A Critical Review of the Enhanced Recovery of Rare Earth Elements from Phosphogypsum.

The increasing demand for rare earth elements (REEs), especially from new and innovative technology, has strained their supply, which makes the exploration of new REE sources necessary, for example, the recovery of REEs from phsophogypsum (PG). PG is a byproduct during the wet production of phosphoric acid, which is an attractive secondary resource for REEs due to a large amount of REEs locked in them. In most cases, REEs contained in PG are mainly encapsulated in the gypsum crystal, leading to a low leaching efficiency. Therefore, it is particularly important to use various methods to enhance the leaching of REEs from PG. In this review, we summarized and classified various enhanced leaching methods for the recovery of REEs from PG, and the advantages and disadvantages of different methods were compared. A joint method of recrystallization and RIL may be a promising enhanced leaching approach for the recovery of REEs from PG. Recrystallization could achieve both the complete REE release and simultaneous preparation of industrial materials with high value added, such as high-strength α-hemihydrate gypsum by phase transformation of PG, and the RIL technology could adsorb the releasing REEs and realize their efficient extraction. Such a combination appears to show significant advantages because of high REE recovery, as well as high value-added product preparation at low cost.

Selective removal of phosphate from aqueous solution by MIL-101(Fe)/bagasse composite prepared through bagasse size control.

MIL-101(Fe)/sugarcane bagasse (SCB) with high adsorption capacity and selectivity toward phosphate was prepared through in-situ synthesis method. Effects of bagasse size on the morphology and performances of the composites were investigated, and adsorption behavior and mechanism of phosphate on the composite prepared at the optimum bagasse size were studied. Results showed that composite prepared with bagasse size of 200-300 mesh (MIL-101(Fe)/SCB3) showed much higher adsorption capacity than SCB, blank MIL-101(Fe) and the composites prepared with the other bagasse size, which was due to the more positively charged surface and the more exposed adsorption active sites including FeOHx and exchangeable Cl-. Co-ions experimental results illustrated that the as prepared MIL-101(Fe)/SCB3 showed high adsorption affinity toward phosphate, and the common cationic and anionic ions exhibited negligible effects on phosphate adsorption capacity and rate. The optimum pH range for phosphate adsorption on MIL-101(Fe)/SCB3 was from 3.0 to 10.0, and in this range Fe release was less than 0.03%. Adsorption mechanism showed that phosphate was adsorbed mainly through electrostatic force, ion-exchange, and inner-sphere surface complex. Simulated wastewater treatment experiment showed that MIL-101(Fe)/SCB3 could efficiently remove phosphate from aqueous solution.

Simultaneous removal of cationic and anionic dyes from aqueous solution by double functional groups modified bagasse.

Double functional groups modified bagasse (DFMBs), a series of new zwitterionic groups of carboxyl and amine modified adsorbents, were prepared through grafting tetraethylenepentamine (TEPA) onto the pyromellitic dianhydride (PMDA) modified bagasse using the DCC/DMAP method. DFMBs' ability to simultaneously remove basic magenta (BM, cationic dye) and Congo red (CR, anionic dye) from aqueous solution in single and binary dye systems was investigated. FTIR spectra and Zeta potential analysis results showed that PMDA and TEPA were successfully grafted onto the surface of bagasse, and the ratio of the amount of carboxyl groups and amine groups was controlled by the addition of a dosage of TEPA. Adsorption results showed that adsorption capacities of DFMBs for BM decreased while that for CR increased with the increase of the amount of TEPA in both single and binary dye systems, and BM or CR was absorbed on the modified biosorbents was mainly through electrostatic attraction and hydrogen bond. The adsorption for BM and CR could reach equilibrium within 300 min, both processes were fitted well by the pseudo-second-order kinetic model. The cationic and anionic dyes removal experiment in the binary system showed that DMFBs could be chosen as adsorbents to treat wastewater containing different ratios of cationic and anionic dyes.

Study on the occurrence state of main components of phosphogypsum dihydrate and its impurity distribution.

The dihydrate phosphoric acid process is the mainstream technique. However, the phosphogypsum (PG) produced contains high levels of impurities such as phosphorus and fluorine, severely constraining its valorization. In order to elucidate the occurrence patterns of phosphorus and fluorine impurities in PG, this study employed analytical methods including XRF, XRD, AMICS (Automated Mineralogy Integrated with Chemistry System), XPS, and chemical element balance analysis. We investigated the occurrence states of phosphorus, fluorine, silicon, iron, and aluminum elements in PG from wet-process phosphoric acid production, as well as the distribution characteristics of phosphorus and fluorine impurities. Additionally, we utilized Density Functional Theory (DFT) calculations to determine the binding energies of major minerals with water-soluble phosphate and fluoride groups, and analyzed the zeta potentials of gypsum and quartz mineral surfaces. The results indicate that the main mineral phases in PG are gypsum, quartz, potassium silicate minerals, aluminosilicate minerals, and hematite, predominantly occurring in monomineralic forms. Phosphorus impurities primarily exist in calcium silicate and hematite minerals, while fluorine is mainly associated with gypsum and potassium silicate minerals. DFT calculations demonstrate strong binding energies between calcium silicate and hematite minerals with PO4 3-, as well as between gypsum and quartz minerals with F-. The acidic conditions in the separation tank during wet-process phosphoric acid production may contribute to the distinctive distribution characteristics of phosphorus and fluorine impurities in PG.

The Impurity Removal and Comprehensive Utilization of Phosphogypsum: A Review.

Phosphogypsum (PG), a byproduct during the phosphoric acid production process, also known as the wet process, contains complex and diverse impurities, resulting in low utilization and considerable accumulation. This leads to a massive waste of land resources and a series of environmental pollution problems. Given the current urgent ecological and environmental situation, developing impurity removal processes with low energy consumption and high efficiency, exploring valuable resource recovery, preparing high value-added PG products, and broadening the comprehensive utilization ways of PG are significant strategies to promote the sustainable consumption of PG and sustainable development of the phosphorus chemical industry. This review comprehensively summarizes the advantages and disadvantages of existing PG impurity removal and utilization technologies and probes into the future development direction, which provides references and ideas for subsequent PG research.

Rational design of dense microporous carbon derived from coal tar pitch towards high mass loading supercapacitors.

The compact carbon materials with huge specific surface area (SSA) and proper pore structure are highly desirable towards high-performance supercapacitors at the cell level. However, to well balance of porosity and density is still an on-going task. Herein, a universal and facile strategy of pre-oxidation-carbonization-activation is employed to prepare the dense microporous carbons from coal tar pitch. The optimized sample POCA800 not only possesses a well-developed porous structure with the SSA of 2142 m2 g-1 and total pore volume (Vt) of 1.540 cm3 g-1, but also exhibits a high packing density of 0.58 g cm-3 and proper graphitization. Owing to these advantages, POCA800 electrode at areal mass loading of 10 mg cm-2 shows a high specific capacitance of 300.8 F g-1 (174.5 F cm-3) at 0.5 A g-1 and good rate performance. The POCA800 based symmetrical supercapacitor with a total mass loading of 20 mg cm-2 displays a large energy density of 8.07 Wh kg-1 at 125 W kg-1 and remarkable cycling durability. It is revealed that the prepared density microporous carbons are promising for practical applications.

Ar plasma assists in enhanced oxygen evolution kinetics of MOG-derived multicomponent transition metal sulfides.

Transition metal sulfides are low-cost oxygen evolution reaction (OER) electrocatalysts that can potentially substitute noble metal catalysts. However, the adsorption process of their OER is impeded by their intrinsic poor catalytic activity. Constructing heterojunction and vacancy defects in transition metal sulfides is an efficient method to promote the process of oxygen evolution. Herein, a facile approach based on in situ sulfurization of metal-organic gels (MOGs) and a short-time plasma treatment was developed to fabricate vacancy-modified polymetallic sulfides heterojunction. The synergistic effect of the multi-component heterojunction and sulfur vacancy contributed greatly to improving the electron migration efficiency and OER ability of the electrocatalyst. As a result, the optimum oxygen evolution activity was achieved with appropriate surface vacancy concentrations by regulating the plasma radio frequency powers. The plasma-treated catalyst under 400 W showed the best OER performance (lower overpotential of 235 mV in 1 M KOH solution with the Tafel slope of 31 mV dec-1) and good durability over 11 h of chronopotentiometry testing. This work sheds new light on constructing multimetal-based heterojunction electrocatalysts with rich vacancy defects for oxygen evolution reactions.

[Separation and purification of total flavonids of Astragalus membranaceus with ethanol/phosphate aqueous two-phase system].

OBJECTIVE: To study separation and purification of flavonids with ethanol/phosphate aqueous two-phase system. METHOD: The diversity of phase separation ability and the distribution of target products in various systems were taken as indicators to analyze aqueous two-phase extraction systems and phase diagrams formed by ethanol and some common salts, screen out EtOH/ K2HPO4 system as the optimla system for extracting total flavonids, and study the impact of proportion of components in EtOH/K2 HPO4 system on the partition coefficient and phase ratio of flavonids. RESULT AND CONCLUSION: The EtOH/K2 HPO4 system with omegaEtOH 36.05% and omegaKHPO4 18.20% has been proved as the optimal conditions for separating and purifying total flavonoids of Astragalus (TFA). Under this optimal condition, the partition coefficient and the extraction yield of TFA reached 10.33 and 96.6%, respectively. After extraction, the contents of A. membranaceus saponins and A. membranaceus polysaccharides in top and bottom phases were determined at the same time, showing that A. membranaceus saponins in the removal rate reached 92.01%, and A. membranaceus polysaccharides were totally concentrated in bottom water phase, indicating a removal rate of 100%. Therefore, this is beneficial to separate and purify total flavonids from A. membranaceus crude extracts.

Polyaminophosphoric Acid-Modified Ion-Imprinted Chitosan Aerogel with Enhanced Antimicrobial Activity for Selective La(III) Recovery and Oil/Water Separation.

In this study, polyaminophosphoric acid (PA)-functionalized ion-imprinted chitosan (CS) aerogel was fabricated for the first time, exhibiting good antibacterial property for selective La(III) recovery and oil/water separation. The as-prepared PA-CS-IIA-2 shows a remarkable adsorption capacity of 114.6 mg/g toward La(III) and high selectivity in the competitive adsorption systems, which is attributed to its abundant imprinting sites and surface functional groups. Benefiting from the amphiphilic property, the PA-CS-IIA-2 also exhibits an excellent adsorption performance for the extractant, oils, and organic solvents. Besides, the PA-CS-IIA-2 presents excellent regeneration and reusability characteristics. Moreover, compared with CS, the PA-CS-IIA-2 exhibits a significantly improved antibacterial activity originating from the PA component. Most importantly, the PA-CS-IIA-2 aerogel is capable of removing multiple pollutants all together and effectively inhibiting bacteria in the complex wastewater environments. Therefore, this study paves the way for developing high-performance rare-earth capture materials with multiple functions to meet diverse applications.

Pore structure regulation of hierarchical porous carbon derived from coal tar pitch via pre-oxidation strategy for high-performance supercapacitor.

Carbon materials with rational pore structure have attracted tremendous attention in high-performance supercapacitor applications. However, designing and constructing such carbon materials with excellent performances via a simple and low-cost route is still a challenge. Herein, the nitrogen self-doped oxygen-rich hierarchical porous carbons (OTSx-PC) derived from coal tar pitch are constructed via a facile strategy of air pre-oxidation-activation. The air pre-oxidation treatment can effectively regulate the small-sized mesopore structure (2-4 nm) of samples. The optimal OTS350-PC sample exhibits a high specific capacitance of 298 F g-1 at 0.5 A g-1, and delivers a high energy density of 14.9 Wh kg-1 at a power density of 0.15 kW kg-1 with remarkable cycling stability in KOH aqueous electrolyte. This excellent electrochemical performance is attributed to its ultrahigh specific surface area (SSA, 2941 m2 g-1), huge total pore volume (Vt, 1.9 cm3 g-1), rational pore structure and reasonable heteroatom configuration, which ensure sufficient charge storage, rapid electrolyte ions diffusion, as well as the contributed pseudocapacitance. This research not only offers a facile route for high-value utilization of coal tar pitch but also provides the cost-effective and excellent porous carbons for supercapacitor with high performance.

Effects of Ion Characteristics on the Leaching of Weathered Crust Elution-Deposited Rare Earth Ore.

To reveal the ion-exchange mechanism in the leaching process of weathered crust elution-deposited rare earth ores with different leaching agents, the effects of a variety of cations and anions at different concentrations on the leaching process were investigated, including Al3+, Fe3+, Ca2+, Mg2+, Na+, K+, NH 4 + and Cl-, NO 3 - , and SO 4 2 - . Meanwhile, the relationships between different concentrations of cations and anions and leaching efficiency were investigated, as was the relationship between different concentrations of cations and anions and zeta potential. The effect of different ions on the swelling of clay minerals during leaching process was also investigated. The results shown that NH 4 + was the most affected electrolyte cation in terms of rare earth leaching efficiency during the leaching process of weathered crust elution-deposited rare earth ore among three different cationic valence states, and the leaching efficiency was 86.93% at the optimal leaching concentration. The influence of the three anions on the leaching efficiency of rare earth was NO 3 - > Cl - > SO 4 2 - , and the leaching efficiency of rare earth were 83.21, 81.52, and 80.12% at the optimal leaching concentration, respectively. The NH 4 + had the greatest effect on the zeta potential of weathered crust elution-deposited rare earth ore, and the zeta potential was -18.1 mV at the optimal leaching concentration. Additionally, the order of the effect of three anions on zeta potential was SO 4 2 - > NO 3 - > Cl - . Combined with the effect on the rare earth leaching process, anions and cations were considered separately, and NH 4 + and Cl- were selected; the relationship between the rare earth leaching efficiency and zeta potential conforms to the follow equations: NH 4 + :Y = -0.48X2 - 13.51X - 1.58, R 2 = 0.98133 and Cl-:Y= -1.22X2 - 17.64X + 23.29, R 2 = 0.99010. It was also found in the swelling experiment of the weathered crust elution-deposited rare earth ore that the swelling ratio of clay minerals was the lowest when the cation and anion were NH 4 + and Cl- and the swelling ratios were 1.874 and 2.015%, respectively.

Effects of surface modification on heavy metal adsorption performance and stability of peanut shell and its extracts of cellulose, lignin, and hemicellulose.

Effects of surface modification by carboxyl group on Pb2+ adsorption performances and stability of peanut shell and its extracts (cellulose, lignin, and hemicellulose) were investigated. Stability of the biosorbents was measured by determining organic compound release amount (TOC). Results showed that adsorption capacity of peanut shell and the extract was poor and stability of them was not good enough. Amount of organic compound released from the unmodified sorbents followed the order: cellulose > lignin > peanut shell > hemicellulose. Hemicellulose was the main organic compound release resource for the raw peanut shell. Due to the poor stability of the raw materials, peanut and its extract could not be used directly in the practical waste water treatment. After modification, adsorption capacity of peanut shell, cellulose, lignin, and hemicellulose increased by 4- to 6-folds. Stability of the modified sorbents also increased significantly, and TOC determined for the modified peanut shell, cellulose, and hemicellulose was lower than 4.0 mg L-1 in the optimum pH range from 4.0 to 6.0 even using for 30 days, which was lower than the drinking water standard in China. Modified peanut shell and its extract except for lignin could be used safely in pH ranged from 4.0 to 6.0. Surface modification could improve the adsorption performances and stability of the biosorbents.

[Separation and purification of flavonoids from Smilax glabra by macroporous adsorption resin].

OBJECTIVE: To investigate the process of separating and purifying flavonoids from Smilax glabra. METHOD: With the yield of flavonoids as index, the optimum process of separating and purifying flavonoids from S. glabra Roxb was screened by static and dynamic adsorption tests. RESULT: The static saturated adsorption capacity of D101 macroporous resin to flavonoids of S. glabra was 45.6 mg x g(-1) (dry resin). The optimum conditions of dynamic adsorption and elution were as that the pH, the concentration, the adsorption velocity of the extracting solution, and the adsorption capacity were 6.00 +/- 0.20, 4.2 mg x mL(-1), 2 mL x min(-1) and 15 mL, respectively. The adsorbed resin column was washed by 100 mL 60% ethanol with pH value of 8.00 +/- 0.20 at the eluting velocity of 3 mL x min(-1) after washed by 100 mL distilled water. CONCLUSION: The flavonoids of S. glabra was able to be easily separated and purified by D101 macroporous resin under the optimum conditions above, and the recovery rate was higher than 90%. The content of obtained flavonoids reached 62.6%, which was 2 times of the content before purification.

A two-step leaching method designed based on chemical fraction distribution of the heavy metals for selective leaching of Cd, Zn, Cu, and Pb from metallurgical sludge.

For selective leaching and highly effective recovery of heavy metals from a metallurgical sludge, a two-step leaching method was designed based on the distribution analysis of the chemical fractions of the loaded heavy metal. Hydrochloric acid (HCl) was used as a leaching agent in the first step to leach the relatively labile heavy metals and then ethylenediamine tetraacetic acid (EDTA) was applied to leach the residual metals according to their different fractional distribution. Using the two-step leaching method, 82.89% of Cd, 55.73% of Zn, 10.85% of Cu, and 0.25% of Pb were leached in the first step by 0.7 M HCl at a contact time of 240 min, and the leaching efficiencies for Cd, Zn, Cu, and Pb were elevated up to 99.76, 91.41, 71.85, and 94.06%, by subsequent treatment with 0.2 M EDTA at 480 min, respectively. Furthermore, HCl leaching induced fractional redistribution, which might increase the mobility of the remaining metals and then facilitate the following metal removal by EDTA. The facilitation was further confirmed by the comparison to the one-step leaching method with single HCl or single EDTA, respectively. These results suggested that the designed two-step leaching method by HCl and EDTA could be used for selective leaching and effective recovery of heavy metals from the metallurgical sludge or heavy metal-contaminated solid media.

Novel combined method of biosorption and chemical precipitation for recovery of Pb2+ from wastewater.

A novel combined biosorption-precipitation process has been designed and applied to recycle Pb2+ from low concentration lead containing wastewater. Pb2+ was firstly removed selectively from wastewater by pyromellitic dianhydride (PMDA) modified sugarcane bagasse (SB) fixed-bed column, and then, it was desorbed into the concentrated eluate and recycled by adding chemical precipitant. Adsorption performance of the column and optimum desorption and precipitation condition for Pb2+ were investigated in detail. Results showed that the as-prepared column could efficiently remove Pb2+ from aqueous solution and optimum condition for Pb2+ precipitation in eluate was at pH 3.0 and molar ratio of precipitant to Pb2+ of 5:1 by using Na3PO4 as precipitant. Recovery experiment illustrated that Pb2+ was selectively removed from wastewater containing ions of Pb2+, Zn2+, Cd2+, Ca2+, K+, and Na+ through competitive substitution adsorption on the modified SB, and mass ratio of the five metal ions in eluate was 96.8:0.7:0.7:0.7:0.5:0.5. Pb2+ in this concentrated and purified eluate solution was recycled efficiently by adding Na3PO4. The combined method had great potential in application of heavy metal recovery from wastewater.

[Extracting flavonoids from Smilax glabra by microwave-assisted method].

OBJECTIVE: To optimize the process of extracting flavonoids from Smilax glabra. METHODS: Flavonoids were extracted from Smilax glabra by microwave-assisted method, and the extracting time, microwave power, ethanol concentration, solid-solvent ratio and extracting temperature were optimized through single factor experiment and orthogonal test. RESULTS: The optimun process parameters of extracting flavonoids were as follows: the extracting time, microwave power, ethanol concentration, solid-liquid ratio and extracting temperature were 5 minutes, 600 W, 60%, 1:30 and 80 degrees C, respectively. CONCLUSION: The proposed method has been applied stably and reliably to the extraction of flavonoids from Smilax glabra which has the advantages of high recovery and short extraction time. The recovery, the yield and the content of falvonoids are 93.2%, 2.66% and 32.6%, respectively.

[The optimum condition on baicalin extraction from Scutellaria baicalensis].

OBJECTIVE: To optimize the extraction process of baicalin from Scutellaria Baicalensis Georgi with ethanol. METHODS: The extraction process of baicalin from Scutellaria Baicalensis Georgi was optimized with the aid of orthogonal experiments. RESULTS: The optimal parameters were obtained as follows: extracting temperature, extracting time, ethanol concentration and mechanical stirring speed were 80 degrees C, 1 h, 60% and 400 r/min, respectively. The extracting rate of baicalin was 83.90%. CONCLUSION: Comparing ethanol circumfluence, boiling water, ultrasonic wave and semi-bionic on extracting baicalin from Scutellaria baicalensis Georgi, the ethanol circumfluence extraction is better than other technological conditions on extracting baicalin.

Selective adsorption and recycle of Cu2+ from aqueous solution by modified sugarcane bagasse under dynamic condition.

Tetraethylenepentamine modified sugarcane bagasse was prepared and applied to test its feasibility in removing and recovering Cu2+ from wastewater under dynamic condition. Results showed that the Cu2+ could be selectively absorbed from wastewater by the modified SCB fixed bed column. To understand the adsorption mechanism, Cd2+ had been selected as the model interfering ion to investigate how co-ions influence the adsorption of Cu2+ on the sorbent. It was observed that the adsorption capacity of the sorbent for Cu2+ (0.26 mmol g-1) was significantly higher than that of Cd2+ (0.03 mmol g-1), even when the Cd2+ initial concentration was 100 times higher than that of Cu2+ in the binary system. This finding indicated that the presence of Cd2+ in the solution exerted negligible influence on the adsorption of Cu2+ on the modified SCB. The selectivity of the modified sorbent was further confirmed in the Cu/Cd/Mg/Pb/K quinary system. Further analysis to dynamic adsorption experiment illustrated that, due to the presence of amine groups, the modified SCB showed strong coordination ability to Cu2+, which allowed the other adsorbed ions (e.g., Cd2+) desorbed. This high adsorption selectivity toward Cu2+ suggested that this prepared sorbent would be a promising candidate for removing and recovering Cu2+ from wastewater.

Simultaneous removal of cationic and anionic dyes by the mixed sorbent of magnetic and non-magnetic modified sugarcane bagasse.

Magnetic carboxyl groups modified (MMS) and non-magnetic amine groups modified (AMS) sugarcane bagasse were prepared and mixed to remove cationic and anionic dye simultaneously from aqueous solution. For comparison, the adsorption performances of MMS, AMS and the mixed sorbent for basic magenta (cationic dye) and congo red (anionic dye) were investigated in the binary system. Zeta potential analysis showed that MMS was negatively charged and AMS was positively charged in the investigated pH range. The adsorption capacities of MMS for basic magenta and congo red were 1.24 and 0.04mmolg(-1), while those of AMS were 0.04 and 1.55mmolg(-1), respectively. Both of MMS and AMS had high adsorption capacity and affinity toward opposite-charged dye but low adsorption capacity and affinity toward similar-charged dye. Adsorption experiments in the binary system showed that only the mixed sorbent could remove the two dyes simultaneously from aqueous solution (removal efficiencies >90%). The amounts of basic magenta and congo red absorbed on the mixed sorbent both increased linearly with the increase of their initial concentrations in the investigated range. The dye loaded mixed magnetic and non-magnetic sorbents could be separated by a magnet. MMS and AMS could be regenerated by using acid and alkaline eluents, respectively. After regeneration, the MMS and AMS could be mixed again and used repeatedly. The mixed sorbent had great potential in practical dye waste water treatment.

Development of a novel fluorescent probe for nitric oxide detection: 8-(3',4'-diaminophenyl)-difluoroboradiaza-S-indacence.

A novel fluorescent probe, 8-(3',4'-diaminophenyl)-difluoroboradiaza-S-indacence (DABODIPY), was designed and synthesized for monitoring nitric oxide production, which features high photostability and no pH dependency over a wide pH range. The fluorescence of 8-(3',4'-diaminophenyl)-difluoroboradiaza-S-indacence is very low, however, when the probe traps nitric oxide (NO) in the presence of dioxygen, the strong fluorescent triazole form is obtained, which offers the advantages of specificity, and sensitivity for direct detection of NO. Calibration using various concentrations of NO showed the method has good linearity (0.08-4.00 micromol l(-1)) and its detection limit is 10 nmol l(-1) (s/n = 3). The proposed method has been used to monitor the release of NO from S-nitrosocysteine, a NO-releasing agent.