Toxicity of nanodiamonds to white rot fungi Phanerochaete chrysosporium through oxidative stress.

Nanodiamonds (NDs) are produced with large scale and applied in many areas, thus the environmental impacts and hazards of NDs should be systematically investigated. In this study, we evaluated the interaction between detonation NDs and white rot fungus Phanerochaete chrysosporium and the impact on the fungus decompositions activities. NDs did not influence the biomass gain of P. chrysosporium and the culture medium pH values. The mycelia of P. chrysosporium were destroyed upon the direct contact with NDs, while the rest retained the fibrous structure. Ultrastructural observations suggested that small aggregates of NDs seldom entered the fungus cells, but the break of cell wall and the loss of cytoplasm were induced by NDs. Under both optical and electron microscopes, the aggregation of colloidal ND particles was observed, which was the possible reason of low toxicity. High concentrations of NDs inhibited the laccase activity and manganese peroxidase activity of P. chrysosporium, which led to the decrease of decomposition activity for pollutants. Colloidal ND particles were not well dispersed in sawdust degradation evaluations, so no inhibitive effect was observed for wood degradation. The toxicological mechanism of NDs was assigned to oxidative stress. The results collectively suggested that NDs had low toxicity to white rot fungi and could be applied safely. The colloid dispersion/aggregation of nanoparticles in biological systems should be carefully considered during the design of safe nanomaterials.

Carboxylated graphene oxide-chitosan spheres immobilize Cu2+ in soil and reduce its bioaccumulation in wheat plants.

Due to the strong interaction with pollutants and the huge adsorption capacity, graphene adsorbents are widely applied in water decontamination. However, graphene adsorbents are seldom used in soil remediation, because the adsorptive sites on graphene would be occupied by soil components. In this study, we prepared carboxylated graphene oxide-chitosan (GO-COOH/CS) spheres for the immobilization of Cu2+ from water and soil. The pores in GO-COOH/CS allowed the internal diffusion of Cu2+ solution, while they blocked the direct contact between the solid soil and the adsorptive sites on graphene sheets. Therefore, the high adsorption capacity of GO-COOH/CS spheres (78 mg/g) was largely retained for the soil Cu2+ fixation. The partition coefficient (PC) for Cu2+ adsorption onto GO-COOH/CS spheres was 4.2 mg/g/µM at Ce of 0.48 mg/L and qe of 31 mg/g, while the PC value decreased to 0.096 mg/g/µM at Ce of 91.4 mg/L and qe of 78 mg/g. At initial Cu2+ concentrations of 120 mg/L and lower, the fixation efficiencies were all higher than 99% and the corresponding free Cu2+ concentrations in leachates were lower than 1.0 mg/L. The Cu2+ fixation on GO-COOH/CS spheres largely reduced its bioaccumulation in wheat roots from 127.8 µg/g to 51.2 µg/g. The toxicity evaluations suggested that GO-COOH/CS spheres were of low toxicity to wheat seedlings and did not amplify the toxicity of Cu2+. The implications to the design of graphene adsorbents for soil remediation are discussed. Overall, our results collectively indicated that porous GO-COOH/CS spheres were high-performance adsorbents for the immobilization of Cu2+ to reduce Cu2+ bioaccumulation in plants.

Adsorptive decontamination of Cu2+-contaminated water and soil by carboxylated graphene oxide/chitosan/cellulose composite beads.

Graphene adsorbents have been applied to remove diverse pollutants from aqueous systems. However, the mechanical strength of most graphene adsorbents is low and the fragile graphene sheets are released into the environment. In this study, we prepared carboxylated graphene oxide/chitosan/cellulose (GCCSC) composite beads with good mechanical strength for the immobilization of Cu2+ from both water and soil. The proportional limit of GCCSC beads was 3.2 N, a much larger value than graphene oxide beads (0.2 N). The largest pressure for GCCSC beads recorded before brittle failure was 26 N. The Cu2+ adsorption capacity of GCCSC beads was 22.4 mg/g in aqueous systems at initial Cu2+ concentration of 40 µg/mL, which is competitive with many efficient adsorbents. The partition coefficient (PC) for the Cu2+ adsorption onto GCCSC beads was 1.12 mg/g/µM at Ce of 0.83 mg/L and qe of 14.3 mg/g. The PC decreased to 0.055 mg/g/µM at Ce of 26.0 mg/L and qe of 22.4 mg/g. The adsorption kinetics of Cu2+ on GCCSC beads were moderately fast and required approximately 3 h to reach equilibrium with a k2 of 0.0021 g/(mg·min). A lower temperature and higher pH slightly increased the adsorption capacity of GCCSC beads. The ionic strength did not influence the adsorption. The porous structure of GCCSC beads blocked the direct contact between soil and the graphene surface; thus, a high Cu2+ immobilization efficiency was achieved by GCCSC beads applied to soil. The implications for the design of high-performance graphene adsorbents for water and soil remediation are discussed.

Biotransformation of Pristine and Oxidized Carbon Nanotubes by the White Rot Fungus Phanerochaete chrysosporium.

Carbon nanomaterials are widely studied and applied nowadays, with annual production increasing. After entering the environment, the complete degradation of these carbon nanomaterials by microorganisms is proposed as an effective approach for detoxification and remediation. In this study, we evaluated the degradation of pristine multiwalled carbon nanotubes (p-MWCNTs) and oxidized multiwalled carbon nanotubes (o-MWCNTs) by the white rot fungus Phanerochaete chrysosporium, which is a powerful decomposer in the carbon cycle and environmental remediation. Both p-MWCNTs and o-MWCNTs were partially oxidized by P. chrysosporium as indicated by the addition of oxygen atoms to the carbon skeleton in the forms of C=O and O-H bonds. The fungal oxidation led to the shortening of MWCNTs, where precipitated o-MWCNTs showed more short tubes. During the transformation, the defects on the tubes became detached from the carbon skeleton, resulting in decreases of the ID/IG (intensity of D-band/ intensity of G-band) values in Raman spectra. The transformation mechanism was attributed to the enzymatic degradation by laccase and manganese peroxidase excreted by P. chrysosporium. The results collectively indicated that MWCNTs could be transformed by P. chrysosporium, but complete degradation could not be achieved in a short time period. The implications on the environmental risks of carbon nanomaterials are discussed.

Carboxylation as an effective approach to improve the adsorption performance of graphene materials for Cu2+ removal.

Graphene materials are high-performance adsorbents for water and soil remediation, whose oxygen containing groups bind to metal ions intensely. In this study, we prepared carboxylated graphene oxide (GO-OCH2COOH) sponge and investigated the adsorption behaviors of Cu2+ on it by both experimental and computational approaches. Carboxylation largely improved the adsorption capacity from 23.8mg/g for graphene oxide (GO) sponge to 93.8mg/g for GO-OCH2COOH. The efficient adsorption was due to the strong interaction between Cu2+ and carboxyl groups (especially in -OCH2COOH form) according to the density functional theory calculation, while epoxy and hydroxyl groups contributed lowly. The fast adsorption process was achieved within 30min, corresponding to a large k2 value of pseudo-second order model (0.061mg/g/min). The adsorption was spontaneous and exothermic according to thermodynamics analyses. The binding strength of Cu2+ on GO-OCH2COOH was so strong that pH and ionic strength had mild impact. The strong binding sites were not recyclable, but the weaker ones (more than 40%) could be regenerated by simple washing. Our results highlighted the importance of chemical design in graphene adsorbents and the potential of GO-OCH2COOH in heavy metal fixation from water and soil.

Fungal transformation of graphene by white rot fungus Phanerochaete chrysosporium.

The wide applications of graphene materials require the thorough investigation on their biosafety and environmental risks. Transformation of graphene materials is a fundamental issue in their environmental risk evaluations. The enzymatic degradation of graphene is widely reported using peroxidases, but the information on the fungal transformation of graphene is still unavailable. Herein, we incubated reduced graphene oxide (RGO) in the white rot fungus Phanerochaete chrysosporium culture system for 4 weeks and investigated the transformation of RGO by multiple techniques. P. chrysosporium efficiently added oxygen to RGO and decreased the its carbon contents accordingly. The ID/IG ratios of RGO showed statistically increases upon the transformation by P. chrysosporium according to Raman spectroscopy, suggesting the increase of defects on carbon skeleton. The negatively charged oxygen containing groups exfoliated the graphene sheets as indicated by the larger layer distance according to the X-ray diffraction spectra and the increased roughness under scanning electron microscopy. The transformation was more obvious in the RGO separated from the fungal balls than the precipitates in the culture medium. The mechanism of transformation was attributed to the enzymatic degradation by P. chrysosporium. The environmental implication of the fungal transformation of graphene materials and the potential of using fungi to reduce the environmental risks of graphene materials are discussed.

Toxicity of carbon nanotubes to white rot fungus Phanerochaete chrysosporium.

Carbon nanotubes (CNTs) are widely used in diverse areas with increasing annual production, thus the environmental impact of CNTs needs thorough investigation. In this study, we evaluated the effect of pristine multi-walled CNTs (p-MWCNTs) and oxidized multi-walled CNTs (o-MWCNTs) on white rot fungus Phanerochaete chrysosporium, which is the decomposer in carbon cycle and also has many applications in environmental remediation. Both p-MWCNTs and o-MWCNTs had no influence on the dry weight increase of P. chrysosporium and the pH value of culture system. The fibrous structure of P. chrysosporium was disturbed by p-MWCNTs seriously, while o-MWCNTs had litter influence. The ultrastructural changes were more evident for P. chrysosporium exposed to p-MWCNTs and only p-MWCNTs could penetrate into the cell plasma. The chemical composition of P. chrysosporium was nearly unchanged according to the infrared spectra. The laccase activity was suppressed by p-MWCNTs, while o-MWCNTs showed stimulating effect. The decoloration of reactive brilliant red X-3B was not affected by both CNT samples. However, serious inhibition of wood degradation was observed in the p-MWCNTs exposed groups, suggesting the potential threat of CNTs to the decomposition of carbon cycle. The implication to the environmental risks and safe applications of carbon nanomaterials is discussed.

Toxicity of Pristine and Chemically Functionalized Fullerenes to White Rot Fungus Phanerochaete chrysosporium.

Fullerenes are widely produced and applied carbon nanomaterials that require a thorough investigation into their environmental hazards and risks. In this study, we compared the toxicity of pristine fullerene (C60) and carboxylated fullerene (C60-COOH) to white rot fungus Phanerochaete chrysosporium. The influence of fullerene on the weight increase, fibrous structure, ultrastructure, enzyme activity, and decomposition capability of P. chrysosporium was investigated to reflect the potential toxicity of fullerene. C60 did not change the fresh and dry weights of P. chrysosporium but C60-COOH inhibited the weight gain at high concentrations. Both C60 and C60-COOH destroyed the fibrous structure of the mycelia. The ultrastructure of P. chrysosporium was changed by C60-COOH. Pristine C60 did not affect the enzyme activity of the P. chrysosporium culture system while C60-COOH completely blocked the enzyme activity. Consequently, in the liquid culture, P. chrysosporium lost the decomposition activity at high C60-COOH concentrations. The decreased capability in degrading wood was observed for P. chrysosporium exposed to C60-COOH. Our results collectively indicate that chemical functionalization enhanced the toxicity of fullerene to white rot fungi and induced the loss of decomposition activity. The environmental risks of fullerene and its disturbance to the carbon cycle are discussed.