Molecular-weight-dependent effects of humic acid on hexavalent chromium accumulation and reduction in green microalgae.

Dissolved organic matter (DOM) in natural waters comprises a highly heterogeneous mixture of complex compounds that coexist with microalgae and critically regulate metal uptake, yet the molecular-weight-dependent effects of DOM on algal metal detoxification remain largely unresolved. Humic acid (HA), the dominant DOM component, is known to influence microalgal heavy-metal removal, but its specific molecular-weight role in the migration and transformation of low concentrations of hexavalent chromium [Cr(VI)] has not been elucidated. In this study, binary ("algae-HA molecular weight") and ternary ("algae-HA molecular weight-Cr(VI)") systems were constructed using Chlorella vulgaris and four HA fractions (0.45 μm-30 kDa, 10-30 kDa, 5-10 kDa, and <5 kDa). Results revealed that at 0.5 mg/L Cr(VI), the HA fractions exerted distinct regulatory effects on algal physiology and Cr behavior. Decreasing HA molecular weight substantially enhanced total Cr removal by 5.4%-29.9%, driven predominantly by extracellular adsorption (75.78%-80.85%). The < 5 kDa fraction exhibited the highest bioavailability, significantly promoting algal growth, metabolic activity, and Cr uptake. This fraction achieved the maximum total Cr removal (48.46%) and Cr(VI) reduction (60.75%), facilitated by strong complexation of carboxyl and hydroxyl groups with algal surface sites. Extracellular polymeric substances (EPS) accounted for 14.50% of Cr removal and reduced Cr(VI) by 78.25%, revealing a synergistic adsorption-reduction pathway. This study provides mechanistic evidence that HA molecular weight governs microalgal Cr(VI) adsorption, reduction, and sequestration, offering a new conceptual framework for optimizing microalgae-DOM systems in heavy-metal remediation.