Cr(VI) Formation Related to Cr(III)-Muscovite and Birnessite Interactions in Ultramafic Environments

Chromium is abundantly and primarily present as Cr­(III) in ultramafic rocks and serpentine soils. Chromium­(III) oxidation involving chromite (FeCr₂O₄) via interactions with birnessite has been shown to be a major pathway of Cr­(VI) production in serpentine soils. Alternatively, Cr­(III)-bearing silicates with less Cr­(III) may provide higher Cr­(VI) production rates compared to relatively insoluble chromite. Of the potential Cr­(III)-bearing silicates, Cr­(III)-muscovite (i.e., fuchsite) commonly occurs in metamorphosed ultramafic rocks and dissolution rates may be comparable to other common Cr­(III)-bearing phyllosilicates and clays. Here, we examine the formation of Cr­(VI) related to Cr­(III)-muscovite and birnessite (i.e., acid birnessite) interactions with and without humic matter (HM) via batch experiments. Experimentally, the fastest rate of Cr­(VI) production involving Cr­(III)-muscovite was 3.8 × 10^–1 μM h^–1 (pH 3 without HM). Kinetically, Cr­(III)-muscovite provides a major pathway for Cr­(VI) formation and Cr­(VI) production rates may exceed those involving chromite depending on pH, available mineral surface areas in solution, and the abundance of Cr­(III) present. However, when HM is introduced to the system, Cr­(VI) production rates decrease by as much as 80%. This highlights that HM strongly decreases but may not completely suppress the formation and mobilization of Cr­(VI). A Sri Lankan serpentine soil was utilized to provide context with regards to the experimental results. Despite Cr­(VI) in the soil solids and Cr­(VI) formation being favorable from Cr­(III)-bearing minerals, no detectable Cr­(VI) was released into soil solutions potentially due to the abundance of HM. Overall, the dynamic interactions of Cr­(III)-bearing silicates and birnessite provide a kinetically favorable route of Cr­(VI) formation which is tempered by humic matter

Interface interactions between insecticide carbofuran and tea waste biochars produced at different pyrolysis temperatures

<p>Biochars showed a potential as adsorbents for organic contaminants, however, have not been tested for carbofuran, which has been detected frequently in water. This study provides evidences for the use of infused tea residue derived biochar for carbofuran removal. Biochars were produced at 300, 500 and 700 °C by slow pyrolysis and were characterized by proximate and ultimate analysis, FT-IR, SEM, BET and pore size distribution. Pyrolysis temperature showed a pronounced effect on biochar properties. The maximum carbofuran removal was achieved at pH 5. Freundlich and Temkin models best fit the equilibrium data. Biochars produced at 700 °C showed the highest sorption intensity. The adsorption process was likely to be a favorable chemisorption process with electrostatic interactions between carbofuran molecules and biochar surface. Acid-base interactions, electrophilic addition reactions and amide bond formations are the possible mechanisms of carbofuran adsorption. Overall, biochars prepared from tea waste can be utilized as effective adsorbents for removal of aqueous carbofuran.</p

Removal of antimonate and antimonite from water by schwertmannite granules

<p>In order to overcome the drawbacks of small particle-sized adsorbents, schwertmannite powder was fabricated into granules in the present study. These granules were evaluated for Sb(III) and Sb(V) removal from water and intraparticle mass transfer resistance of Sb(III) and Sb(V) onto the porous adsorbent was modeled. Schwertmannite granules (SG) exhibited capacities of 32.9 mg/g for Sb(III) and 23.2 mg/g for Sb(V), respectively, which are superior to many reported granular adsorbents and even powder adsorbents. Mass transfer was separately modeled using the pore volume diffusion model and surface diffusion model. The film diffusion coefficients, <i>k</i>_L, range from 1.09 × 10⁻⁵ to 3.08 × 10⁻⁵ cm/s. The pore diffusion coefficients, <i>D</i>_ep, range from 6.20 × 10⁻⁵ to 10.85 × 10⁻⁵ cm²/s, and the surface diffusion coefficients, <i>D</i>_s, range from 1.12 × 10⁻⁹ to 3.57 × 10⁻⁹ cm²/s. The concentration decay data-sets were successfully fitted with these best obtained parameters. Sb(III) was effectively removed over a wide pH range, while the removal of Sb(V) was pH dependent and could be enhanced by lowering solution pH. Sb(III)-loaded SG was regenerated with 91.2% re-adsorption capacity retained after five cycles when using 0.6% NaOH as the stripping solution. The desorption of Sb(V) was not as successful as Sb(III). Before breakthrough (5 μg/L) occurred, 1,690 and 712 bed volumes (BVs) of Sb(III), and 769 and 347 BVs of Sb(V) were treated when operating at space velocity values of 2 and 6 h⁻¹, respectively. Considering the low cost and the granular form of schwertmannite, the adsorbent is a promising modestly priced adsorbent and can be easily used in packed bed or filter units for practical application.</p

Natural Solar Irradiation Produces Fluorescent and Biodegradable Nanoplastics

Nanoplastics (NPs) have raised global concern owing to their potential health effects. Herein, after simulated and natural solar irradiation, polyethylene, polypropylene, polystyrene, and poly(vinyl chloride) nanoplastics (PVC NPs) were observed to exhibit enhanced fluorescence, particularly PVC NPs. Furthermore, the role of photoaged NPs as a potential fluorescence indicator was evaluated by exposing a model aquatic organism Daphnia magna to these NPs. Our results revealed that photoaged NPs exhibited strong fluorescence owing to the generation of conjugated π bonds, which can achieve π–π* electron transition with low energy consumption. Photogenerated fluorescence also enabled the photoaged NPs to act as efficient fluorescent tracers, which can help track NP migration in various organisms. The results of two-photon laser confocal scanning microscopy revealed that the photoaged NPs could translocate across biological barriers and accumulate in extraintestinal tissues in addition to being ingested and excreted. Moreover, compared with pristine NPs, the photoaged NPs underwent biodegradation more easily, probably because of increased hydrophilicity due to photogenerated oxygen-containing moieties. Therefore, in addition to producing fluorescent NPs without the attachment of external fluorescent dyes, the natural photoaging process can promote the migration and degradation of photoaged NPs in food chains

Thermodynamic Analysis of Nickel(II) and Zinc(II) Adsorption to Biochar

While numerous studies have investigated metal uptake from solution by biochar, few of these have developed a mechanistic understanding of the adsorption reactions that occur at the biochar surface. In this study, we explore a combined modeling and spectroscopic approach for the first time to describe the molecular level adsorption of Ni­(II) and Zn­(II) to five types of biochar. Following thorough characterization, potentiometric titrations were carried out to measure the proton (H⁺) reactivity of each biochar, and the data was used to develop protonation models. Surface complexation modeling (SCM) supported by synchrotron-based extended X-ray absorption fine structure (EXAFS) was then used to gain insights into the molecular scale metal–biochar surface reactions. The SCM approach was combined with isothermal titration calorimetry (ITC) data to determine the thermodynamic driving forces of metal adsorption. Our results show that the reactivity of biochar toward Ni­(II) and Zn­(II) directly relates to the site densities of biochar. EXAFS along with FT-IR analyses, suggest that Ni­(II) and Zn­(II) adsorption occurred primarily through proton-active carboxyl (−COOH) and hydroxyl (−OH) functional groups on the biochar surface. SCM-ITC analyses revealed that the enthalpies of protonation are exothermic and Ni­(II) and Zn­(II) complexes with biochar surface are slightly exothermic to slightly endothermic. The results obtained from these combined approaches contribute to the better understanding of molecular scale metal adsorption onto the biochar surface, and will facilitate the further development of thermodynamics-based, predictive approaches to biochar removal of metals from contaminated water

An efficient phosphorus scavenging from aqueous solution using magnesiothermally modified bio-calcite

<p>Bio-calcite (BC) derived from waste hen eggshell was subjected to thermal treatments (calcined bio-calcite (CBC)). The BC and CBC were further modified via magnesiothermal treatments to produce modified bio-calcite (MBC) and modified calcined bio-calcite (MCBC), respectively, and evaluated as a novel green sorbent for P removal from aqueous solutions in the batch experiments. Modified BC exhibited improved structural and chemical properties, such as porosity, surface area, thermal stability, mineralogy and functional groups, than pristine material. Langmuir and Freundlich models well described the P sorption onto both thermally and magnesiothermally sorbents, respectively, suggesting mono- and multi-layer sorption. Langmuir predicted highest P sorption capacities were in the order of: MCBC (43.33 mg g⁻¹) > MBC (35.63 mg g−¹) > CBC (34.38 mg g⁻¹) > BC (30.68 mg g⁻¹). The MBC and MCBC removed 100% P up to 50 mg P L⁻¹, which reduced to 35.43 and 39.96%, respectively, when P concentration was increased up to 1000 mg L⁻¹. Dynamics of P sorption was well explained by the pseudo-second-order rate equation, with the highest sorption rate of 4.32 mg g⁻¹ min⁻¹ for the MCBC. Hydroxylapatite [Ca₁₀(PO₄)₆(OH)₂] and brushite [CaH(PO₄)·2H₂O] were detected after P sorption onto the modified sorbents by X-ray diffraction analysis, suggesting chemisorption as the operating sorption mechanism.</p

Pyrogenic carbon and its role in contaminant immobilization in soils

<p>Pyrogenic carbon (PyC), including soil native PyC and engineered PyC (biochars), is increasingly being recognized for its potential role as a low-cost immobilizer of contaminants in soils. Published reviews on the role of soil native PyC as a sorbent in soils have so far focused mainly on organic contaminants and paid little or no attention to inorganic contaminants. Further, a comprehensive review on the production of both natural PyC and engineered PyC (biochars), mechanisms involved, and factors influencing their role as soil contaminant immobilizer is so far not available. The objective of this review is thus to systematically summarize the sources, formation, and properties of PyC, including its quantification in soils, followed by their roles in the immobilization of both organic and inorganic contaminants in soils. Effectiveness of PyC on bioavailability, leaching, and degradation of soil contaminants was summarized. Notably, the mechanisms and factors (for the first time) influencing the immobilization processes for soil contaminants were also extensively elucidated. This review helps better understand and design PyC for soil contaminant immobilization.</p

Distribution and Accumulative Pattern of Tetracyclines and Sulfonamides in Edible Vegetables of Cucumber, Tomato, and Lettuce

Veterinary antibiotics can be released to environment by the animals’ excretions, which thereby poses human health and ecological risks. Six antibiotics (tetracycline, oxytetracycline, chlortetracycline, sulfamethazine, sulfamethoxazole, and sulfadimethoxine) at three concentrations (5, 10, and 20 mg kg^–1 soil) were employed in pots filled with a loamy sand upland soil. Three types of vegetable seedlings, including cucumber (Cucumis sativus), cherry tomato (Solanum lycopersicum), and lettuce (Lactuca sativa), were also cultivated during 45 d in the greenhouse. All antibiotics taken up by tested plants showed negative effects on growth. Relatively high levels of tetracyclines and sulfonamides (SAs) were detected in the nonedible parts, roots, and leaves of cucumber and tomato, but fruit parts accumulated them lower than acceptable daily intake. Indeed, cucumber roots accumulated SAs by up to 94.6% of total addition (at 5 mg kg^–1 soil)

Arsenic(V) biosorption by charred orange peel in aqueous environments

<p>Biosorption efficiency of natural orange peel (NOP) and charred orange peel (COP) was examined for the immobilization of arsenate (As(V)) in aqueous environments using batch sorption experiments. Sorption experiments were carried out as a function of pH, time, initial As(V) concentration and biosorbent dose, using NOP and COP (pretreated with sulfuric acid). Arsenate sorption was found to be maximum at pH 6.5, with higher As(V) removal percentage (98%) by COP than NOP (68%) at 4 g L⁻¹ optimum biosorbent dose. Sorption isotherm data exhibited a higher As(V) sorption (60.9 mg g⁻¹) for COP than NOP (32.7 mg g⁻¹). Langmuir model provided the best fit to describe As(V) sorption. Fourier transform infrared spectroscopy and scanning electron microscopy combined with energy dispersive X-ray spectroscopy analyses revealed that the –OH, –COOH, and –N-H surface functional groups were involved in As(V) biosorption and the meso- to micro-porous structure of COP sequestered significantly (2-times) higher As(V) than NOP, respectively. Arsenate desorption from COP was found to be lower (10%) than NOP (26%) up to the third regeneration cycle. The results highlight that this method has a great potential to produce unique ‘charred’ materials from the widely available biowastes, with enhanced As(V) sorption properties.</p

Phosphate-assisted phytoremediation of arsenic by <i>Brassica napus</i> and <i>Brassica juncea</i>: Morphological and physiological response

<p>In this study, we examined the potential role of phosphate (P; 0, 50, 100 mg kg⁻¹) on growth, gas exchange attributes, and photosynthetic pigments of <i>Brassica napus</i> and <i>Brassica juncea</i> under arsenic (As) stress (0, 25, 50, 75 mg kg⁻¹) in a pot experiment. Results revealed that phosphate supplementation (P100) to As-stressed plants significantly increased shoot As concentration, dry biomass yield, and As uptake, in addition to the improved morphological and gas exchange attributes and photosynthetic pigments over P0. However, phosphate-assisted increase in As uptake was substantially (up to two times) greater for <i>B. napus</i>, notably due to higher shoot As concentration and dry biomass yield, compared to <i>B. juncea</i> at the P100 level. While phosphate addition in soil (P100) led to enhanced shoot As concentration in <i>B. juncea</i>, it reduced shoot dry biomass, primarily after 50 and 75 mg kg⁻¹ As treatments. The translocation factor and bioconcentration factor values of <i>B. napus</i> were higher than <i>B. juncea</i> for all As levels in the presence of phosphate. This study demonstrates that phosphate supplementation has a potential to improve As phytoextraction efficiency, predominantly for <i>B. napus</i>, by minimizing As-induced damage to plant growth, as well as by improving the physiological and photosynthetic attributes.</p