Previous studies on reduction of Cr(VI) by humic acids (HAs) have seldom used the extracts from composted animal manure. For greater yields of HAs and resource reclamation of animal manure, cattle manure was used as the composting material in our study. The capacity of humic acids extracted from composted cattle manure (HAs cm ) to reduce Cr(VI) was tested under the influence of environmental factors (pH, illumination with light and dissolved oxygen). And the non-biological detoxification mechanism was investigated by using three-dimensional fluorescence, Fourier transform infrared spectroscopy (FTIR) and X-ray absorption near-edge structure (XANES) spectroscopy. The results indicated that Cr(III) after the reduction of Cr(VI) formed an outer sphere complex with -OH and inner sphere complex with carboxyl groups in HAs cm .
Excess sludge was used to produce volatile fatty acids (VFAs) by
fermentation, which has recently attracted much attention because
the sludge-derived VFAs were able to function as a kind of ideal carbon
source for wastewater biological treatment. This study specifically
targeted the mechanism behind the effects of humic acid (HA) on promoting
production of VFAs from excess sludge during acidogenic fermentation.
The HA was characterized using Fourier-transform infrared (FTIR) spectrum, 13C nuclear magnetic resonance (13C NMR), and 3D
excitation–emission matrix (3D-EEM). When HA was present at
1.20 g/g of total chemical oxygen demand, the content of VFAs produced
was maximal (1752 mg COD/L) and was about 2.15 times that of the control
without HA (815 mg/L). Mechanism exploration revealed that the dominant
population of microbial community in the sample with added HA had
shifted to Firmicutes, which was related to the acid production. More
importantly, the electron transfer between nicotinamide adenine dinucleotide
hydrogen (NADH) and nicotinamide adenine dinucleotide (NAD+) was enhanced by HA, contributing to improve the VFAs’ accumulation.
In contrast, the activities of the key enzymes, including acetic kinase
(AK) and phosphotransferase (PTA), in the acidogenic stage had little
change with HA addition, which meant that HA could not stimulate acidification
by the way of changing enzymatic activities.
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