increased from 52.53 to 62.11 mg g-1 (+18.2%), showing that enhancement magnitude also depends on the adsorbent. Pore filling, hydrogen bonding, π-π interactions, and M-O coordination are the governing mechanisms. Both materials maintain stable adsorption performance in real water matrices, with LDO@BC showing particular robustness. The opposite effects of the same dye on two different adsorbents, and the concentration-dependent switch on LDO@BC, demonstrate that the impact of coexisting dyes is governed by both the dye type and the adsorbent surface chemistry, providing new insights for designing adsorbents for complex wastewater.