Cement paste column for simultaneous removal of fluoride, phosphate, and nitrate in acidic wastewater

Study on the effect of an ultrasound assisted reaction on the crystallization properties of recovered cryolite

During the treatment of spent cathode carbon from electrolytic aluminum, a large amount of fluoride containing wastewater is generated. By adding different sodium source and aluminum source reagents, under the conditions of different addition order, pH, temperature and time, the effects of conventional static reaction, stirring reaction and ultrasonic assisted reaction on the crystallization properties of recovered cryolite were investigated. The results showed that under the optimum reaction conditions (sodium source: NaCl, aluminum source: AlCl3, the molar ratio of AlCl3 to NaCl is 1 : 3, addition order: first addition of AlCl3 and then NaCl, pH is 8.57, time is 40 min, temperature at room temperature), the removal efficiency of fluoride ions was the highest when ultrasound assisted treatment was used. The cryolite products with ultrasound assisted crystallization and without ultrasound assisted crystallization were characterized using SEM and TEM. The results showed that the crystal particles obtained by ultrasound assisted crystallization were relatively concentrated, and the morphology was regular and the surface was smooth. Design Expert orthogonal software was used to design the response surface test, it was found that ultrasound time has the most significant impact on the content of recovered cryolite among single factors, and the interaction between ultrasound frequency and ultrasound power, ultrasound power and ultrasound time was highly significant among multiple factors.

Simultaneous removal of fluoride and phosphate from semiconductor wastewater via chemical precipitation of calcium fluoride and hydroxyapatite using byproduct of recycled aggregate

Semiconductor wastewater with high concentrations of fluoride and phosphate is an environmental issue that cannot be ignored. Moreover, the byproduct of recycled aggregates, concrete fines, cannot be reused in concrete manufacturing, which is a key issue to address for the sustainable development of the concrete industry. The objective of this study was to tackle the crucial environmental issues of these two industries by developing concrete fines as an alternative material to treat semiconductor wastewater. The chemical precipitation of calcium fluoride and hydroxyapatite in the presence of concrete fines was determined as the mechanism underpinning the removal of fluoride and phosphate in wastewater. Owing to the wide range of contaminant concentration and solution pH and the possibility of multi-stage treatment, the effects of the initial contaminant concentration (F: 100-1000 mg/L; P: 20-200 mg/L) and solution pH (pH: 2-7) on the removal reactions were determined. The highest F and P removal percentages were more than 99%, and the final F and P concentrations met the effluent standard (F: 15 mg/L, P: 1.3 mg/L). The removal reactions of F and P are generally in competition, and the removal of F has priority over the removal of P. The pseudo-second-order model can describe the kinetics of the removal reactions well. The formation of fluorapatite can reduce the F concentration below the concentration achievable by CaF2 precipitation alone. Furthermore, using the byproduct of recycled aggregates instead of conventional chemicals to treat semiconductor wastewater is promising in terms of reducing CO2 emissions, and prospective applications are discussed. This study can lead to the development of a sustainable and clean process for semiconductor wastewater treatment using byproducts from the concrete industry.

Performance of biochar mixed cement paste for removal of Cu, Pb and Zn from stormwater

Using biochar as a partial replacement of Portland cement in cementitious materials is a promising solution to mitigate negative environmental impacts. However, current studies in available literature primarily focus on the mechanical properties of composites made with cementitious materials and biochar. Therefore, this paper reports the effects of the type of biochar, the percentage of biochar addition, and the particle size of the biochar on the removal efficiency of Cu, Pb, and Zn, as well as the effect of contact time on the removal efficiency of Cu, Pb, and Zn, along with the compressive strength. The peak intensities of OH^-, CO₃^2- and Calcium Silicate Hydrate (Ca-Si-H) peaks increase with increasing biochar addition levels, reflecting increased hydration product formation. The reduction of particle size of biochar causes the polymerization of the Ca-Si-H gel. However, no significant changes were observed in heavy metal removal, irrespective of the percentage of biochar addition, the particle size of biochar, or the type of biochar added to the cement paste. Adsorption capacities above 19 mg/g, 11 mg/g and 19 mg/g for Cu, Pb and Zn were recorded in all composites at an initial pH of 6.0. The Pseudo second order model best described the kinetics of the Cu, Pb, and Zn removal. The rate of adsorptive removal increases with the decrease in the density of the adsorbents. Over 40% of Cu and Zn were removed as carbonates and hydroxides through precipitation, whereas over 80% of Pb removal was via adsorption. Heavy metals bonded with OH^-, CO₃^2- and Ca-Si-H functional groups. The results demonstrate that biochar can be used as a cement replacement without negatively impacting heavy metal removal. However, neutralization of the high pH is needed before safe discharge.

Influence of concrete material of runoff collection containers on monitoring of nitrogen and phosphorus pollutants

The accurate monitoring of N and P surface runoff losses from farmland is crucial to control agricultural nonpoint source pollution. A pond constructed with concrete material (CM) is a common collection container used during field experiments in China, but the adsorption characteristics of concrete may cause a considerable underestimation of surface runoff losses from farmland. To characterize any neglected error caused by the collection container material, a laboratory experiment was conducted comparing the N and P contents of runoff samples collected from CM and plastic material (PM) containers. The results indicated that CM containers significantly lowered N and P sample contents compared with PM containers, which was attributed to the adsorption capacity of pollutants by CM containers. This was confirmed by scanning electron microscopy (SEM) images of particles retained in CM containers. In an attempt to alleviate this error, three common water-repellent materials were applied to CM containers that significantly limited the pollutant adsorption of CM containers. Moreover, it was shown that there was no significant difference between the calculated concentration of runoff losses and the total amount of pollutants. To calibrate the observational error from CM containers, stepwise multiple regression models of different forms of N and P pollutants were developed. The results of this study suggest that treating CM containers with water repellent is an effective measure for improving the accuracy of new-built monitor points of agricultural nonpoint source pollutants. In addition, the calibration of observational error from CM containers and delayed sampling is essential to estimate agricultural nonpoint source pollution load via the surface runoff from farmland based on data from monitor points.

Retention of phosphorus and fluorine in phosphogypsum for cemented paste backfill: Experimental and numerical simulation studies

The solidification/stabilization of phosphogypsum using cemented paste backfill (OCPB) provides a low-cost and alternative in-situ technique for recycling phosphogypsum stockpiles. But the OCPB is far from obtaining steady states in which the pollutants would redistribute as a response to dynamic environmental conditions. Further, the associated chemical interactions and the mineralogy information of the solubility-controlling phases of contaminants (fluorine and phosphorus) have not been thoroughly studied or fully understood. In this study, a framework coupling the chemical, mineralogical, and morphological analyses is used to determine the fluoride and phosphate retention mechanisms of immobilized OCPB. Then the pH-dependent leaching tests and numerical simulation is applied as a useful tool to identify the minerals controlling stabilized OCPB leaching behavior. The overall findings proved that aluminate-rich calcium silicate hydrates play an essential role in fluoride and phosphate retention. Both experimental and simulational acid neutralization and leaching curves indicate that the cementitious matrix works as a strong buffering material ensuring high pH conditions that are necessary for fluorine and phosphorus retention. Although discrepancies were observed in absolute fluorine and phosphorus leaching values at highly acidic conditions, the simulations are able to describe highly amphoteric leaching behavior. The simulation suggests that the aluminum species and calcium phosphates governed the solubility of fluorine and phosphorus, respectively. The results of this work would have implications for predicting the leaching behavior of OCPB in detrimental and multiple environments.

Simultaneous and efficient removal of fluoride and phosphate in phosphogypsum leachate by acid-modified sulfoaluminate cement

The high concentration of fluoride and phosphate in phosphogypsum leachate is harmful to the environment and ecosystem. Thus, there is a need to develop feasible materials or technologies to remove both fluoride and phosphate in acidic phosphogypsum leachate. In this study, sulfoaluminate cement (SC) was used to simultaneously remove fluoride and phosphate in wastewater based on its moderate alkalinity and rich content of metal elements (Ca, Al and Fe, etc). The acidized sulfoaluminate cement (ASC) composite was prepared through modifying SC with hydrochloric acid, which can increase the specific surface areas of the raw SC, as well as the activity of the metal elements in SC. Compared with other coagulants, ASC showed excellent removal performance for fluoride and phosphate, such as higher removal efficiency, better effluent quality, and accelerated settling rate. The fluoride and phosphate removal performances of ASC herein were investigated at different dosages, pH values, coexisting substances, and initial concentrations. As a result, ASC exhibited wide pH adaptability and satisfactory selectivity for fluoride and phosphate. The possible removal mechanisms of fluoride and phosphate by ASC included chemisorption, ion exchange, and precipitation. The main end products associated with fluoride were fluorite (CaF₂), aluminum fluoride (AlF₃), and iron trifluoride (FeF₃). The main final products amid phosphate removal, on the other hand, were brushite (CaHPO₄·2H₂O), aluminophosphate ((H₃O)·AlP₂O₆(OH)₂), silicocarnotite (Ca₂SiO₄·Ca₃(PO₄)₂) and iron phosphate (Fe(H₂PO₄)₃). More importantly, ASC can effectively treat the phosphogypsum leachate at a wide range of concentrations, and the concentrations of phosphate and fluoride in the effluents were lower than 0.5 mg P L^-1 and 4 mg L^-1, respectively. To sum up, ASC is a competitive candidate to treat wastewater with high fluoride and phosphate content, such as phosphogypsum leachate.

Environmental management and potential valorization of wastes generated in passive treatments of fertilizer industry effluents

A phosphogypsum stack located in SW Spain releases highly acidic and contaminated leachates to the surrounding estuarine environment. Column experiments, based on a mixture of an alkaline reagent (i.e., MgO or Ca(OH)₂) dispersed in an inert matrix (dispersed alkaline substrate (DAS) technology), have shown high effectiveness for the treatment of phosphogypsum leachates. MgO-DAS and Ca(OH)₂-DAS treatment systems achieved near total removal of PO₄, F, Fe, Zn, Al, Cr, Cd, U, and As, with initial reactive mass:volume of leachate treated ratios of 3.98 g/L and 6.35 g/L, respectively. The precipitation of phosphate (i.e., brushite, cattiite, fluorapatite, struvite and Mn₃Zn(PO₄)₂·2H₂O) and sulfate (i.e., despujolsite and gypsum) minerals could control the solubility of contaminants during the treatments. Therefore, the hazardousness of these wastes must be accurately assessed in order to be properly managed, avoiding potential environmental impacts. For this purpose, two standardized leaching tests (EN-12457-2 from the European Union and TCLP from the United States) were performed. According to European Union (EN-12457-2) regulation, some wastes recovered from DAS treatments should be classified as hazardous wastes because of the high concentrations of SO₄ or Sb that are leached. However, according to United States (US EPA-TCLP) legislation, all DAS wastes are designated as non-hazardous wastes. Moreover, the solids generated in the DAS systems could constitute a promising secondary source of calcite and/or P. This research could contribute to worldwide suitable waste management for the fertilizer industry.

Removal of fluoride ions from aqueous solution by metaettringite

Fluoride contamination is a major problem in wastewater treatment. Metaettringite (which has previously shown enhanced anion adsorption) was investigated as a possible adsorbent to remove fluoride from low-concentration solution (25 mg-F/L). The fluoride removal properties of ettringite and metaettringite were first compared at pH 10, and metaettringite was found to be more effective. The dominant reaction mechanism for fluoride adsorption in metaettringite was found to be recrystallization of metaettringite by rehydration; this was accompanied by precipitation of calcium fluoride. The adsorption kinetics followed the pseudo-second order model. Metaettringite was also able to remove fluoride effectively in low pH environment (i.e., at pH 3.5). The influence of coexistence of sulfate ions in solution on the fluoride removal performance was investigated, and a small decrease in performance was noted. The residual fluoride concentrations obtained with higher doses of metaettringite were lower than those specified by the Japanese effluent standard (non-coastal areas: 8 mg-F/L; coastal areas: 15 mg-F/L). The fluoride removal capacity of metaettringite was compared with those of other solid materials. The observed maximum capacity was 174.7 mg-F/g-metaettringite. In the case of high fluoride concentration solution, the main removal mechanism will be changed to calcium fluoride precipitation. In general, metaettringite is regarded as promising material for fluoride removal in wastewater treatment.

Leaching behavior and environmental safety evaluation of fluorine ions from shotcrete with high-fluorine alkali-free liquid accelerator

High-fluorine alkali-free liquid accelerator (AF_-hf, F^- concentration was about 2.31g/L) was still used in engineering because of its low cost, excellent stability, and coagulation-promoting effect. The main purpose of this study was to explore the leaching behavior of fluorine ions in shotcrete for tunnel lining with high-fluorine alkali-free liquid accelerator and whether there was fluoride pollution. The setting time and mechanical properties of cement paste and mortar with AF_-hf were tested. Under different environmental conditions, F^- leaching concentration from sprayed concrete was studied comparatively. Moreover, XRD and SEM were used to analyze the crystal composition and micro morphology of hydration products. The experimental results showed that with the increase of AF_-hf dosage, the setting time of cement paste was greatly shortened, and later strength of mortar and shotcrete could meet the construction requirements. In addition, when the leaching solution type (including Na₂CO₃, Na₃PO₄, Na₂SO₄, and NaNO₃) and testing conditions (particle size, soaking temperature, leaching solutions) were different, F^- leaching concentration changed regularly, and the minimum value was more than 20 mg/L, which might cause fluorine pollution to groundwater and soil. After shotcrete samples were soaked, the CaF₂ peaks' intensity was relatively weaker and Ca(OH)₂ decreased obviously. Meanwhile, cement hydration products became looser and abundant of flaky C-S-H gel transformed into fibrous and chained structure.

Simultaneous removal of nitrate and phosphate in groundwater using Ca-citrate complex

Eutrophication can be caused by excessive input of nutrients, such as nitrate and phosphate, to surface water. Nutrients in groundwater can enter surface water by means of base flow, requiring treatment before they reach surface water bodies. While some studies have attempted to remove nitrate and phosphate, methods for simultaneous removal in groundwater have rarely been reported. In this study, we propose an innovative treatment method to simultaneously remove nitrate and phosphate in groundwater based on an injection of Ca-citrate complex. A total of five batch experiments with different conditions were conducted to identify the removal mechanisms of nitrate and phosphate and to evaluate the use of alternative organic materials, such as lactate. The results showed that Ca-citrate complex can remove nitrate and phosphate simultaneously. Nitrate was removed through denitrification by denitrifying bacteria which used citrate as a carbon source. The removal mechanisms for phosphate were precipitation of phosphate minerals (e.g., hydroxyapatite) and adsorption. The results also showed that reactive materials based on Ca-lactate complex were able to remove nitrate and phosphate. This study suggests that nitrate and phosphate in groundwater can simultaneously be removed using organic-based calcium complexes, proposing a promising remedial method to alleviate potential eutrophication in surface water as well as groundwater contamination.

Performance of permeable pavement systems on stormwater permeability and pollutant removal

Permeable pavement is an effective means for stormwater runoff control and pollutant removal. However, relatively few studies have examined the characteristics of permeable brick and corresponding permeable pavement system (PPS). In this work, the permeable pavement systems consisted of surface permeable brick layer (concrete or ceramic) with structural layer (including a cement mortar layer, a permeable concrete layer, and a gravel layers) were selected as typical cases to assess their permeability and runoff pollutant removal performance by laboratory experiments. The results indicated that PPS had obvious outflow hysteresis effect. The PPS with ceramic brick layer reached the saturation flow rate earlier and showed larger outflow rate than that with concrete brick layer. Both types of PPSs had a relatively high efficiency (83.8-95.2%) in removing suspended solids (SS) in stormwater runoff mainly due to the interception and filtration of the surface brick layer, whereas the structural layer of the PPS played a vital role in the removal of total phosphorus (TP). The percentage of total nitrogen (TN) removal efficiency via ceramic brick layer accounted for via corresponding PPS was obviously larger than that of concrete brick layer. The PPS also displayed a certain chemical oxygen demand (COD) removal ability: around 14.0-27.0% for concrete type and 20.9-28.9% for ceramic type. Subsequently, a multi-objective evaluation model was implemented based on the analytic hierarchy process (AHP) method to identify the optimal scheme in relation to four indices: permeability, environmental benefit, compressive strength, and comprehensive economic cost. The results showed, insofar, the ceramic PPS is preferred with a better economic performance. Our study attempts to select optimal designs of PPS and provides insight into the permeable capacity and the efficiency of pollutant removal in PPS.

Current Applications of Recycled Aggregates from Construction and Demolition: A Review

A literature review comprising 163 publications published over a period of 26 years from 1992 to 2018 is presented in this paper. This review discusses the generation and recycling of construction and demolition waste (CDW) as well as its main uses as raw materials for the construction engineering sector. This review pays attention to the use of CDW aggregates for sand, pavements/roads, bricks, ceramics, cementitious materials, and concrete productions, as well its uses as eco-friendly materials for water decontamination. The physical-chemical and mechanical characteristics of recycled aggregates play an important role in their correctly chosen applications. The results found in this literature survey allow us to conclude that recycled aggregates from CDW can be successfully used to produce construction materials with quality comparable to those produced with natural aggregates. We concluded that the use of CDWs as raw materials for manufacturing new construction materials is technically feasible, economical, and constitutes an environmentally friendly approach for a future construction and demolition waste management strategy.

Design and optimization of sustainable passive treatment systems for phosphogypsum leachates in an orphan disposal site

The optimization of the dispersed alkaline substrate (DAS) technology was investigated to achieve the treatment of highly acidic and polluted effluents from a phosphogypsum pile in an orphan site of SW Spain. This phosphogypsum disposal area is located on the Tinto river marsh soils, where it acts as a source of pollution for the estuarine environment, releasing high concentrations of metal(loid)s and radionuclides, which degrade the surrounding waters. The methodology consists of flowing the leachates through columns loaded with a combination of a fine-grained alkaline reagent scattered in a non-reactive matrix to raise the water pH while decreasing the solubility of dissolved contaminants. Seven columns were built, one for each of the alkaline reagent used: limestone, barium carbonate, biomass ash, fly ash, MgO, Mg(OH)₂, and Ca(OH)₂. The Ca(OH)₂-DAS and MgO-DAS treatment systems showed the highest effectiveness, reaching near-total removal for PO₄, F, Fe, Zn, Cu, Al, Cr, and U with initial reagent mass:treated volume ratios of 36.3 g/L and 7.57 g/L, respectively. Total As removal was only achieved in the Ca(OH)₂-DAS treatment. Phosphate precipitation was the main mechanism responsible for pollutants removal. Geochemical modeling using PHREEQC code and mineralogical evidence confirmed the precipitation of these minerals. This study forms the basis of an effective and environmentally sustainable treatment system for phosphogypsum leachates to reduce the impact of the fertilizer industry worldwide.

Phosphorus removal from wastewater by waste concrete: influence of P concentration and temperature on the product

This study investigated the feature of phosphorus uptake by low-cost waste concrete. Adsorption isotherms, metal dissolution, influence of P concentration and temperature, as well as adsorbent regeneration were investigated. Chemical extraction, SEM, XRD, FTIR, and XPS were employed to determine the products of P sequestration. Results demonstrated that phosphate adsorption fitted the Langmuir isotherm model well, with estimated maximum phosphate adsorption capacity of 80.5 mg/g (10 °C). Of adsorbed phosphate, 72.1% could be desorbed when 0.1 M citrate buffer was used as eluant, and waste concrete could be recovered and reused for 4 times by the combination of eluting and roasting. Mechanisms including Ca/alkali dissolution, surface adsorption, and chemical precipitation are involved in the sequestration of phosphorus from wastewater by waste concrete. Weakly adsorptive phosphorus and Ca-P precipitate were the main products. P concentration was the major factor that affected P removal capacity and the product types, while temperature had certain effect at low P concentration. The dominant product was weakly adsorptive phosphorus for low P concentration at low temperature, which was substituted by Ca-P precipitate as temperature or P concentration increased. The increase of P concentration assisted both the increase of P removal potential and the formation of Ca-P precipitate to crystal DCPD.

Experimental and model study for fluoride removal by thermally activated sepiolite

The present investigation demonstrates the preparation of thermally activated sepiolite for effective removal of fluoride via adsorption from an aqueous solution. The thermal treatments on sepiolite were conducted at different temperatures (300-950 °C) for 4 h in an N₂ atmosphere, and the thermally activated sepiolite was characterized using a field emission scanning electron microscope (FESEM), X-ray diffractometry (XRD), X-ray fluorescence (XRF), a differential scanning calorimetry-thermogravimetric analyzer (DSC-TGA), and a surface area analyzer. Sepiolite that was treated at 950 °C was shown to have a higher fluoride removal efficiency than other temperatures. The fluoride removal was evaluated under different experimental conditions such as solution pH, adsorbent dose, reaction time, initial concentration, temperature, presence of co-existing ions, and reuses. The kinetic and equilibrium adsorption results were well described by the pseudo-second-order kinetic model and Langmuir isotherm, respectively, and adsorption of fluoride onto thermally activated sepiolite was endothermic and spontaneous in nature. The Langmuir maximum adsorption capacity (169.95 mg/g) was superior to the literature value. The thermally activated sepiolite was also effective in a continuous flow system for treating fluoride. Thus, this thermally activated sepiolite is expected to be used as an effective adsorbent for the removal of fluoride in water.

Building global change resilience: Concrete has the potential to ameliorate the negative effects of climate-driven ocean change on a newly-settled calcifying invertebrate

Global climate change is driving sea level rise and increasingly frequent storm events, which are negatively impacting rapidly-growing coastal communities. To mitigate these impacts, coastal infrastructure must be further protected by upgrading hard defences. We propose that incorporating pH-buffering materials into these upgrades could safeguard marine organisms from the adverse effects of ocean acidification and ocean warming during the vulnerable transition from planktonic larvae to benthic juveniles. To test this, we examined the effects of ocean warming (24 or 27 °C), ocean acidification (pH 8.1, 7.9, 7.7), and substratum (concrete, greywacke, granite) in all combinations on the settlement success of an ecologically and commercially important sea urchin, Tripneustes gratilla. Low pH (7.9, 7.7) generally reduced the quantity and size of juveniles four weeks post-settlement, although this was partially ameliorated by increased temperature (24 vs. 27 °C). In the warmed and acidified treatments, settlement rates were lower on concrete than granite or greywacke, but two weeks post-settlement, juveniles on concrete were larger, and had longer spines and higher survival rates than on greywacke or granite, respectively. The benefits provided by concrete to newly-settled juveniles may be related to alkali chemicals leaching from concrete buffering low pH conditions in surrounding seawater and/or increased availability of bicarbonate in the boundary layers around its surface. Our results highlight the potential for pH-buffering materials to assist marine organisms in coping with the effects of changing ocean conditions, but further research is required to understand the generality and mechanism(s) driving the beneficial effects of concrete and to test pH-buffering materials in the field.

Alginate-modified biochar derived from Ca(II)-impregnated biomass: Excellent anti-interference ability for Pb(II) removal

A novel biochar modified with sodium alginate was prepared using Ca(II)-impregnated biomass, and used to remove metals from aqueous solutions. The maximum adsorption capacity for Pb(II) was estimated to be 1.225 mmol/g (253.6 mg/g), which is far more than that of most adsorbents. Moreover, the modified biochar had a great anti-interference ability for effective removal of Pb(II) from multi-metal system. The biochar still had strong ability to adsorb Pb(II) when the initial concentrations of interfering ions were 5 times higher than that of Pb(II). Functional groups and minerals of the biochar worked for Pb(II) removal and the anti-interference ability. On the one hand, carboxyl could complex with Pb(II) through monodentate and bidentate bridging; on the other hand, Pb(II) was easier to form a precipitate with minerals than other metals. This study suggested that the novel biochar had the potential for practical application in effective removal of Pb(II) from wastewater.

Biochar composites with nano zerovalent iron and eggshell powder for nitrate removal from aqueous solution with coexisting chloride ions

Biochar (BC) was produced from date palm tree leaves and its composites were prepared with nano zerovalent iron (nZVI-BC) and hen eggshell powder (EP-BC). The produced BC and its composites were characterized by SEM, XRD, BET, and FTIR for surface structural, mineralogical, and chemical groups and tested for their efficiency for nitrate removal from aqueous solutions in the presence and absence of chloride ions. The incidence of graphene and nano zerovalent iron (Fe0) in the nZVI-BC composite was confirmed by XRD. The nZVI-BC composite possessed highest surface area (220.92 m2 g-1), carbon (80.55%), nitrogen (3.78%), and hydrogen (11.09%) contents compared to other materials. Nitrate sorption data was fitted well to the Langmuir (R 2 = 0.93-0.98) and Freundlich (R 2 = 0.90-0.99) isotherms. The sorption kinetics was adequately explained by the pseudo-second-order, power function, and Elovich models. The nZVI-BC composite showed highest Langmuir predicted sorption capacity (148.10 mg g-1) followed by EP-BC composite (72.77 mg g-1). In addition to the high surface area, the higher nitrate removal capacity of nZVI-BC composite could be attributed to the combination of two processes, i.e., chemisorption (outer-sphere complexation) and reduction of nitrate to ammonia or nitrogen by Fe0. The appearance of Fe-O stretching and N-H bonds in post-sorption FTIR spectra of nZVI-BC composite suggested the occurrence of redox reaction and formation of Fe compound with N, such as ferric nitrate (Fe(NO3)3·9H2O). Coexistence of chloride ions negatively influenced the nitrate sorption. The decrease in nitrate sorption with increasing chloride ion concentration was observed, which could be due to the competition of free active sites on the sorbents between nitrate and chloride ions. The nZVI-BC composite exhibited higher nitrate removal efficiency compared to other materials even in the presence of highest concentration (100 mg L-1) of coexisting chloride ion.

Phosphate adsorption onto thermally dehydrated aluminate cement granules

Phosphorus is the main element for eutrophication of water bodies. Aluminate cement is a cheap building material rich in aluminium and calcium which have significant effects on phosphate adsorption. This study aimed at the investigation of removal behavior of phosphate by thermally dehydrated aluminate cement granules, treated at different temperatures, and the adsorption mechanisms. It was found that 600 °C was the optimal temperature, producing excellent granules with a particle size of 0.6–1.5 mm (T600), giving a great adsorption capacity of phosphate of 49.1 mg P per g and presenting fast and high initial adsorption, reaching a capacity of 23.7 mg P per g within 30 min at 20 °C. The phosphate adsorption process was dominated by chemical adsorption, mainly through inner-sphere complexion and phosphate precipitation on the surface of the adsorbent. Compared with other phosphate adsorbents, T600 may be an economical and efficient adsorbent.

Thermally dehydrated aluminate cement granules show a large phosphate adsorption capacity of 49.1 mg P per g and fast and high initial adsorption.

Perfluorooctane sulfonate adsorption on powder activated carbon: Effect of phosphate (P) competition, pH, and temperature

Powdered activated carbon (PAC), as an adsorbent, was applied to remove perfluorooctane sulfonate (PFOS) from aqueous solution. Laboratory batch experiments were performed to investigate the influences of phosphate (P) competition, temperature, and pH for PFOS adsorption onto PAC. The results showed that higher temperature favored PFOS adsorption in single and binary systems. The kinetic data fitted very well to the pseudo second-order kinetic model. Thermodynamically, the endothermic enthalpy of the PFOS adsorption in single and binary systems were 125.07 and 21.25 kJ mol^-1, respectively. The entropy of the PFOS adsorption in single and binary systems were 0.479 and 0.092 kJ mol^-1 K^-1, respectively. And the Gibbs constants were negative. These results indicated that the adsorption processes were spontaneous. The adsorption isotherms of PFOS agreed well with the Langmuir model. In the single system, PFOS adsorption decreased with increased pH value. The difference in the amount of PFOS adsorption between the single and binary systems increased at higher pH. Frustrated total internal reflection (FTIR) demonstrated that P competition increased the hydrophilicity of the PAC and the electrostatic repulsion between PFOS and PAC, then the PFOS adsorption amount decreased. It also demonstrated that, at higher temperature, increased PFOS adsorption was mainly due to the higher diffusion rate of PFOS molecules and greater number of active sites opened on the PAC surface.

Co-adsorption of perfluorooctane sulfonate and phosphate on boehmite: Influence of temperature, phosphate initial concentration and pH

The co-presence of perfluorooctane sulfonate (PFOS) and phosphate in wastewater of various industries has been detected. Removing PFOS and phosphate simultaneously before discharging sewage into natural water can decrease effectively the environmental risk caused by the combined pollution of PFOS and phosphate. In this study, laboratory batch experiments were conducted for investigating the co-adsorption of PFOS and phosphate on boehmite and the influences of temperature, phosphate initial concentration and pH on the co-adsorption. The adsorption thermodynamics and kinetics of PFOS and phosphate on boehmite were also investigated completely and systematically. The results showed that lower temperature favored the co-adsorptions of PFOS and phosphate. The adsorption of PFOS and phosphate on boehmite agreed well with the Langmuir isotherm and the adsorption parameters of thermodynamics are ΔH=-16.9 and -20.0kJmol^-1 (PFOS and phosphate), ΔS=-5.69 and -7.63Jmol^-1 K^-1 (PFOS and phosphate) and ΔG <0 (PFOS and phosphate). It demonstrated that the co-adsorption of PFOS and phosphate on boehmite is a spontaneously exothermic process. Moreover, the co-adsorption process can be described well by a pseudo-second-order kinetic model. With increasing phosphate initial concentration, more phosphate could be adsorbed on boehmite, while the adsorption of PFOS decreased at phosphate initial concentration of less than 30mgL^-1 and increased at that of larger than 30mgL^-1. In the co-adsorption process, the adsorption amount of PFOS decreased with pH increasing, but that of phosphate changed little.

Performance and characterization of a non-sintered zeolite porous filter for the simultaneous removal of nitrogen and phosphorus in a biological aerated filter (BAF)

A novel non-sintered zeolite porous filter (ZPF) and commercially available ceramsite (CAC) are used to investigate the simultaneous removal of nitrogen and phosphorus from city wastewater treated by biological aerated filter (BAF) reactors.

Use of phosphorus-sorbing materials to remove phosphate from greenhouse wastewater

High phosphate content in wastewater is currently a major issue faced by the North American greenhouse industry. Phosphate-sorbing material filters could provide a means of removing phosphate from wastewater prior to discharge to the environment, but the characterization of economically viable materials and specific recommendations for greenhouse wastewater are not available. Batch and column experiments were used to examine the capacity of two calcium-based waste materials, basic oxygen furnace slag and a concrete waste material, to remove phosphate from greenhouse nutrient solution at varied operating conditions. Material columns operating at a hydraulic retention time (HRT) of 3 h consistently removed >99% of influent phosphate at a concentration of 60 mg/L over repeated applications and demonstrated high phosphate retention capacity (PRC) of 8.8 and 5.1 g P/kg for slag and concrete waste, respectively. Both materials also provided some removal of the micronutrients Fe, Mn and Zn. Increasing HRT to 24 h increased P retention capacity of slag to >10.5 g P/kg but did not improve retention by concrete waste. Decreasing influent phosphate concentration to 20 mg/L decreased PRC to 1.64 g P/kg in concrete waste columns, suggesting fluctuations in greenhouse wastewater composition will affect filter performance. The pH of filter effluent was closely correlated to final P concentration and can likely be used to monitor treatment effectiveness. This study demonstrated that calcium-based materials are promising for the removal of phosphate from greenhouse wastewater, and worthy of further research on scaling up the application to a full-sized system.

Simultaneous removal of nitrate, hydrogen peroxide and phosphate in semiconductor acidic wastewater by zero-valent iron

The zero-valent iron (ZVI) wastewater treatment has been applied to simultaneous removal of nitrate, hydrogen peroxide and phosphate in semiconductor acidic wastewaters. The simultaneous removal occurs by the reactions performed due to the sequential transformation of ZVI under the acidic condition. Fortunately the solution pH of semiconductor acidic wastewaters is low which is effective for the sequential transformation of ZVI. Firstly the reduction of nitrate is taken place by electrons generated by the corrosion of ZVI under acidic conditions. Secondly the ferrous ion generated by the corrosion of ZVI reacts with hydrogen peroxide and generates ·OH radical (Fenton reaction). The Fenton reaction consists of the degradation of hydrogen peroxide and the generation of ferric ion. Finally phosphate precipitates out with iron ions. In the simultaneous removal process, 1.6 mM nitrate, 9.0 mM hydrogen peroxide and 1.0 mM phosphate were completely removed by ZVI within 100, 15 and 15 min, respectively. The synergy among the reactions for the removal of nitrate, hydrogen peroxide and phosphate was found. In the individual pollutant removal experiment, the removal of phosphate by ZVI was limited to 80% after 300 min. Its removal rate was considerably improved in the presence of hydrogen peroxide and the complete removal of phosphate was achieved after 15 min.

Mechanisms of phosphorus removal by cement-bound ochre pellets

Hydrous ferric oxide (here termed 'ochre') sludge, an abundant waste product produced from the treatment of acid mine drainage (AMD), was used in this study for the removal of phosphorus (in the form of phosphate ions) from contaminated waters. The phosphorus uptake capacities of both raw and pelletized AMD solids were compared using batch and column tests. Addition of a cement binder to the AMD solids during pellet production led to significantly increased P-loading of the resultant solids compared to the raw sludge. Additionally, the pellets were found to continue to remove P in tests up to 7 d in duration whereas the unbound AMD sludge appeared to approach equilibrium with phosphate solution after approximately 60 min of contact time. In line with previous studies P uptake by the AMD solids was found to be primarily via adsorption. By contrast calcium phosphate precipitation was found to be the dominant removal mechanism for the cement-bound ochre pellets with a relatively small proportion of removal attributable to the AMD solids. SEM-EDX analysis of the surface of used pellets showed a Ca:P molar ratio close to that of hydroxyapatite (HAP). Continuous column tests on these pellets showed a rapid decrease in P removal capacity by the pellets over time, attributable to the formation of a passivating HAP surface layer.

Properties of synthetic monosulfate as a novel material for arsenic removal

Monosulfate was examined as a novel material for As(V) removal since its layered double hydroxide structure was expected to possess a high capacity for anion exchange. Phase-pure monosulfate was synthesized by hydration at 80-90°C for 36 h using a stoichiometric mixture of tricalcium aluminate (calcined at 1300°C) and gypsum. The analyses of PXRD, WDXRF, and FE-SEM confirmed the successful synthesis of highly pure monosulfate with a negligible impurity of ettringite. Batch experiments were carried out to investigate the kinetics of As(V) removal by monosulfate. A close relationship between As(V) uptake and sulfate release was observed. The intercalation of arsenate in the interlayer of monosulfate was confirmed by PXRD and FT-IR analyses. From a series of equilibrium batch experiments, it is seen that initial sorption of As(V) on monosulfate follows Langmuir isotherm, whereas further injection of As(V) caused transformation of monosulfate to ettringite, which was confirmed by FE-SEM micrographs. However, after the transformation, the solid phases in the equilibrium experiments were found to significantly lose their ability to take up As(V) in exchange for sulfate. A possible explanation for this result was hypothesized and discussed in the context of the literature.

Competitive adsorption characteristics of fluoride and phosphate on calcined Mg-Al-CO3 layered double hydroxides

With synthetic wastewater, competitive adsorption characteristics of fluoride and phosphate on calcined Mg-Al-CO(3) layered double hydroxides (CLDH) were investigated. A series of batch experiments were performed to study the influence of various experimental parameters, such as pH, contact time, and order of addition of the anions on the competitive adsorption of fluoride and phosphate on CLDH. It was found that the optimal pH is around 6 and it took 24 h to attain equilibrium when fluoride and phosphate were simultaneous added. The order of addition of anions influenced the adsorption of fluoride and phosphate on CLDH. The kinetic data were analyzed using the pseudo first-order and pseudo second-order models and they were found to fit very well the pseudo second-order kinetic model. Data of equilibrium experiments were fitted well to Langmuir isotherm and the competitive monolayer adsorption capacities of fluoride and phosphate were found to be obviously lower than those of single anion at 25 °C. The results of X-ray diffraction, Scanning Electron Microscopy with energy-dispersive X-ray analyses, and ATR-FTIR demonstrate that the adsorption mechanism involves the rehydration of mixed metal oxides and concomitant intercalation of fluoride and phosphate ions into the interlayer to reconstruct the initial LDHs structure.

Phosphate adsorption on granular palygorskite: batch and column studies

A method to prepare granular palygorskite (GPA) was put forward in this research, and its potential use to remove phosphate species from aqueous solution was assessed. Batch experiments were performed to study the adsorption equilibrium and influence of contact time and pH on the adsorption and desorption of phosphate onto GPA in water. The maximum phosphate adsorption capacity of GPA was 13.1 mg/g. Kinetic data revealed that more than 90% of phosphate was adsorbed onto GPA within 2 hours. Phosphate adsorption capacity was 0.10 mg/g in column experiments, and co-existing anions could decrease phosphate removal. The saturated column was regenerated by 0.2 mol/L sodium hydroxide, and the GPA could be reused in phosphate removal. The data obtained from both batch and column studies indicated that GPA could be used effectively to remove phosphate from water.

Uptake of fluoride by nano-hydroxyapatite/chitosan, a bioinorganic composite

A bioinorganic composite namely nano-hydroxyapatite/chitosan (n-HApC) composite which could be employed for technology development was prepared and studied for its defluoridation efficiency. It has been observed that there was a slight enhancement in the defluoridation capacity (DC) of n-HApC composite (1560mgF(-)/kg) than nano-hydroxyapatite (n-HAp) which has a DC of 1296mgF(-)/kg. The sorbents were characterized with XRD and TEM studies. The fluoride sorption was explained with Freundlich and Langmuir isotherms. Thermodynamic parameters such as DeltaG degrees , DeltaH degrees , DeltaS degrees and Ea were calculated in order to understand the nature of sorption. The sorption process was found to be controlled by pseudo-second-order and pore diffusion models. Field studies were carried out with the fluoride containing water sample collected from a fluoride-endemic area in order to test the suitability of the sorbents at field conditions.