Comparison study of phosphorus adsorption on different waste solids: Fly ash, red mud and ferric-alum water treatment residues

New design to enhance phosphonate selective removal from water by MOF confined in hyper-cross-linked resin

Although polymeric anion exchange resins can remove phosphonates, they lack selectivity for target phosphonates and are susceptible to interference by anions and other substances. Here, we developed a novel strategy via confining MIL-101(Fe)-NH2 inside commercial resins IRA-900 for high-efficient and precise phosphonate removal, accompanying with the improvement of the stability and recovery of MIL-101(Fe)-NH2. The obtained nanocomposite MIL-101(Fe)-NH2@IRA-900 (MFNI) exhibited significantly enhanced phosphonate removal in the presence of competing anions (Cl-, SO42-, NO3- and CO32-) and natural organic matter (humic acid) at high concentrations (2-4 times of phosphonate concentration). Moreover, MFNI displayed the highest phosphonate adsorption capacity (12.9 mg P/g) and the fastest adsorption kinetics (120 min) than hydrated ferric oxides modified IRA-900 (HFOI) (6.7 mg P/g, 180 min), MIL-101(Fe)-NH2 (7.6 mg P/g, 240 min) and IRA-900 (5.6 mg P/g, 360 min). Such higher adsorption affinity and anti-interference ability came from the synergistic effect of the host IRA-900 (hydrogen-bond interaction and electrostatic attraction) and the embedded MIL-101(Fe)-NH2 (ligand exchange). The depleted MFNI could be regenerated with a binary NaOH-NaCl solution and reused without significant loss of capacity. Column adsorption runs by using MFNI indicated the fresh MFNI could achieve 100 % removal of PPOA in 10.5 h continuously feeding, which offered the possibility of achieving potential large-scale applications. In general, a new MOF-confined design approach was practiced to achieve selective elimination of phosphates and to improve the stability and recovery of MOF.

Recent trends in the use of fly ash for the adsorption of pollutants in contaminated wastewater and soils: Effects on soil quality and plant growth

Fly ash is one of the largest types of industrial wastes produced during the combustion of coal for energy generation. Finding efficient and sustainable solutions for its reuse has been the subject of substantial research worldwide. Here, we review the recent research data related to (i) the use of fly ash as a low-cost adsorbent for pollutants in wastewater and soils and (ii) its implications in soil-plant system. Fly ash showed prominent adsorption capacity for pollutants in water especially when it was activated or applied in composites. In addition to direct pollutant binding in soils, fly ash can enhance the soil pH indirectly increasing metals' immobilization reducing their plant uptake. Its non-selective adsorptive nature may lead to the co-adsorption of nutrients with pollutants which merits to be considered. Owing to its considerable nutrient contents, fly ash can also improve soil fertility and plant growth. The effects of fly ash on soil physico-chemical properties, microbial population and plant growth are critically evaluated. Fly ash can also contain potentially toxic contaminants (toxic metals, hydrocarbons, etc.) which could have harmful impacts on soil health and plant growth. Identifying the levels of inherent pollutants in fly ash is crucial to evaluate its suitability as a soil amendment. Negative effects of fly ash can also be addressed by using co-amendments, biological agents, and most importantly by an adequate calibration (dose and type) of fly ash based on site-specific conditions. Research directions are identified to promote the research regarding its use in wastewater treatment and agriculture.

Efficient low-concentration phosphate removal from sub-healthy surface water by adsorbent prepared based on functional complementary strategy

The remediation of low-concentration phosphorus polluted surface water (LP-SW) is one of most challenging environmental issues worldwide. Adsorption is more suitable for LP-SW remediation due to its low cost and operability. Based on the strategy of functional complementation among industrial solid wastes (ISWs), ISW-based phosphate absorbent material (PAM) was prepared from coal ash (CA, binder), rich‑calcium (Ca) carbide slag (CS, active component) and iron salt (functional reagent) by optimizing materials ratios and roasting conditions. PAM prepared under optimal conditions (Fe/CC-2opt) had good phosphate adsorption efficiency. Notably, Fe/CC-2opt not only ensured that the effluent met Environmental Quality Standards for Surface Water (pH = 6.0-9.0), but also facilitated the formation of brushite instead of hydroxyapatite due to FeSO4 addition. Compared with hydroxyapatite, brushite had greater potential application value as fertilizer due to its solubility and high P/Ca ratio. The possible mechanisms of phosphate adsorption by PAM included surface precipitation, surface complexation, electrostatic adsorption and release of Ca2+/OH-. Preparation cost of PAM was 80 US$/ton, and treatment cost was 0.07 US$/g P. Regeneration efficiency of PAM was still above 80 % after five cycles. The design idea and result of this study provide theoretical basis and technical support for the preparation of PAM with low cost, commercial production and great adsorption capacity.

Red mud recycling by Fe and Al recovery through the hydrometallurgy method: a collaborative strategy for aluminum and iron industry

In this work, a collaborative strategy for the aluminum and iron industry based on red mud recycling through the hydrometallurgy method was proposed. In this method, Fe³⁺ and Al³⁺ were firstly separated from the red mud by using H₂SO₄ as a leaching agent, which was by-produced from the sintering process of an iron and steel industry. Multiple influence factors on the leaching process were investigated, with the H₂SO₄ addition amount showing the strongest influence on the leaching rates of Al and Fe. The main components of the filter residue were CaSO₄, TiO₂, and SiO₂, which could be reused as additives in the building materials. Subsequently, the final Fe recovery product was obtained through the co-precipitation, Fe/Al separation, and Fe(OH)₃ calcination. In the final product, the content of Fe₂O₃ reached 82.87%, and the iron grade was 58.01%, meeting the requirement being raw materials for sinter production.

Adsorptive removal of phosphorus from aqueous solutions using natural and modified coal solid wastes

An invaluable utilization approach for industrial wastes is to employ them as effective adsorbents for environmental pollutants. This study aimed to investigate the phosphorus (P) adsorption behavior of coal wastes and zeolite in three forms of pristine powder (CP and ZP), nanoparticles (CNP and ZNP), and Fe (III)-modified nanoparticles (MCNP and MZNP). The adsorbents were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) analyses. The effects of pH, initial P concentration, and contact time were studied under batch mode. Results showed an optimum pH range of 2-6 for the P adsorption process. The pseudo-second-order kinetic model and the Langmuir isotherm described the P adsorption data well. The P adsorption capacity of the studied adsorbents was enhanced after modifications. However, the coal-based modified adsorbents represented higher P adsorption performances rather than the zeolite ones. The maximum P adsorption capacity (Q_max) values were obtained as 0.36, 3.23, and 30.48 mg g^-1 for CP, CNP, and MCNP, and 0.80, 2.84, and 6.99 mg g^-1 for ZP, ZNP, and MZNP, respectively. The surface complexation, ligand exchange, and electrostatic attraction processes were identified as the main P adsorption mechanisms by the studied adsorbents.

Removal and recovery of phosphorus from secondary effluent using layered double hydroxide-biochar composites

Removal of phosphorus (P) from wastewater and its recovery as a fertilizer are solutions to both P pollution control and resource recycling for agriculture. In this study, various layered double hydroxide biochar composites (LDH/BCs), namely, Zn-Al-LDH/BC, Mg-Al-LDH/BC, and Mg-Fe-LDH/BC, were synthesized to remove P from secondary effluents and then applied as fertilizers. Batch experiments showed that LDH/BCs could adsorb P in fast kinetics, with adsorption capacities ranging 35.19-55.76 mg P/g. A dynamic experiment was performed under different column heights and flow rates, and the results fitted well with Thomas model (R² > 0.90). These LDH/BCs effectively removed P in the continuous mode, even when treating secondary effluents. Furthermore, when the used LDH/BCs applied as fertilizers, the adsorbed Mg-Al-LDH/BC and Mg-Fe-LDH/BC stimulated crop growth; however, Zn-Al-LDH/BC did not. These differences were attributed to not only the availability of P, but also the stimulation or inhibition of photosynthetic pigment synthesis in crops by adsorbents. Overall, we synthesized LDH/BCs, which effectively removed and recovered P from secondary effluents, and investigated the factors influencing the effects of LDH/BCs on crops. We suggest that both P availability and physiological influences of adsorbents on crops should be considered when using adsorbents as fertilizers.

Unveiling the role of dissolved organic matter on phosphorus sorption and availability in a 5-year manure amended paddy soil

Dissolved organic matter (DOM) is an active component of organic manure that is widely used in agroecosystems to increase nutrient availability and consequently enhance crop yields. However, the ways in which soil DOM characteristics are influenced by organic manure and how it contributes to crop yield and soil P availability remains unclear. Here, we conducted a 5-year field experiment and demonstrated that partial replacement of chemical P fertilizer with swine manure could maintain high rice yield and soil available P levels and increase P fertilizer use efficiency (PUE) in comparison to chemical fertilization, even when the total P input was reduced. This suggests that organic manure application can significantly mobilize soil P and increase P availability. Structural equation modeling analysis indicated that the soil pH and humification degree of DOM, rather than DOM content, directly decreased maximum P adsorption capacity. The combined results of the optical spectroscopy and ultrahigh-resolution mass spectroscopy obtained from the laboratory validation experiment based on the DOM-removed soil demonstrated that manure-derived DOM competing with P for adsorption was one of the main reasons for the increase in soil P availability and that the effective DOM components were N-containing lignins, tannins, and condensed polycyclic aromatics with higher O/C and lower H/C ratios. Overall, our results provide solid evidence that soil DOM characteristics are influenced by manure application and facilitate soil P availability, which could help guide the sustainable P management and manure application in agroecosystems.

Ecological floating bed for decontamination of eutrophic water bodies: Using alum sludge ceramsite

In this study, a combined ecological floating bed (C-EFB) with alum sludge ceramsite (ASC) was designed to improve the water purification effect of traditional ecological floating beds (T-EFBs). During the ASC preparation stage, alum sludge was shaped into a ball, air-dried, and fired under 600 °C. The physical and chemical properties of the ASC meet the requirements of Artificial Ceramsite Filter Materials for Water Treatment (CJ/T229-2008). This study investigated the increased capability of this new-type artificial substrate (ASC) on the removal of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total phosphorus (TP), and total nitrogen (TN) from eutrophic landscape water. Compared with the T-EFB, the C-EFB owns a higher purification efficiency. The highest average efficiency of COD, NH4+-N, TN and TP removals during the four operating stages was 78.2%, 58.1%, 46.7% and 53.2%, respectively, in the C-EFB, which were all higher than those of 53.5%, 32.4%, 27.2% and 25.8%, respectively, for the T-EFB. Among them, the C-EFB showed a higher advantage in the removal of TP. The results showed that the potential benefits of utilizing ASC in seriously eutrophic bodies of water.

Improved holding and releasing capacities of coal gangue toward phosphate through alkali-activation

Activation is an effective method to improve adsorption capacity of coal gangue, however, most activated gangues reported have limited adsorption capacity for phosphate because of the electronegativity, affecting their use in agricultural production and environmental protection. In order to enhance the phosphate absorption capacity of coal gangue through alkali-activation, three alkali-activated silicate coal gangue according to fine-size (AS-FCG¹), medium-size (AS-MCG²), and large-size (AS-LCG³) were prepared through grinding, calcining (800 °C) and stimulating with Ca(OH)₂ solution (0.1 M), and raw coal gangue (RCG⁴) was used as control. The results revealed that AS-FCG has the best modified effects, its maximum phosphate adsorption capacity, according to Langmuir model, was 11.796 mg g^-1, which was 4.41, 8.16 and 73.73 times higher than that of AS-MCG, AS-LCG and RCG, respectively. The adsorption penetration time of AS-FCG packed column was over 30 h, which was longer than that of other samples. Besides, when the AS-FCG packed column loaded with phosphate was desorbed with distilled water, the phosphate concentration of filtrate was low and rapidly tended to 0 mg L^-1, while when it was desorbed with NaHCO₃ solution, the concentration remained high sustainably. The best adsorption performance of AS-FCG were associated with generation of calcium silicate hydrate gel and ettringite, which provided more Ca²⁺, Al³⁺, hydroxyl and larger specific surface area. It is suggested that alkali-activation is able to truly realize the recycling of solid waste, AS-FCG is an inexpensive, durable and eco-friendly material, which could not only be used to hold phosphate, but also release it slowly as fertilizer.

Anionic Dye Removal by Polypyrrole-Modified Red Mud and Its Application to a Lab-Scale Column: Adsorption Performance and Phytotoxicity Assessment

In this study, polypyrrole-modified red mud (PRM) was prepared for the efficient removal of anionic dyes (methyl orange and Congo red) from aqueous solutions. The phytotoxicity (bean sprouts) of the dye solution before and after dye removal was investigated. Adsorption kinetics confirmed that the adsorption of methyl orange (MO) and Congo red (CR) on PRM was controlled by chemical reactions between the functional groups of polypyrrole and dyes. From Langmuir isotherm fitting, we found the theoretical adsorption capacities of MO and CR on PRM were 194.1 and 314.9 mg/g, respectively. The adsorption progress of MO and CR on PRM was found to be spontaneous and endothermic. The column studies demonstrated that, under dynamic flow, the PRM can efficiently remove MO and CR from aqueous solution, with adsorption capacities of 31.08 and 55.04 mg/g, respectively. In the toxicity test, the phytotoxicity of the column effluents (after dye removal) was significantly lowered compared to the initial dye influents. After the removal of MO and CR, the average root length of bean sprouts was increased from 3.30 cm to 5.18 cm and from 3.01 cm to 7.00 cm, respectively. These findings highlighted the efficient removal of dyes by PRM from aqueous solution, demonstrating the possible application of PRM for the removal of dye from dye-contaminated wastewaters.

Efficient removal of nitrogen and phosphorus in aqueous solutions using modified water treatment residuals-sodium alginate beads

A high-performance sorbent, modified water treatment residuals-sodium alginate beads (WTR-SA beads), was prepared through a series of salt and combined thermal roasting composite modification between water treatment residuals and sodium alginate. The properties of modified WTR-SA beads composites were characterized by SEM-EDS, FT-IR, XRD, and BET. The adsorption performance of WTR-SA beads was investigated in removing nitrogen and phosphorus from wastewater. Compared to the unmodified WTR, the removal efficiency of nitrogen and phosphorus onto the modified WTR-SA beads was increased from 22.34 and 77.13% to 95.14 and 98.31%, respectively. The adsorption capacities of nitrogen and phosphorus onto the modified WTR-SA beads were reach a maximum of 2.52 mg/g and 6.45 mg/g, respectively. The adsorption behavior can be well described using a quasi-second-order kinetic model and Langmuir isotherm model. The thermodynamic properties of nitrogen adsorption indicated that the adsorption was spontaneous and exothermic. On the contrary, the adsorption process of phosphorus is an endothermic reaction. The adsorption of nitrogen by modified WTR-SA beads is mainly carried out through ion exchange and hydroxyl complexation, and ion exchange plays a major role in it. While, the adsorption of modified WTR-SA beads on phosphorus is affected by three actions: ligand exchange, chemical precipitation, and ion exchange, which ligand exchange is the main effect. Based on these results, it can be concluded that the modified WTR-SA beads are a high efficiency adsorbent for removing nitrogen and phosphorus from domestic and industrial wastewater.

Applicability of ferric(III) hydroxide as a phosphate-selective adsorbent for sewage treatment

This research was undertaken to evaluate the usability of ferric(III) hydroxide for phosphate removal from sewage. Batch adsorption experiments, partly fixed bed column experiments, were conducted to study the influence of various factors, competing anions and contact time on the adsorption of phosphate on ferric(III) hydroxide. Processing ferric iron in the form of akaganeite (β-FeOOH) greatly increased the adsorption capacity for phosphate. The optimum phosphate removal was observed in the pH_eq ≤ 6.0. All results from this study demonstrate the potential usability of β-FeOOH as a good phosphate-selective adsorbent for the phosphate removal system for a sewage treatment plant.

Removal of inorganic arsenic from aqueous solution by Fe-modified ceramsite: batch studies and remediation trials

During sediment remediation, adsorbent addition is an effective technology for the removal of contaminants but the cost is often high. In this study, a low-cost adsorbent, ceramsite, made from contaminated riverbed sediment was synthesized. The Fe-modified ceramsite (FMC) was used as adsorbent to remove arsenate from aqueous solutions and reduce the inorganic arsenic release from contaminated sediments. Kinetic studies showed that chemisorption mainly governed the adsorption process while batch studies yielded the theoretical adsorption capacity for arsenate of 10.63 mg/g at pH = 7 condition. Co-existing anions and pH have no significant impact on the adsorption process. In the regeneration studies, 91, 86, and 80% of the adsorption capacity were recovered in 3 cycles. In-situ remediation trials revealed that the addition of the adsorbent to sediment surface significantly reduced the release of inorganic arsenic into aqueous system, with a reduction efficiency of 86%. Furthermore, the species of the arsenic in the surface layer was significantly inactivated from an active state to a stable state. These findings highlight the application of the FMC as a facile and cost-effective adsorbent for containment of arsenic in solutions and sediments, demonstrating that they are highly applicable for practical cases.

Fabrication of Ceramsite Adsorbent from Industrial Wastes for the Removal of Phosphorus from Aqueous Solutions

A more applicable adsorbent was fabricated using industrial wastes such as red mud, fly ash, and riverbed sediments. The heavy metal inside the raw materials created metal hydroxy on the adsorbent surface that offered elevated adsorption capacity for phosphorus. The required equilibrium time for the adsorption is only 10 min. The theoretical maximum adsorption capacity of the adsorbent was 9.84 mg·g−1 inferred from the Langmuir adsorption isotherm. Higher solution pH favored phosphorus adsorption. Kinetics study showed that the adsorption could be better fitted by the pseudo-second-order kinetic model. The presence of coexisting anions had no significant adverse impact on phosphorus removal. The speciation of the adsorbed phosphorus indicated that the adsorption to iron and aluminum is the dominating adsorption mechanism. Moreover, a dynamic adsorption column experiment showed that, under a hydraulic time of 10 min, more than 80% of the phosphorus in the influent was removed and the surplus phosphorus concentration was close to 0.1 mg L−1. The water quality after adsorption revealed its applicability in real treatment. Consequently, the adsorbent synthesized from industrial wastes is efficient and applicable due to the high efficiency of phosphorus removal and eco-friendly behavior in solutions.

Synergistic Recapturing of External and Internal Phosphorus for In Situ Eutrophication Mitigation

In eutrophication management, many phosphorus (P) adsorbents have been developed to capture P at the laboratory scale. Existing P removal practice in freshwaters is limited due to the lack of assessment of the possibility and feasibility of controlling P level towards a very low level (such as 10 μg/L) in order to prevent the harmful algal blooms. In this study, a combined external and internal P control approach was evaluated in a simulated pilot-scale river–lake system. In total, 0.8 m3 of simulated river water was continuously supplied to be initially treated by a P adsorption column filled with a granulated lanthanum/aluminium hydroxide composite (LAH) P adsorbent. At the outlet of the column (i.e., inlet of the receiving tanks), the P concentration decreased from 230 to 20 µg/L at a flow rate of 57 L/day with a hydraulic loading rate of 45 m/day. In the receiving tanks (simulated lake), 90 g of the same adsorbent material was added into 1 m3 water for further in situ treatment, which reduced and maintained the P concentration at 10 µg/L for 5 days. The synergy of external and internal P recapture was demonstrated to be an effective strategy for maintaining the P concentration below 10 µg/L under low levels of P water input. The P removal was not significantly affected by temperature (5–30 °C), and the treatment did not substantially alter the water pH. Along with the superior P adsorption capacity, less usage of LAH could lead to reduced cost for potation eutrophication control compared with other widely used P adsorbents.

Application of red-mud based ceramic media for phosphate uptake from water and evaluation of their effects on growth of Iris latifolia seedling

In the present study, we have produced red-mud based ceramic media (RMCM) as an adsorbent for removal of phosphate from aqueous solutions, and their application in a constructed wetland. Phosphate adsorption to RMCM was investigated by varying initial phosphate concentration, contact time, and temperature. Adsorption of phosphate to the surface of RMCM was confirmed by scanning electron microscope and energy dispersive X-ray spectroscopy. The surface area and pore volume of RMCM decreased significantly after phosphate adsorption. Experimental equilibrium data followed Langmuir and Sips model better than the Freundlich model. Kinetic data followed both pseudo first order and pseudo second order reactions. Thermodynamics suggested the phosphate adsorption process onto RMCM to be endothermic and spontaneous, and physisorption dominated. Fourier transform infrared spectrumof phosphate adsorbed RMCM did not show any PO specific bands thus ruling out role of chemical forces in phosphate adsorption. Overall, phosphate adsorption on RMCM was driven by physisorption. The RMCM promoted biomass growth and increased the surface area of roots in Iris latifolia. Together with RMCM, I. latifolia augmented removal of phosphate from aqueous solution. Based on their phosphate removal performance and plant-growth promoting effects, we believe that RMCM can be effectively used in constructed wetlands.

Soil phosphorus dynamics following land application of unsaturated and partially saturated red mud and water treatment residuals

The secondary use of P-sorbing industrial by-products as a fertilizer or soil conditioner is gaining increased attention, particularly in light of diminishing reserves of rock phosphate traditionally used to manufacture P fertilizer. This study examined applications of red mud (RM) and water treatment residuals (WTR) at two levels of P saturation (i.e. 'as received' and partially saturated) in a soil incubation and runoff plot study. When incubated with soils ranging in texture and initial P concentration, P-sorbing residuals that were less enriched with P decreased water-extractable soil P (WEP) concentration to a greater extent than more P saturated residuals. In contrast to WTR treatments, not all of the RM applications decreased soil WEP concentrations below those of the control soils. The runoff study investigated soil P dynamics when partially P-saturated RM and WTR's were surface applied to grass plots at 2 t ha^-1 on Day 0, followed by three rainfall simulations (7 cm h^-1 for 30 min, Days 2, 7 and 28) and at 3 t ha^-1 on Day 70 followed by two more rainfall simulations (Days 77 and 96). Application of residuals at these rates did not significantly increase dissolved reactive P (DRP) in runoff compared with unamended controls during the study. Forage cuttings taken 90 days after the first rainfall simulation indicated that nutrient uptake was not compromised by the application of the residuals. Overall results indicate that WTRs may be a more suitable soil amendment than RM residuals given their greater ability to reduce soil WEP across a range of soils without simultaneously increasing Mehlich-3 extractable soil P concentrations above the upper threshold limit (150 mg P kg^-1), and their minimal impact on plant nutrient uptake.

Biofilm development dynamics and pollutant removal performance of ceramsite made from drinking-water treatment sludge

Alum-sludge ceramsite and denitrifying bacteria (XP-1, XP-2, CL-1, CL-3) were used as substrate and constructed biofilm for enhancing the removal of pollutants from wastewater. The results showed that, due to the large specific surface area, the maximum growth rate was 0.49 mg/(g·day) on the sludge ceramsite, and the mass of biofilm attached onto sludge ceramsite was 5.98 times higher than that when using commercial ceramsite as substrate. Better removal performance could be achieved with the combination of sludge ceramsite and bacteria, viz. 98.6%, 91.0%, and 85.8% reduction in total phosphorus (TP), total nitrogen (TN), and chemical oxygen demand (COD), respectively. Pseudo-first-order kinetics, pseudo-second-order kinetics, Monod kinetics, and multiple Monod kinetics combined with continuous-flow-stirred tank reactor (CFSTR) behavior were used to investigate the dynamics of the pollutant removal processes. The decrease in band brightness for bacteria attached onto sludge ceramsite was 11.5%, while it was more than 35.7% on commercial ceramsite during wastewater treatment according to results from denaturing gradient gel electrophoresis (DGGE). Sludge ceramsite played an important role in maintaining quantities and activities of denitrifying bacteria, and application of sludge ceramsite substrate and denitrifying bacteria was a reliable method to enhance the removals of phosphorus, nitrogen, and COD from domestic wastewater. PRACTITIONER POINTS: Alum-sludge ceramsite was a good substrate for phosphorus adsorption and denitrifying bacterial growth. There was 5.98 times more biofilm on sludge ceramsite than on commercial ceramsite The biofilm of denitrifying bacteria on sludge ceramsite was more stable. High removals of TP (98.6%), TN (90.1%) and COD (85.81%) were achieved.

Hydrothermal Conversion of Red Mud into Magnetic Adsorbent for Effective Adsorption of Zn(II) in Water

Red mud, a Fe-rich waste generated from the aluminum industry, was recovered as an adsorbent for wastewater treatment. The separation process of red mud from water after adsorption, including centrifugation and filtration, was complicated. This study demonstrated an alternative option to recycle red mud for preparing magnetic adsorbent via a facile hydrothermal route using ascorbic acid as reductant. Red mud is weakly magnetized and consists of andradite, muscovite, hematite, and cancrinite. After hydrothermal treatment, andradite in red mud was reductively dissolved by ascorbic acid, and transformed into magnetite and morimotoite. With increasing hydrothermal temperature, the dissolution of andradite accelerated, and the crystallite size of magnetite increased. When the hydrothermal temperature reached 200 °C, the prepared adsorbent P-200 showed a desirable saturation magnetization of 4.1 Am2/kg, and could be easily magnetically separated from water after adsorption. The maximum adsorption capacity of P-200 for Zn2+ was 89.6 mg/g, which is eight-fold higher than that of the raw red mud. The adsorption of Zn2+ by P-200 fitted the Langmuir model, where cation exchange was the main adsorption mechanism. The average distribution coefficient of Zn2+ at low ppm level was 16.81 L/g for P-200, higher than those of the red mud (0.3 L/g) and the prepared P-120 (1.48 L/g) and P-270 (5.48 L/g), demonstrating that P-200 had the best adsorption capacity for Zn2+ and can be served as a practical adsorbent for real-world applications. To our knowledge, this is the first study to report the conversion of red mud into a magnetic adsorbent under mild conditions.

Nitrogen and phosphorus removal from wastewater by sand with wheat straw

Wheat straw amendment to sandy soil has the potential to remove nutrients from wastewater. This study investigated the ability of wheat straw to remove inorganic nitrogen (N) and phosphorus (P) from wastewater when mixed into sand at different rates. Wastewater from a sewage treatment plant was added to sand alone and amended with different wheat straw rates 2.5, 5, 7.5, 10, and 12.5 g wheat straw kg^-1 so that the sand was covered with about 15 cm of wastewater. Leaching was carried out after 4, 8, and 16 days and inorganic N and P were analysed after leaching in both the leachate and sand, as well as N₂O and CO₂ release. In the amended sand, nitrate was about fourfold lower throughout the experiment compared to sand alone. Ammonium was twofold higher than sand alone at 12.5 g straw kg^-1 throughout the experiment and on day 16 also at ≥ 5 g straw kg^-1. Leachate inorganic N concentration was up to 70-fold higher in sand alone than in amended soils irrespective of straw rate. On day 16, P leaching was about threefold lower and P retention was 40% higher in all amended treatments than sand alone. The redox potential in sand alone was higher than with straw amendments. With straw amendment, the release of CO₂ per day was six times higher than with sand alone and increased with straw rates, but very little N₂O and CH₄ was released throughout the experiment. It can be concluded that amendment of sand with wheat straw can remove large proportions of inorganic N and P from wastewater, even at low straw rates. Likely mechanisms for retention are dissimilatory nitrate reduction and subsequent binding of ammonium to straw for N, and binding to the straw and microbial uptake for P.

Evaluation of industrial by-products and natural minerals for phosphate adsorption from subsurface drainage

Agricultural subsurface drainage has been recognized as an important pathway for phosphorus transport from soils to surface waters. Reactive permeable filters are a promising technology to remove phosphate from subsurface drainage. Three natural minerals (limestone, zeolite, and calcite) and five industrial by-products (steel slag, iron filings, and three recycled steel by-products) were evaluated for phosphate removal from subsurface drainage using batch adsorption experiments. Phosphate adsorption onto these materials was characterized by Langmuir isotherm and second-order kinetic models. The adsorption capacities increased by factors of 1.2-2.5 when temperature was increased from 5°C to 30°C. Industrial by-products exhibited phosphate adsorption capacities that were one order of magnitude higher than natural minerals. Medium-sized steel chips exhibited high phosphate adsorption capacities (1.64-3.38 mg/g) across different temperatures, pH values, organic matter concentrations, and real drainage water matrixes. The strong chemical bonds between phosphate and steel by-products prevented the release of adsorbed phosphate back to the solution. The steel by-product filter can be paired with a woodchip bioreactor for nitrate and phosphate removal. It is suggested that the phosphate filter be connected to a woodchip bioreactor after the startup phase to minimize the impact of dissolved organic matter on phosphate adsorption. The results of this study suggest that the low-cost steel by-products examined could be used as effective adsorption media for phosphate removal from subsurface drainage.

A study by response surface methodology (RSM) on optimization of phosphorus adsorption with nano spherical calcium carbonate derived from waste

A nano spherical CaCO₃ (NSC) derived from solid waste (precipitated from tris(α-chloropropyl) phosphate and triethyl phosphate mixed wastewater) was prepared as adsorbent for phosphorus removal from aqueous solution. Response surface methodology (RSM) was used to develop an approach for the evaluation of phosphorus adsorption process, and Box-Behnken design was performed to investigate the effects of various experimental parameters (temperature, contact time, initial pH and dosage of absorbent) on phosphorus adsorption. The model results of experimental data gave a high correlation coefficient (R² = 0.9658), and a predictive model of quadratic polynomial regression equation and optimum level values were established successfully. It was found that the adsorption efficiency and adsorption capacity reached 97.05% and 123.79 mg/g, respectively, under conditions of temperature of 45 °C, initial pH 5.3, contact time of 11 h, and absorbent amount of 392 mg/L. X-ray diffraction (XRD) analysis testified new phase, Ca₁₀(PO4)₆CO₃, was produced in the adsorption process. Apart from that, adsorption behavior fitted well with the Langmuir isotherm model and logistic growth model. The thermodynamic study indicated that phosphorus removal by NSC as adsorbent was a spontaneous, endothermic, and mainly chemical adsorption process.

Enhanced phosphate removal from wastewater by using in situ generated fresh trivalent Fe composition through the interaction of Fe(II) on CaCO3

Excessive existences of nutrients such as phosphate in the aqueous environment remain as a heavy concern although many researches have been reported for dealing with their removal. Based on the understanding toward the interactions of Fe compounds with phosphate and carbonate from many available researches, we designed a very simple and efficient approach for phosphate removal by using in situ generated fresh trivalent Fe composition through the interaction of Fe(II) as FeSO₄ on CaCO₃. Addition and agitation of Fe(II) and CaCO₃ simultaneously to phosphate solution allowed an amorphous Fe(III)-P or Ca-Fe(III)-P precipitation, with a phosphate removal rate close to 100%, to reduce the residual phosphorus concentration less than 0.03 mg/L from 100 mg/L, reaching the discharge limit, even with the addition amounts of CaCO₃ as low as a stoichiometric ratio of CaCO₃/PO₄^3- at 0.9 and ratio of Fe(II)/PO₄^3- at 1.5, and the percent of P₂O₅ in the precipitate was as high as 19.4% enough as phosphate source for fertilizer production. Different from the alkaline process with enough OH^- group, the slow hydrolysis of CaCO₃ resulting in low concentration of OH^- group for the formation of Fe(OH)₂, which was oxidized soon by air into trivalent Fe, achieved a continuous generation of fresh ferric composition for phosphate precipitation and could avoid its rapid formation and subsequent transformation into stable FeOOH of large particle size to lose the activity. These results based on the synergistic effect of using CaCO₃ and Fe(II) together may have applications in the treatment of eutrophic wastewater through a process with many advantages of easy operation and low-cost besides the high removal efficiency with phosphate percentage inside the precipitate high enough to serve for fertilizer production.

Investigation of Taguchi optimization, equilibrium isotherms, and kinetic modeling for phosphorus adsorption onto natural zeolite of clinoptilolite type

Adsorption process plays an important role in the removal of phosphorus from aqueous solutions. A laboratory experiment was conducted to investigate the adsorption characteristics of phosphorus onto natural zeolite and to find out the relative importance of some controllable treatments in phosphorus adsorption process using the Taguchi optimization methodology. Results showed that the adsorption of phosphorus in the presence of Fe3+ and Al3+ was higher than that in the absence of these two cations probably due to the adsorption of phosphorus-bearing anions on opposite charges of these cations. Also, increase in contact time tended phosphorus adsorption to be increased. The addition of base and acid treatments caused an increase and a decrease, respectively, on phosphorus adsorption. The order of effectiveness of treatments on the values of phosphorus adsorption was as follows: acid/base treatment >sorbent to sorbate ratio > modification with aluminium (Al)/iron (Fe) >contact time >phosphorus concentration. Phosphorus adsorption data well fitted to the Freundlich isotherm model. The pseudo-second order was the best model describing phosphorus adsorption kinetics. According to the results reported herein, it is assumed that the main mechanism controlling phosphorus adsorption onto natural zeolite is chemisorption.

Experimental study and simulation of phosphorus purification effects of bioretention systems on urban surface runoff

Excessive phosphorus (P) contributes to eutrophication by degrading water quality and limiting human use of water resources. Identifying economic and convenient methods to control soluble reactive phosphorus (SRP) pollution in urban runoff is the key point of rainwater management strategies. Through three series of different tests involving influencing factors, continuous operation and intermittent operation, this study explored the purification effects of bioretention tanks under different experimental conditions, it included nine intermittent tests, single field continuous test with three groups of different fillers (Fly ash mixed with sand, Blast furnace slag, and Soil), and eight intermittent tests with single filler (Blast furnace slag mixed with sand). Among the three filler combinations studied, the filler with fly ash mixed with sand achieved the best pollution reduction efficiency. The setting of the submerged zone exerted minimal influence on the P removal of the three filler combinations. An extension of the dry period slightly promoted the P purification effect. The combination of fly ash mixed with sand demonstrated a positive purification effect on SRP during short- or long-term simulated rainfall duration. Blast furnace slag also presented a positive purification effect in the short term, although its continuous purification effect on SRP was poor in the long term. The purification abilities of soil in the short and long terms were weak. Under intermittent operations across different seasons, SRP removal was unstable, and effluent concentration processes were different. The purification effect of the bioretention system on SRP was predicted through partial least squares regression (PLS) modeling analysis. The event mean concentration removal of SRP was positively related to the adsorption capacity of filler and rainfall interval time and negatively related to submerged zones, influent concentration and volume.

Granulation of drinking water treatment residuals as applicable media for phosphorus removal

Recycling drinking water treatment residuals (DWTR) show promise as a strategy for phosphorus (P) removal; however, powdered DWTR is not an ideal practical medium due to clogging. This study granulates DWTR by entrapping powdered DWTR in alginate beads. Results show that granular DWTR has an appreciable amount of mesopores along with a Brunauer-Emmett-Teller (BET) surface area of 43.8 m²/g and total pore volume of 0.049 cm³/g. Most metals (e.g., Al, Ba, Be, Cd, Co, Cr, Mn, Ni, Pb, and Zn) in granular DWTR became more stable and granular DWTR could be considered non-hazardous material. Further analysis indicates that the granular DWTR has strong P adsorption capability with a maximum adsorption capacity of 19.70 mg/g as estimated by the Langmuir model. Good P adsorption may be attributed to the formation of Fe-PO₄ and Al-PO₄ associated with the amorphous state of enormous iron and aluminum in granular DWTR. More importantly, granular DWTR exhibits good mechanical stability and maintained its shape with weight loss below 12.49% after three recycling rounds. Overall, granular DWTR appears to serve as better media for phosphorus removal in water treatment structures such as wetlands.

Study on adsorption of methylene blue by a novel composite material of TiO2 and alum sludge

A composite material of TiO₂ and alum sludge (TiO₂@AS) is reported in this paper. The samples of alum sludge (AS) and TiO₂@AS were characterized using a field emission scanning electron microscope (FE-SEM), an energy dispersive spectrometer (EDS) and an Ultima IV X-ray diffractometer (XRD). In order to study the adsorption capacity and the adsorption mechanism of methylene blue by TiO₂@AS in aqueous solution, three indexes — pH, adsorbent dosage and initial concentration of methylene blue — were investigated to evaluate the adsorption capacity of TiO₂@AS. Moreover, thermodynamic, kinetic and isothermal model analyses of the adsorption process were carried out. The results showed that pH has little effect on the adsorption capacity. The maximum adsorption efficiency occurred at an optimized pH value of 11 for the aqueous solution. The adsorbed amount of methylene blue on TiO₂@AS increased with the initial concentration of adsorbate and decreased with an increase in TiO₂@AS dosage. The adsorption kinetics of methylene blue by TiO₂@AS were in good agreement with the quasi-second-order kinetic model (R² ≥ 0.9964), indicating that chemisorption is the main rate-controlling step. The Freundlich isotherm equation can best describe the experimental data (R² = 0.9788). It is revealed that the adsorption is mainly multilayer adsorption. Because ΔG < 0 and ΔH > 0, the adsorption is a spontaneous endothermic process. Furthermore, the TiO₂@AS can be regenerated under ultraviolet light, thus prolonging the service life of AS and facilitating a solution to the problem of adsorption plugging.

A composite material of TiO₂ and alum sludge (TiO₂@AS) is reported in this paper.

Statistical optimization and batch studies on adsorption of phosphate using Al-eggshell

This work provides a simple and convenient method to manufacture the sorbent of Al-eggshell. The influence of AlCl3 concentration and pH values as well as the dosage of sorbent and their interactions on adsorption of phosphate was investigated. Therefore, Box–Behnken design coupled with response surface method was adopted to explore the empirical model for phosphate species removal. It was observed that there is an optimal point, C(AlCl3)(0.29 mol/l)–pH(6.12)–dosage(6.72 g/l), for the goal of maximizing phosphate species removal. The second-order polynomial model for phosphate reduction was given as Removal(%) = 96.43 +10.82X1 + 4.29X2 − 0.70X3 + 2.06X1X2 − 1.72X1X3 +8.24X2X3 − 13.10X12 − 17.26X22 − 1.72X32. Contour pictures implied that the interaction between pH values and sorbent dosage was the strongest, followed by C(AlCl3) versus dosage and C(AlCl3) versus pH. The adsorption of phosphate data had a good agreement with the Freundlich isotherm equation at 313 and 323 K. Otherwise, Langmuir–Freundlich model described the best fitness at the temperature of 293, 298, and 303 K. The process of adsorption of phosphate on Al-eggshell fitted a pseudo-second-order kinetic equation, which indicates the exothermic reaction. In conclusion, the present work suggests Al-eggshell as an efficient and environmental friendly sorbent for phosphate species adsorption from aqueous solutions.

The Internal Recycle Reactor Enhances Porous Calcium Silicate Hydrates to Recover Phosphorus from Aqueous Solutions

In this experiment, the porous calcium silicate hydrates (P-CSHs) were prepared via a hydrothermal method and then modified by polyethylene glycol (PEG). The modified P-CSHs combined with an internal recycle reactor could successfully recover the phosphorus from electroplating wastewater. The modified P-CSHs were characterized by X-ray diffraction (XRD), N2 adsorption-desorption isotherms, and Fourier transform infrared spectroscopy (FT-IR). After compared with different samples, the modified P-CSHs-PEG2000 sample had larger specific surface area of 87.48 m2/g and higher pore volume of 0.33 cm3/g, indicating a high capacity for phosphorus recovery. In the process of phosphorus recovery, the pH value of solution was increased to 9.5, which would enhance the recovery efficiency of phosphorus. The dissolution rate of Ca2+ from P-CSH-PEG2000 was fast, which was favorable for phosphorus precipitation and phosphorus recovery. The effects of initial concentration of phosphorus, P-CSHs-PEG2000 dosage, and stirring speed on phosphorus recovery were analyzed, so the optimal operation conditions for phosphorus recovery were obtained. The deposition was analyzed by XRD, N2 adsorption-desorption, and SEM techniques; it was indicated that the pore volume and surface area of the P-CSHs-PEG2000 were significantly reduced, and the deposition on the surface of P-CSHs-PEG2000 was hydroxyapatite.