Diffusional Properties of Methanogenic Granular Sludge: ¹ H NMR Characterization

Abstract

The diffusive properties of anaerobic methanogenic and sulfidogenic aggregates present in wastewater treatment bioreactors were studied using diffusion analysis by relaxation time-separated pulsed-field gradient nuclear magnetic resonance (NMR) spectroscopy and NMR imaging. NMR spectroscopy measurements were performed at 22°C with 10 ml of granular sludge at a magnetic field strength of 0.5 T (20 MHz resonance frequency for protons). Self-diffusion coefficients of H 2 O in the investigated series of mesophilic aggregates were found to be 51 to 78% lower than the self-diffusion coefficient of free water. Interestingly, self-diffusion coefficients of H 2 O were independent of the aggregate size for the size fractions investigated. Diffusional transport occurred faster in aggregates growing under nutrient-rich conditions (e.g., the bottom of a reactor) or at high (55°C) temperatures than in aggregates cultivated in nutrient-poor conditions or at low (10°C) temperatures. Exposure of aggregates to 2.5% glutaraldehyde or heat (70 or 90°C for 30 min) modified the diffusional transport up to 20%. In contrast, deactivation of aggregates by HgCl 2 did not affect the H 2 O self-diffusion coefficient in aggregates. Analysis of NMR images of a single aggregate shows that methanogenic aggregates possess a spin-spin relaxation time and self-diffusion coefficient distribution, which are due to both physical (porosity) and chemical (metal sulfide precipitates) factors.

Magnetic resonance microscopy of iron transport in methanogenic granules

“…Note that the cracks in the core can develop, e.g. due to biomass decay during a starvation period [14] and are thus not primarily formed to facilitate transport processes within the biofilm matrix. The observation of that one or two cracks per a biofilm sample that did not influence the metal transport was also reported by Phoenix and Holmes [24] in phototrophic biofilm suggesting that the development of these cracks is a common feature for biofilms.…”

“…D ejf of water has been previously investigated in comparable methanogenic granules as used in the present study and it ranged from 70% to 80% of D ejf in free water [14], suggesting that the D ejf of metals in biofilms is restricted more than the D ejf of water. The value of D ejf in biofilms depends on the physical-chemical properties of both the solute and biofilm [35].…”

“…Magnetic Resonance Imaging (MRI) can measure metal transport in porous matrixes non-destructively and non-invasively based on changes in spin-lattice (T 1 ) or spin-spin (T 2 ) relaxation times due to the presence of paramagnetic metal ions [8][9][10][11][12][13]. Although MRI measurements have often been applied to reveal structural and diffusional properties of microbial biofilms [14][15][16][17][18][19], only a few studies have focused on metal-biofilm interactions [20][21][22][23][24]. A pioneering study was presented by Nestle and Kimmich [25], measuring metal sorption in well defined, highly homogeneous alginate gels and immobilized cells.…”

Nickel and cobalt sorption on anaerobic granular sludges: kinetic and equilibrium studies

“…Magnetic resonance imaging (MRI) experiments allow the reaction-diffusion process of paramagnetic materials to be studied in a nondestructive manner with high spatial (several 10 µm) and temporal (several min) resolution. 50,53 …”

NMR investigation of water diffusion in different biofilm structures

“…Diffusion strongly depends on biofilm systems, growth conditions, and biofilm geometries. For example, in methanogenic granular sludge internal heterogeneities significantly influenced the mass transfer (Lens et al, ). Strong correlations between the diffusion of substrates and biofilm parameters have been reported (Bishop, Zhang, & Fu, ; Debeer, Stoodley, Roe, & Lewandowski, ; Stewart, ).…”