The effects of LMWOAs on biodegradation of multi-component PAHs in aqueous solution using dual-wavelength fluorimetry

Quantitatively modeling of tetracycline photodegradation in low molecular weight organic acids under simulated sunlight irradiation

As the ubiquitous active components in aquatic environments, low molecular weight organic acids (LMWOAs) have a large influence on the environmental behaviors of contaminants. This research was focused on the effect of different LMWOAs including 11 aliphatic acids and 7 aromatic acids on the photodegradation kinetics of tetracycline (TC), and the development of quantitative structure-activity relationship (QSAR) model. Results showed that TC photodegradation in the presence of LMWOAs fitted pseudo-first-order photolysis kinetics, and the observed photolysis rate constant (k_obs) varied from 0.077 to 0.331 h^-1. The QSAR model was developed by partial-least-squares (PLS) with using a sequential approach with 25 theoretical molecular descriptors. Four descriptors including E_LUMO-E_HOMO, E_LUMO, CCR and Q_max were found to mechanistically and statistically affect k_obs. The high cross validated regression coefficient (Q_cum², 0.898) and high correlation coefficient (R², 0.908) indicated significantly goodness-of-fit and high robustness of the model. The predicted and observed values with high agreement in the defined applicability domain featured accuracy and feasibility of model. This work provided a robust predictive method for estimating the TC photodegradation in the presence of different structures of LMWOAs.

Root exuded low-molecular-weight organic acids affected the phenanthrene degrader differently: A multi-omics study

As a class of highly toxic and persistent organic pollutants, polycyclic aromatic hydrocarbons (PAHs) are an increasingly urgent environmental problem. Low-molecular-weight organic acids (LMWOAs) are important factors that regulate the degradation of PAHs by plant rhizosphere microorganisms, which affect the absorption of PAHs by plant roots. However, the comprehensive mechanisms by which LMWOAs influence the biodegradation of PAHs at cellular and omics levels are still unknown. Here, we systematically analyzed the roles of citric, glutaric and oxalic acid in the PAH-degradation process, and investigated the mechanisms through which these three LMWOAs enhance phenanthrene (PHE) biodegradation by B. subtilis ZL09-26. The results showed that LMWOAs can improve the solubility and biodegradation of PHE, enhance cell growth and activity, and relieve membrane and oxidative stress. Citric acid enhanced PHE biodegradation mainly by improving the strain's cell proliferation and activity, while glutaric and oxalic acid accelerated PHE biodegradation mainly by improving the expression of enzymes and providing energy for the cells of B. subtilis ZL09-26. This study provides new insights into rhizospheric bioremediation mechanisms, which may enable the development of new biostimulation techniques to improve the bioremediation of PAHs.

Low-molecular-weight organic acids impede the degradation of naphthol in iron oxides/persulfate systems: Implications for research experiments in pure conditions

Naphthols are industrial contaminants occurring widely in soils and waters. Remediation of organic pollutants can be done by chemical oxidation using persulfate. However, most research experiments testing degradation of organic pollutants have been done in ideal conditions, e.g. using a pure compound in pure water, and thus are weakly representative of real natural conditions where pollutants occur in complex mixtures of numerous organic compounds. Therefore we tested here the effect of the presence of small organic acids, as typical compounds occurring in natural media, on the degradation of 1-napthol with persulfate and iron oxides. Results show that organic acids decreased naphthol transformation by 3.7% for malic acid, 53.2% for tartaric acid, 72.3% for citric acid and 77% for oxalic acids, in a magnetite/persulfate system during 10 h. Meanwhile, the dissolved Fe species increased gradually with the reaction time; the highest concentration of Fe ions reached to ∼18 μM L^-1 in aqueous phase. Electron paramagnetic resonance technique was applied to determine reactive oxygen species (ROS). The spin density of ·OH, detected as the main ROS, decreased initially, followed by gradually increase, suggesting that organic acids might inhibit the degradation of 1-naphthol by competing with ·OH. These findings disclose the high inhibition of the transformation by organic acids, and thus, more generally, imply that studies using only pure contaminants are weakly representative for remediation of real, natural samples.

Influence of root components of celery on pyrene bioaccessibility, soil enzymes and microbial communities in pyrene and pyrene-diesel spiked soils

Though phytoremediation is deemed as a promising approach to restore polycyclic aromatic hydrocarbon (PAHs) contaminated sites, studies about how the biodegradation of PAHs is enhanced still remains incomprehensive. Effects of root components on pyrene bioaccessibility, soil enzymes and microbial communities were explored in the paper, and their interactions in simulated pyrene and pyrene-diesel spiked microcosms were tried to give a reasonable explanation. Results indicated that root components enhanced the pyrene removal of bioaccessible and adsorbed fractions by 16.10 and 1.80mgkg^-1, respectively, in pyrene-spiked soils at the end of the experiment. By contrast, root components increased the degradation of bioaccessible fraction by only 3.3mgkg^-1 in pyrene-diesel spiked soils. Although the bound fractions of pyrene increased over time in treatments without root components, they remained relatively stable, ranging from 0.02 to 0.03mgkg^-1, in root components amended treatments. Activities of soil enzymes (polyphenol oxidase, catalase, invertase, urease and alkaline phosphatase) varied differently in response to pollutants and root components. Analysis of phospholipid fatty acids revealed that root components increased the biomass of soil microorganisms and altered the microbial structure. Pearson correlation analysis proved positive correlations between all the microbial subgroups and pyrene removal in pyrene-spiked soils, but the degradation of bioaccessible pyrene was only positively related with microorganisms confirmed by monounsaturated fatty acids in pyrene-diesel spiked soils.

Effects of root exudates on gel-beads/reeds combination remediation of high molecular weight polycyclic aromatic hydrocarbons

Changes in root exudates, including low molecular weight organic acids (LMWOAs), amino acids and sugars, in rhizosphere soils during the gel-beads/reeds combination remediation for high molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs) and the degree of the effects on HMW-PAH biodegradation were evaluated in this study. The results showed that the gel-beads/reeds combination remediation notably increased the removal rates of pyrene, benzo(a)pyrene and indeno(1,2,3-cd)pyrene (65.0-68.9%, 60.0-68.5% and 85.2-85.9%, respectively). During the removal of HMW-PAHs, the LMWOAs, particularly maleic acid, enhanced the biodegradation of HMW-PAHs. Arginine and trehalose monitored in reed root exudates promoted the growth of plants and microorganisms and then improved the removal of HMW-PAHs, especially pyrene. However, the contribution of reed root exudates on degradation of 5- and 6-ring PAHs was minor. These results indicated that the utilization of root exudates was certainly not the only important trait for the removal of HMW-PAHs.

In situ visualization and quantitative investigation of the distribution of polycyclic aromatic hydrocarbons in the micro-zones of mangrove sediment

The distribution of polycyclic aromatic hydrocarbons (PAHs) in the micro-zones of mangrove sediment is a predominant factors determining PAH bioavailability. In this study, a novel method for the in situ visualization (via microscope) and quantitative investigation of the PAH distribution in the micro-zones of mangrove sediment was established using microscopic fluorescence spectral analysis combined with derivative synchronous fluorescence spectroscopy (MFSA-DSFS). The MFSA-DSFS method significantly suppressed the background fluorescence signal of the sediment (the S/N values increased by over two orders of magnitude). The proportion of the nonpolar organic carbon content in the particulate organic matter (POM) rather than its content in the total organic matter (TOM) showed a significantly positive correlation with the uneven PAH distribution (Relative DC-M values) evaluated using the established method (p < 0.05). The extent of the uneven PAH distribution in the micro-zones of aged sediment was higher than that in the spiked sediment. Moreover, the distribution pattern of the PAHs within the mangrove sediment changed to become more homogeneous in the presence of low-molecular-weight organic acids (LMWOAs), which primarily contribute to increasing the POM content.

Biodegradation of persistent organics can overcome adsorption-desorption hysteresis in biological activated carbon systems

In Biological Activated Carbon (BAC) systems, persistent organic pollutants can be removed through a combination of adsorption, desorption and biodegradation. These processes might be affected by the presence of other organics, especially by the more abundant easily-biodegradable organics, like acetate. In this research these relations are quantified for the removal of the persistent pharmaceutical metoprolol. Acetate did not affect the adsorption and desorption of metoprolol, but it did greatly enhance the metoprolol biodegradation. At least part of the BAC biomass growing on acetate was also able to metabolise metoprolol, although metoprolol was only converted after the acetate was depleted. The presence of easily-degradable organics like acetate in the feeding water is therefore beneficial for the removal of metoprolol in BAC systems. The isotherms obtained from metoprolol adsorption and desorption experiments showed that BAC systems are subject to hysteresis; for AC bioregeneration to take place the microbial biomass has to reduce the concentration at the AC-biomass interface 2.7 times compared to the concentration at which the carbon was being loaded. However, given the threshold concentration of the MET degrading microorganisms (<0.08 μg/L) versus the average influent concentration (1.3 μg/L), bioregeneration is feasible.

Determination of the Residual Anthracene Concentration in Cultures of Haloalkalitolerant Actinomycetes by Excitation Fluorescence, Emission Fluorescence, and Synchronous Fluorescence: Comparative Study

Polycyclic aromatic hydrocarbons (PAHs) are compounds that can be quantified by fluorescence due to their high quantum yield. Haloalkalitolerant bacteria tolerate wide concentration ranges of NaCl and pH. They are potentially useful in the PAHs bioremediation of saline environments. However, it is known that salinity of the sample affects fluorescence signal regardless of the method. The objective of this work was to carry out a comparative study based on the sensitivity, linearity, and detection limits of the excitation, emission, and synchronous fluorescence methods, during the quantification of the residual anthracene concentration from the following haloalkalitolerant actinomycetes cultures Kocuria rosea, Kocuria palustris, Microbacterium testaceum, and 4 strains of Nocardia farcinica, in order to establish the proper fluorescence method to study the PAHs biodegrading capacity of haloalkalitolerant actinobacteria. The study demonstrated statistical differences among the strains and among the fluorescence methods regarding the anthracene residual concentration. The results showed that excitation and emission fluorescence methods performed very similarly but sensitivity in excitation fluorescence is slightly higher. Synchronous fluorescence using Δλ = 150 nm is not the most convenient method. Therefore we propose the excitation fluorescence as the fluorescence method to be used in the study of the PAHs biodegrading capacity of haloalkalitolerant actinomycetes.

In situ determination of the depuration of three- and four-ringed polycyclic aromatic hydrocarbons co-adsorbed onto mangrove leaf surfaces

A dual-wavelength fiber-optic fluorimetry for the in situ simultaneous determinations of fluorene (Flu), phenanthrene (Phe) and pyrene (Pyr) adsorbed onto the leaf surfaces of living Avicennia marina (Am) seedling were developed and used to study the depuration kinetics of the three PAHs, adsorbed individually or mixed together, onto living Am leaf surfaces. Limits of detection for the in situ measurements of adsorbed Flu, Phe and Pyr were 4.62, 2.75 and 1.38 ng spot(-1), respectively. The depuration kinetics of the three selected polycyclic aromatic hydrocarbons (PAHs) are divided into rapid and slow phases; both phases followed the same first-order kinetics with relative clearance rates of Flu > Phe > Pyr during the rapid phase, and a clearance rate order of Pyr > Flu > Phe during the slow phase. For the three PAHs co-adsorbed on living Am leaf surfaces, a significant synergistic effect was detected during the rapid phase clearance; conversely, an antagonistic effect was observed during the slow phase. However, the synergistic effect dominated during both phases of the depuration process, and the co-adsorption of PAHs promoted the clearance of all three compounds from the mangrove leaf surfaces. These findings demonstrate a novel analytical method for in situ characterization of multiple PAHs adsorbed onto the plant surfaces.

Effects of low molecular-weight organic acids and dehydrogenase activity in rhizosphere sediments of mangrove plants on phytoremediation of polycyclic aromatic hydrocarbons

This work evaluated the roles of the low-molecular-weight organic acids (LMWOAs) from root exudates and the dehydrogenase activity in the rhizosphere sediments of three mangrove plant species on the removal of mixed PAHs. The results showed that the concentrations of LMWOAs and dehydrogenase activity changed species-specifically with the levels of PAH contamination. In all plant species, the concentration of citric acid was the highest, followed by succinic acid. For these acids, succinic acid was positively related to the removal of all the PAHs except Chr. Positive correlations were also found between the removal percentages of 4-and 5-ring PAHs and all LMWOAs, except citric acid. LMWOAs enhanced dehydrogenase activity, which positively related to PAH removal percentages. These findings suggested that LMWOAs and dehydrogenase activity promoted the removal of PAHs. Among three mangrove plants, Bruguiera gymnorrhiza, the plant with the highest root biomass, dehydrogenase activity and concentrations of LMWOAs, was most efficient in removing PAHs.