Abstract
Redox processes in groundwater play an important role in bioavailability, toxicity, and mobility of redox-active elements and contaminants. A recent study has demonstrated that low-molecular-weight fraction (LMWF) of humic substances with great number of redox-active functional groups (RAFGs) exhibits great reducing capacity. However, whether LMWF of natural organic matter (NOM) exhibits high redox capacity still remains unclear. Therefore, this study extracted Pahokee peat NOM (PPNOM) and Leonardite NOM (LNOM) from soils, and then LMWFs in these NOMs were collected using a dialysis method. Electron exchange capacities (EEC) and RAFGs of LMWF NOMs at different Eh were analyzed using a novel electrochemical method and a three-dimensional excitation emission fluorescence (3DEEM) spectroscopy. We found that the reducing capacity in LMWF PPNOM was approximately 5-6 times higher than the bulk NOM, while only 7.8% LMWF PPNOM was accounted for in the bulk NOM. An increasing in EEC (EAC + EDC, where EAC is the electron accepting capacity and EDC is the electron donating capacity) of LMWF PPNOM and LNOM with Eh reduced from −0.49 V to −0.69 V. Additionally, an obvious increase in fluorescent intensities of quinone-like fluorophores before and after being reduced LMWF LNOM is responsible for high EAC of LMWF LNOM. These findings provide a better understanding of relationship between RAFGs Eh in LMWF of NOM, further helping in predicting and protection of groundwater environment and fate of transformation and transport for redox-active contaminants in groundwater.
Highlights
Natural organic matters, as the most important redox-active compounds, participate in transformation and transport of redox-active elements and contaminants in biogeochemical processes in groundwater systems [1,2,3,4]
Dialysis experiments were conducted within 5 days to collect different molecular weight fractions (3500 low-molecular-weight fraction (LMWF), 14000 LMWF, 3500 retentate, and 14000 retentate from bulk Pahokee peat NOM (PPNOM) and Leonardite NOM (LNOM)), and the results on total organic carbon TOC content among different LMWFs are shown in Table 1. e TOC results of LMWF natural organic matter (NOM) showed a continuous increase over the dialysis process, which are in agreement with our previous studies that the continuously released LMWF humic substances (HSs) results in an increase in TOC content [19, 26]
4.1–7.8% LMWF NOMs of bulk LNOM and PPNOM, respectively, were obtained at the end of dialysis, which was obviously higher than only 2% LMWF HA of total bulk PPHA and LHA shown in our previous study. e results suggested that NOM owned high LMWF NOM, and these LMWF NOMs occupied higher carbon contents than HA
Summary
As the most important redox-active compounds, participate in transformation and transport of redox-active elements and contaminants in biogeochemical processes in groundwater systems [1,2,3,4]. The chemical speciation, bioavailability, toxicity, and mobility of these redox-active compounds coupled with degradation of natural organic matter (NOM) [5, 6] are dependent on redox conditions. NOM can mediate electron transfer during the biogeochemical processes including microbial reduction of Fe(III) hydroxides [2, 7, 8] processed with immobilization of arsenic and phosphate [9]. E capacities of electron charging and recharging processes of NOM is related to redox conditions (Eh) [10] and redox-active functional groups (RAFGs). E extent and rate of stimulated microbial Fe[III] reduction is relating to redox property of NOM. Previous electrochemical studies have revealed redox properties of NOM in varied redox potential range of −0.9 V–+1.0 V, which significantly affects transformation and electron transfer rates of pollutants in soil environments [28]
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