Immobilization of Microcystin by the Hydrogel–Biochar Composite to Enhance Biodegradation during Drinking Water Treatment

Microcystin-LR (MC-LR), the most common algal toxin in freshwater, poses an escalating threat to safe drinking water. This study aims to develop an engineered biofiltration system for water treatment, employing a composite of poly(diallyldimethylammonium chloride)–biochar (PDDA–BC) as a filtration medium. The objective is to capture MC-LR selectively and quickly from water, enabling subsequent biodegradation of toxin by bacteria embedded on the composite. The results showed that PDDA–BC exhibited a high selectivity in adsorbing MC-LR, even in the presence of competing natural organic matter and anions. The adsorption kinetics of MC-LR was faster, and capacity was greater compared to traditional adsorbents, achieving a capture rate of 98% for MC-LR (200 μg/L) within minutes to tens of minutes. Notably, the efficient adsorption of MC-LR was also observed in natural lake waters, underscoring the substantial potential of PDDA–BC for immobilizing MC-LR during biofiltration. Density functional theory calculations revealed that the synergetic effects of electrostatic interaction and π–π stacking predominantly contribute to the adsorption selectivity of MC-LR. Furthermore, experimental results validated that the combination of PDDA–BC with MC-degrading bacteria offered a promising and effective approach to achieve a sustainable removal of MC-LR through an “adsorption–biodegradation” process.


Supplemental materials information used in the study
Dry MC-LR (500 g powder) was purchased from MilliporeSigma TM (Burlington, MA, United States), and was dissolved in methanol and then diluted using Milli-Q water to prepare stock solutions.Nodularin solution (10 g/mL in methanol) was obtained from Honeywell Fluka TM (Mexico City, Mexico) and applied as an internal standard of MC-LR analysis by UPLC-MS/MS.Humic acids represent a broad range of structurally complex compounds containing aliphatic, aromatic, and hydrophilic functional groups.The humic acids from different companies may have different characteristics.Therefore, Suwannee river humic acid (SRHA) and Fisher Scientific humic acid (FSHA) were purchased from the International Humic Substances Society and Fisher Scientific respectively to fully investigate the effects of humic acids on MC-LR adsorption.Moreover, flulvic acids represent smaller and less hydrophobic NOM compounds than humic acids, thus, Suwannee river fulvic acid (SRFA) was also purchased from the International Humic Substances Society as a representative of NOM.Pristine BC was supplied by Lewis Bamboo Inc. (Alabama, United States).Poly(diallyldimethylammonium chloride) (PDDA, 20 wt% in water, 1.04 g/mL and 600900 cP at 25C) was acquired from Sigma-Aldrich (St. Louis, MO, United States).All experiment solutions were prepared using deionized water (18.2MΩcm) (Milli-Q, Millipore).

Adsorption kinetics modeling
Adsorption kinetics of MC-LR by PDDA-BC were simulated using pseudo-second-order (Eq.S1) model.
where, q k (g/g) is the adsorption capacity of the adsorbent after equilibrium; q t (g/g) is the adsorption capacity of the adsorbent at time t (min); and K 1 (gg -1 min -1 ) is the pseudo-second-order rate constant.
(S2) 1 where, q i (mg/g) is the adsorption capacity of the adsorbent; ) is the Freundlich affinity coefficient; q m (mg/g) is the Langmuir maximum adsorption capacity; and n is the Freundlich linearity constant.

Biodegradation modeling
Biodegradation of MC-LR by Sphingopyxis sp.m6 was simulated using first-order kinetics model (Eq.S4). (S4) where, C i (mg/L) is the equilibrium concentration; C 0 (mg/L) is the initial concentration; and ) is the rate constant.

Generalized linear modeling
where, E(Y) is the MC-LR removal efficiency; g is the link function that specifies how the MC-LR removal relates to the two mechanisms, and identity link was used herein; x represents the MC-LR adsorption removal that is fitted to pseudo-second-order kinetic model; y represents the MC-LR biodegradation that is described by first-order kinetic model; x  y represents the interaction of adsorption and biodegradation for MC-LR removal; a, b, and c are the constants that are related to the contribution of each part to MC-LR removal.

Figure S7
Figure S7Geometry optimization of MC-LR using Dmol3 package.

Figure S9
Figure S9Geometry optimization of Suwannee river fulvic acid using Dmol3 package.

Figure S10
Figure S10 Optimized configurations of (a) electrostatic interaction, (b) hydrogen bonding, (c) - stacking (face to face), and (d) electrostatic interaction and - stacking (face to face) between SRFA -and PDDA-BC via DFT calculations at pH of approximately 6.

Table S1
Parameters of pseudo-second-order kinetics models for MC-LR adsorption.Table S2Parameters of Langmuir and Freundlich isotherm models for MC-LR adsorption.

Table S3
Comparison of Langmuir maximum adsorption capacity (q m ) and inverse of adsorption kinetics half-life (K 1 q k ) of different adsorbents for MC-LR adsorption.All kinetics experiments were conducted in MC-LR spiked pure water or Milli-Q water.