Supporting Information Interaction between Palladium-Doped Zerovalent Iron Nanoparticles and Biofilm in Granular Porous Media: Characterization, Transport and Viability

Palladium-doped nanoscale zerovalent iron (Pd-NZVI) can induce rapid transformations of a variety of pollutants during in situ remediation of contaminated aquifers. Pd-NZVI stabilized with polymeric surface modifiers has shown substantially improved stability and transport compared to bare Pd-NZVI in model subsurface granular media such as clean quartz sand. The natural subsurface environment is, however, much more complex. For example, biofilms may coat aquifer grain surfaces and thus alter the transport of these reactive nanoparticles. Herein, we compare the transport behavior of Pd-NZVI coated with carboxymethylcellulose or rhamnolipid as surface modifiers, in clean and biofilm-coated sand packed columns. Transport studies suggest that, for both types of nanoparticles tested, the particle attachment efficiency to the collector surface generally increases (up to 6 fold in NaCl and 26 fold in CaCl2) in the presence of biofilm. This result indicates the potential for reduced Pd-NZVI transport in a natural groundwater system. The retentive behavior of biofilm-coated media increases with particle–particle aggregation (e.g., C/C0 ~ 0.15 for a markedly aggregated CMC-coated Pd-NZVI in divalent salt), implicating physical straining as an important retention mechanism. Retained Pd-NZVI on biofilm-laden matrices was characterized using enhanced darkfield hyperspectral imaging. Assessments of biofilm viability imply that the retained surface-modified Pd-NZVI is non-toxic to the cells within the biofilm matrices (viability >95%). Moreover, the coating molecules do not negatively impact the viability of the biofilm bacteria.

Characterization of Pd-NZVI.The hydrodynamic diameter (z-average reported) and electrophoretic mobility (EPM) were determined using dynamic light scattering (DLS) and laser Doppler electrophoresis (ZetaSizer Nano ZS, Malvern).Measurements were carried out for both the influent suspensions injected into the column, and the effluent suspensions collected after two pore volumes.Both sizing and EPM measurements were done at least in triplicate for 2-4 independent samples per treatment at room temperature (~ 22°C).

Fig. S1
Biomass along the column length whereby 0-2 cm is the column influent and 6-8 cm is the column effluent after dissecting the column into four segments following equilibration with background electrolyte (10 mM NaCl or CaCl2).Error bars represent standard deviations of the replicate measurements.The average biomass (average of four segments in Fig. S1) is 0.62±0.17and 0.51±0.09mg/g sand, respectively, in 10 mM NaCl and 10 mM CaCl2 equilibrated column, and the biomass is homogeneously distributed; i.e., an inspection of Fig. S1 does not show spatial biomass variation along the column length.The data presented here is consistent with a previous study where uniform biomass coverage was also observed.For comparision, the αpc value in clean sand was adopted from our previous work. 1Nanoparticle physicochemical properties are included in Table S3.Table S3.Physicochemical properties of the Pd-NZVI used in column transport experiments at 1 g/L.Pd-NZVI elution through each column was calculated from the numerical integration of BTCs The value of elution in clean sand at 3-100 mM NaCl was adopted from our previous work 1 The error bars represent the range of elution occurred in replicate experiments where dc is the mean collector diameter, θ is the bed porosity, L is the packed-bed length, and η0 is the single-collector contact efficiency calculated using the Tufenkji-Elimelech equation 4 Fig. S1Biomass along the column length whereby 0-2 cm is the column influent and 6-8 cm is the column effluent after dissecting the column into four segments following equilibration with background electrolyte (10 mM NaCl or CaCl2).Error bars represent standard deviations of the replicate measurements.The average biomass (average of four segments in Fig.S1) is 0.62±0.17and 0.51±0.09mg/g sand, respectively, in 10 mM NaCl and 10 mM CaCl2 equilibrated column, and the biomass is homogeneously distributed; i.e., an inspection of Fig.S1does not show spatial biomass variation along the column length.The data presented here is consistent with a previous study where uniform biomass coverage was also observed.2

Fig. S3 Fig. S4 .
Fig. S3 Measured electrophoretic mobility (EPM) for the influent (a) RL-coated and (b) CMCcoated Pd-NZVI over a range of IS in monovalent (NaCl) and divalent (CaCl2) salt.Data represents the mean ± 95 % confidence interval.The dashed lines are included to guide the eye.

Fig. S5 .
Fig. S5.Measured breakthrough curves at different IS of NaCl in biofilm-coated sand for (a) RLcoated and (b) CMC-coated Pd-NZVI at 0.15 g/L.

Fig. S7
Fig. S7 Measured breakthrough curves at different IS of CaCl2 in (a, b) clean sand and (c, d) biofilm-coated sand for (a, c) RL-coated and (b, d) CMC-coated Pd-NZVI at 0.15 g/L.

Fig. S9
Fig. S9Growth normalized quantitative estimate of the biofilm formed at the end of growth period (24 hours) measured using crystal violet assay (OD570/OD600).

Fig. S11
Fig. S11 Representative Live/Dead fluorescence microscopy images of biofilm cells grown on 24-wells plate after 1 or 24 hour exposure with 1 g/L CMC-and RL-coated Pd-NZVI suspension, and with the supernatant (separated using super magnet).

Table S1 .
Summary of nanoparticle transport studies conducted using columns packed with biofilm-coated granular materials.

Table S2 .
Dynamic light scattering (DLS) measured hydrodynamic diameter of Pd-NZVI injected into the column experiments (influent suspension) and eluted from the column (effluent suspension).

Table S4 .
Summary of Pd-NZVI elution occurred through the columns packed with either clean or biofilm-coated sand.The particle concentration was 0.15 g/L.