Effects of arbuscular mycorrhizal inoculation on the phytoremediation of PAH-contaminated soil: A meta-analysis.

Inoculation with arbuscular mycorrhizal (AM) fungi can accelerate the phytoremediation process by increasing plant biomass and improving soil physicochemical and biological characteristics. However, a quantitative, data-based conclusion is yet to be derived on the roles of AM fungi in remediating soils polluted by polycyclic aromatic hydrocarbons (PAHs), and the impact factors are unclear. To address these issues, we performed a meta-analysis of 45 articles to estimate the effects of AM inoculation on the phytoremediation of soils polluted by PAHs and to examine the influence of experimental conditions on these effects. Our results showed that AM inoculation significantly decreased the residual soil PAHs concentration at all PAHs levels, and the largest effect of AM treatment was 48.5% compared to the non-mycorrhizal treatment. This should be attributed to increased plant growth and PAHs uptake, and soil biological activity in the rhizosphere induced by AM symbionts. Compared to the non-mycorrhizal treatment, the largest AM effects on the total plant biomass, root PAHs concentration, shoot PAHs concentration, soil bacterial biomass, soil catalase activity, and soil polyphenol oxidase activity were 51.7%, 565%, 53.1%, 141%, 100% and 51.9%, respectively. Although these effects on the above mentioned parameters varied with AM fungi (genus, species, and inoculation mode), soil PAHs (source, concentration, and type), plant type (dicots and monocots), and experimental conditions (experimental duration, soil sterilization and additional factors), few negative AM effects were observed. This study confirmed the feasibility of using AM fungi to enhance the phytoremediation of PAHs-contaminated soil.

Fluoride-immobilized co-processing and resource utilization of aluminum-electrolyzed spent cathode carbon in brick-fired kiln.

Spent cathode carbon (SCC) is hazardous waste from the electrolytic aluminum industry due to its high levels of soluble fluoride, while brick-fired kiln provides the clay and heating conditions needed to immobilize fluoride. However, SCC reusing is still understudied, meanwhile co-processing and resource utilization of SCC in brick-fired kiln were still not reported in the literatures in addition to a Chinese patent of the authors. Here, the effects of firing temperatures, firing time, clay doses and calcium doses on the fluoride-immobilized performance of SCC co-processing were explored in a simulated brick-firing kiln, and their mechanisms were analyzed by SEM and XRD. The results indicated that clay-added co-processing in brick-fired kiln was a preferred choice without required additional additives or operations. The leached fluoride met Chinese standards by clay-added co-processing at ≥ 800 °C/ ≥ 40 g clay/ ≥ 120 min. Clay and calcium-added co-processing in brick-fired kiln was another alternative choice with higher fluoride-immobilization rates. The leached fluoride met Chinese standard (GB5085.3-2007) by clay and calcium-added co-processing at ≥ 500 °C/ ≥ 30 min/ ≥ 5 g clay/ ≥ 0.5 g CaCO3. SEM and XRD indicated that SiO2 in clay reacted with sodium in SCC and formed vitreous analog (Na1.55Al1.55Si0.45O4) to prevent fluoride ion migration and the newly-formed k-Feldspar (K2O.Al2O3.6SiO2) may adsorb fluoride ions in clay-added co-processing. Soluble fluoride NaF in SCC were converted into water-insoluble cuspidine in clay and calcium-added co-processing, in addition to the crystalline phase conversion in clay-added co-processing. Therefore, the risks of finished bricks to human health and the environment were greatly reduced after clay-added or clay and calcium-added treatments.

Solidification/stabilization of spent cathode carbon from aluminum electrolysis by vitric, kaolin and calcification agent: fluorides immobilization and cyanides decomposition.

Spent cathode carbon (SCC) is a hazardous waste containing fluorides and cyanides from aluminum electrolysis. Many literatures have focused on SCC leaching; however, SCC hazard-free treatment remains understudied. This article used 10.0 g raw SCC sample to explore the vitric/kaolin solidification and calcium stabilization of SCC, and analyze their hazard-free mechanisms by the methods of XRD and SEM. The leached fluorides were all below the Chinese identification standard for hazardous wastes (GB5085.3-2007), whether at 750/950 °C for 60 min above 8.0 g vitric, or at 1200 °C for 120 min with above 8.0 g kaolin, or above 700 °C for more than 30 min with above 0.5 g CaCO3. Kaolin/vitric solidification relied on the massive addition of vitric and kaolin to produce glassy or glass-like material (K2O·Al2O3·6SiO2) which may retain fluoride. Calcium stabilization converted soluble fluoride NaF in raw SCC sample into insoluble CaF2. Heating 60 min at 500-1200 °C at oxygen atmosphere decomposed almost of cyanides, with leached cyanides meeting Chinese standard GB5085.3-2007. Mass-loss rates of kaolin addition came from a large amount of adsorbed water and structural water in kaolinite and illite wai lost, and that of CaCO3/CaSO4 addition was attributed to their decomposition into volatile CO2/SO2, while that of CaO was a little negative due to its absorption of water vapor and CO2. In brief, as the effective hazard-free manner of SCC, both kaolin/vitric solidification and calcium stabilization successfully have achieved fluoride immobilization and cyanide decomposition.

Comprehensive study on green and sustainable remediation in the USA: policy system and case experiences.

The implementation of green remediation or sustainable remediation (collectively referred as green and sustainable remediation, GSR) has been promoted by multi-stakeholder collaboration. However, comprehensive analysis of GSR is understudied in previous literatures. Policy system and case experience of GSR in the USA are here been analyzed comprehensively. Results indicate that USEPA, SURF-US, and ITRC advocated GR, SR, and GSR, respectively. For the program types of GSR cases, the government-driving forces (especially USEPA) had significant positive effects than those voluntary cleanups. In situ techniques of soil remediation are more widely used than ex situ ones. All techniques of groundwater remediation are in situ, in addition to pump and treat due to its effectiveness to remedy seriously contaminated sites. The best management practices (BMPs) are preferably implemented in remedial construction stage rather than other stages. The percentages of BMPs related to "optimization," "renewable energy," and "recycling or reusing materials" are relatively high, while the other BMPs are relatively low. GSR benefits achieved by BMPs of environment-orientated may not only reduce the environmental footprint, but also gain in economic and social aspects. Moreover, practitioners should more fully understand the full benefits of a BMP implementation and strengthen the consensus among them. In brief, it is necessary for remediation practitioners to supplement the existing defects in policies and their implementations and to select optimum BMPs in specific contaminated sites. This work offers comprehensive and valuable insight into policies and practices of GSR.

Sustainable remediation of dibenzofuran-contaminated soil by low-temperature thermal desorption: Robust decontamination and carbon neutralization.

Thermal desorption (TD) is generally considered to be an effective but unsustainable technology. Decontamination performance, charring behaviors and physicochemical properties during TD of dibenzofuran-contaminated soil (DCS) are explored. After treatment at 300 °C for 20 min, the dibenzofuran concentration decreases from 3969.37 mg/kg to 17.29 mg/kg, lower than Chinese risk screening value. More than 99% of dibenzofuran in soil are removed at low temperature of 300 °C, meanwhile the organic carbon is partially retained in soil. Removal mechanism of DCS at 300 °C is proposed, including desorption, cracking, and charring. Char material of low H:C ratio is produced by the generation, polymerization and dehydrogenation of aromatic intermediates, and then increases carbon stocks and reduces the carbon footprint of contaminated soil. Meanwhile, due to the char generated, pH, cation exchange capacity and specific surface area of DCS heated at 300 °C are higher than those of raw DCS, promoting ecological restoration and enhancing carbon sink in soil ecosystems. The aforesaid saving energy, reducing carbon footprint and enhancing carbon sink are exactly the main innovative technologies for achieving carbon neutrality. Hence, it may be a contribution to climate change mitigation, in addition to a robust and sustainable remediation of organic contaminated soil.

Pilot-scale microsand-ballasted flocculation of wastewater: turbidity removal, parameters optimization, and mechanism analysis.

The flocs formed during microsand-ballasted flocculation (MBF) have attracted much attention. However, few studies have reported on comprehensive process parameters of MBF and its mechanism is still not well understood. Jar test and pilot-scale continuous experiments were here conducted on two kinds of simulated wastewater, labeled S1 (21.6-25.9 NTU) and S2 (96-105 NTU). Results revealed the hydraulic retention time ratio in the coagulation cell, injection and maturation cell, lamella settler of pilot-scale MBF equipment was 1:3:7.3. The optimum poly aluminum chloride doses for samples S1 and S2 were 0.875 g/L and 1.0 g/L. Besides, the optimum size of microsand was 49-106 µm and the optimum dose was 1.0 g/L. Under aforementioned conditions, the effluent turbidity of S1 was below 0.47 NTU, even lower than the Chinese drinking water standard; that of S2 was below 1.7 NTU, meeting the Chinese recycled water standard. Turbidity removal ranged from 98.0 to 98.8% for S1 and 98.5 to 99.5% for S2 when microsand was added. Therefore, microsand addition enhances MBF performance, where microsand serves as an initial core particle. Some microsand core particles bond together to form a dense core structure of micro-flocs by the adsorption bridging of inorganic polymeric flocculant. Moreover, the size of the largest micro-flocs may be controllable as long as the effective energy dissipation coefficient is adjusted appropriately through specific stirring speeds. This work provides comprehensive pilot-scale process parameters for using MBF to effectively treat wastewater and offers a clearer explanation of the formation mechanism of microsand-ballasted flocs.

Sustainable remediation of lube oil-contaminated soil by low temperature indirect thermal desorption: Removal behaviors of contaminants, physicochemical properties change and microbial community recolonization in soils.

Thermal desorption is widely adopted for the remediation of organic compounds, yet is generally considered a non-green-sustainable manner owing to its energy-intensive nature and potential to deteriorate soil reuse. Here, lube oil-contaminated soils were remediated at 200-500 °C in nitrogen atmosphere, upon which removal behaviors of lube oil and physicochemical properties of soils were explored. Illumina 16S ribosomal RNA (rRNA) and 18S rRNA amplicon sequencing were employed to determine the relative abundances and diversities of bacteria and fungi in soils, respectively. The results indicated that, after heating at 350 °C for 60 min, 93% of the lube oil was reduced, with the residual lube oil concentration lower than the Chinese risk intervention values (GB 36600-2018). The weakly-alkaline, multi-phosphorus and char-rich soils after indirect thermal desorption could provide a nutrient source and favorable habitat space for living organisms, and the decomposition of minerals in soils is more conducive to the survival of organisms. Microbial species in soils after heating at 350 °C became extinct, however, microbial species after 3 days of recolonization were enough to carry out DNA extraction when these soils were exposed to natural grass land. Though the microbial richness and diversity in heated soils after 3 days of recolonization were still little lower than those in contaminated soils, Firmicutes (29.41%) and Basidiomycota (9.33%) became dominant at phyla level, while Planomicrobium (16.37%), Massilia (10.09%), Jeotgalibaca (7.91%) and Psychrobacter (6.84%) were dominant at general level, whose ecological function was more conducive to nutrient cycling and ecological resiliency. Overall, this innovative research provides a new perspective: low temperature indirect thermal desorption may also achieve a sustainable remediation, due to its energy-saving (low temperature), favorable physicochemical properties and the rapid recolonization capacity of microbial communities in heated soils.

AM fungi enhance the function of ecological floating bed in the treatment of saline industrial wastewater.

Treatment of saline wastewater attracts more and more attention due to its negative effects on the environment in China. Although salt removal from high-saline wastewater is well done in many industry factories, few technologies are available to remove salt from low-saline wastewater (total dissolved solids, TDS < 10,000 mg/L). In this study, ecological floating bed (EFB) enhanced by arbuscular mycorrhizal fungi (AMF) Glomus etunicatum was constructed to remove salt from simulated low-saline wastewater. Results showed that AM formation in Canna indica was not affected by salt stress, and a higher mycorrhizal colonization rate was even observed under salt stress relative to the control treatment. In saline wastewater containing TDS 5000 mg/L, EFB with AM inoculation (AM-EFB) removed 15.9% of TDS, 19.9% of COD, 14.2% of TN, 22.5% of TP, and 11.6-23.0% of salt ions (Na, K, Mg, and Ca) more than EFB without AM inoculation (NM-EFB) in September, and 13.0% of TDS, 15.8% of COD, 17.5% of TN, 16.6% of TP, and 8.60-22.2% of salt ions (Na, K, Mg, and Ca) more than NM-EFB in October respectively. AMF increased plant uptake to Na and the translocation of Na from root to shoot, especially at the initial stage of the experiment. Additionally, EFB function declined when environmental temperature declined independent of AM inoculation, but the presence of AM increased EFB function in the treatment of saline wastewater relative to NM-EFB. This study provides a new strategy for the treatment of low-saline wastewater and the EFB application in a low-temperature environment.

Degradation of nitrobenzene by synchronistic oxidation and reduction in an internal circulation microelectrolysis reactor.

The degradation of nitrobenzene by synchronistic oxidation and reduction was investigated using an internal circulation microelectrolysis (ICE) reactor with an active volume of 0.018 m3. Compared with a conventional fixed bed reactor with and without aeration, the ICE reactor exhibited a markedly higher nitrobenzene degradation efficiency. The effects of various operational parameters such as reaction time, aeration rate, initial nitrobenzene concentration, initial pH, and a volume ratio of iron and carbon (Fe/C) were also investigated. The optimal operating conditions (reaction time = 60 min, aeration rate = 5 × 10-4 m3/s, initial concentration of nitrobenzene = 300 mg/L, pH = 3.0, Fe/C = 1:1) gave removal efficiencies of nitrobenzene and chemical oxygen demand of 98.2% and 58%, respectively. The biodegradability index of the treated nitrobenzene solution was 0.45, which is 22 times that of the original solution. The reaction intermediates were identified through high-performance liquid chromatography, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, gas chromatography-mass spectrometry, and ion chromatography. The primary intermediates were determined to be aniline, phenol, and carboxylic acids, indicating that nitrobenzene was synchronously oxidized and reduced in the ICE reactor. Based on the identified intermediates, a possible pathway for nitrobenzene degradation in the ICE reactor is proposed.

[Microstructure morphology and flocculation mechanism of the decolorizing flocculant poly-aluminum (III)-magnesium (II)-sulfate].

Crystal structures and morphologic characteristics of poly-aluminum (III)-magnesium (II)-sulfate (PMAS) were discussed using X-radial diffraction and infrared spectrogram and the flocculation mechanism was explored here based on comparisons between decolorization effect of landfill leachate secondary effluent and the system Zeta potential. The results indicated that PMAS was a kind of macromolecular composite polymer of aluminum and magnesium based on -OH bonding. The flocculation mechanism of PMAS was mainly charge neutralization and co-precipitation netting. Charge neutralization was the main mechanism at low dose. Its flocculation behaviors were different along with different pH of wastewater at high doses, namely, it was mainly charge neutralization at low pH and co-precipitation netting at high pH and both of them work together at neutral condition.

Analysis and removal of organic pollutants in biologically treated landfill leachate by an inorganic flocculent composite of Al(III)-Mg(II).

A novel inorganic flocculent composite of Al(III)-Mg(II) poly-magnesium-aluminum-sulfate (PMAS) is used to remove organic matter from biologically treated leachate in some landfills in Beijing, China. Jar-test experiments are employed to determine the optimum conditions for the removal of organic matter, which is represented as UV(254). Under optimum conditions, the removal of COD, BOD(5), and color is also determined. Moreover, gas chromatography coupled with mass spectrometry (GC-MS) is used to analyze the organic matter in the biologically treated leachate before and after treatment by the coagulant. The experimental results indicate that the removal of COD, BOD(5), UV(254), and color by coagulation with PMAS can reach above 65%, 60%, 85%, and 85%, respectively, under optimal conditions. This greatly weakens its pollution extent and improves its visual appeal. Forty-one kinds of organic pollutants in the biologically treated leachate were determined. Some of them belong to the Black List of water environmental preferred controlled pollutants, as judged by the United States and China. The species of alkyl, alkene, acyclic alcohol, and acyclic acyl amines are about 85% removed, some of them are removed completely, while the species of acids, esters, and ketones are removed at about 65%. Those contaminants with benzene rings, such as aromatic hydrocarbons, hydroxybenzene, aromatic alcohol, and aromatic acyl amine, are about 50% partially removed.

Filtration by a novel nanofiber membrane and alumina adsorption to remove copper(II) from groundwater.

The elevated level of heavy metals in groundwater poses a substantial risk potentially to local resource users and the natural environment. Micellar-enhanced filtration (MEF) and alumina adsorption are considered from the viewpoint of copper(II) removal in groundwater, by taking copper(II) as an example. In MEF, copper(II) cations are collected electrostatically on micelles of sodium dodecyl benzene sulfonate (SDBS) and separated from the mother liquor by filtration using a novel nanofiber membrane prepared from chloridized polyvinyl chloride by high-voltage electrospinning process. After MEF with 10-layer filtration and SDBS concentration of 5 mmol/L, the removal of copper(II) in groundwater is above 70%. However, the final solution contains a large amount of surfactant causing serious second contamination in groundwater. This problem is overcome by alumina adsorption, where negatively charged surfactants are adsorbed on positively charged alumina particles and then recovered by conventional filtration. The hybrid process of MEF and alumina adsorption is successfully applied to removing almost 100% of copper(II) from groundwater. Finally, the characterization of the membrane and filtration mechanism are presented here.