From wastes to functions: preparation of layered double hydroxides from industrial waste and its removal performance towards phosphates

To control eutrophication and recover phosphate from wastewater, a calcium carbide slag and red mud composite material (CR-LDH) was prepared using industrial waste as raw material for phosphorus adsorption. The morphology and structure of synthesized CR-LDH were characterized by FT-IR, SEM, EDS, and XRD measurements. Bath adsorption test results showed that the optimal dosages of adsorbent and pH for phosphate were 5 g·L−1 and pH of 7, respectively. The experimental data could be well described by pseudo-second-order kinetic and Langmuir isotherm models, suggesting that the adsorption process of CR-LDH with respect to phosphate was a chemical and monolayer process. The theoretical maximum adsorption capacity obtained by Langmuir isotherm model was 16.06 mg·g−1 at 25 °C. The intra-particle diffusion model fitting results indicated that the adsorption of phosphate by CR-LDH was controlled by both liquid membrane diffusion and intra-particle diffusion. Phosphate was bound to CR-LDH via synergistic effect of physical adsorption, ion exchange, anion intercalation, and chemical precipitation as evidenced from a combination of microscopic analysis and adsorption mechanism study. The actual phosphate-containing wastewater investigation showed that CR-LDH not only exhibited good removal effect on phosphate, but also could greatly reduce turbidity, COD, and ammonia nitrogen, which was suitable for disposal of practical wastewater. The COD, turbidity, and NH4+-N could be reduced by 42.39%, 77.20%, and 20.71%, respectively. These results indicate that CR-LDH can be considered as potential adsorbent for the treatment of phosphate-containing wastewater, which will be helpful to achieve the goal of “treating waste with waste and turning waste into treasure”.


Introduction
As a non-renewable and irreplaceable resource, phosphorus is essential element for cell growth in plants, animals, or bacteria, and thus necessary for maintaining the stability of the whole ecosystem (Jiang et al. 2019a;Seftel et al. 2018). However, excess of phosphorous (≥0.02 mg·L −1 ) will cause eutrophication in widespread water body , along with a series of issues such as the reduction of water economic value and the restriction of sustainable development of the society and economy. Hence, it is of vital importance for controlling redundant phosphate to a low level by cost-effective approaches . Generally speaking, phosphorus in wastewater usually exists in the main forms of organic phosphate, orthophosphate, and polymeric (condensed) phosphate (Bacelo et al. 2020), but the total phosphorus content and the mass fraction of various forms of phosphorus vary depending on the source of the wastewater. Therefore, the most critical step in phosphorus removal of most wastewater is to consider how to remove orthophosphate.
Until now, various approaches have been adopted for the treatment of phosphorus-containing wastewater such as chemical precipitation, crystallization, biological processes, and adsorption method (Bacelo et al. 2020). Among them, adsorption with the characteristic of simple operation, low cost, and environment-friendly (Kong et al. 2019;Kong et al. 2021) is generally considered to be the most effective method for the treatment of phosphorus-containing wastewater. The key to the popularization and application of adsorption method lies in the research and development of cheap and efficient new adsorbents. Many kinds of adsorbents such as layered double hydroxides (LDHs) (Dox et al. 2019;Liu et al. 2019;Seftel et al. 2018;Zhang et al. 2019), zeolite (Li et al. 2021;Razmi and Ghasemi-Fasaei 2018), corn bracts (Jiang et al. 2019b), lignite (Samaraweera et al. 2021), aluminummodified eggshells (Guo et al. 2017), sludge (Tie et al. 2013), MgO nanoparticles biochar composites (Zhu et al. 2021), MOFs, and paper mill sludge-based calcium-containing porous biochar (Wang et al. 2021) have been reported for the removal of phosphorus from wastewater.
LDHs, also known as anionic clays, include hydrotalcite and hydrotalcite-like compounds, and the main body of LDHs is generally composed of hydroxides of two metals. LDHs have the properties of ion exchange between layers and permanent positive charge. After calcination, the product of LDHs have a huge specific surface area and can return to the original layered structure in a certain environment. LDHs are deemed as an excellent adsorbent for the removal of organic and inorganic pollutants from wastewater. In terms of phosphate removal, Zhang et al. (2021) successfully synthesized layered double hydroxides with nitrate via co-precipitation method, and the experimental results showed that the multilayer structure of the synthesized LDHs exhibited good adsorption ability towards phosphate. Jia et al. (2018) exfoliated and incorporated MgAlFe-LDHs in polyether sulfone membranes, and the adsorption rate of the obtained membranes were faster than the unexfoliated ones. The adsorption capacity of obtained membranes with respect to phosphate was 5.61 mg·g −1 . Seftel et al. (2018) used modified ZnAl-LDHs to adsorb phosphate from aqueous solutions. The results showed that anionic exchange was the main adsorption mechanism for the phosphate adsorbed by non-calcined LDH, while the adsorption process of LDH materials towards phosphate was attributed to structural reconstruction and precipitation. However, the LDHs used in most of the above publications were synthesized using chemical agents. If LDHs can be prepared from industrial waste, the production cost of adsorbent will be greatly reduced, and the industrial waste can be reused for recycling. To our best knowledge, the literature focus on using industrial waste residue to prepare LDHs for the removal of phosphate from wastewater had rarely been reported.
The purpose of this study is to make full use of industrial waste residues to develop new adsorbent for the removal of phosphate in wastewater, which can reduce the environmental pollution caused by industrial waste residues, and achieve the purpose of effective treatment of phosphatecontaining wastewater. Herein, we prepared a novel CR-LDH adsorbent with hydrocalumite and magnesia structure by calcium carbide slag and red mud via a simple method. Calcium carbide slag is a calcium-containing solid industrial waste residue produced during the hydride process of chloralkali industry, and the main chemical components are CaO, Al 2 O 3 , Fe 2 O 3 , SiO 2 , MgO, etc. Red mud is a kind of solid waste residue in aluminum industry formed from bauxite (Hu et al. 2017). The influencing factors of adsorbent's dosage, pH, and co-existing ions on the removal performance of phosphate were discussed. The adsorption kinetics, isotherm, and thermodynamics were systematically studied. The adsorption mechanism was proposed by combining the experimental investigation and characterization of CR-LDH before and after adsorption of phosphate. In addition, the applicability of CR-LDH for the removal of phosphate in practical phosphatecontaining wastewater was also investigated.

Chemicals and materials
The carbide slag was taken from an acetylene plant in Fuxin City, Liaoning Province. The red mud was provided by an aluminum plant in Liaocheng City, Shandong Province. The red mud used in this study is "Bayer red mud" which accounts for more than 90% of the world's total red mud. The raw carbide slag and red mud were crushed and ground through 200-mesh sieve, and then dried to constant weight in 105°C oven to remove the moisture and maintain the original properties of the materials. The picture of raw calcium carbide slag powder and red mud powder is presented in Fig. S1a and Fig. S1b of Supplementary Material, respectively. The chemical composition of raw calcium carbide slag powder and red mud powder is provided in Table S1 and Table S2. Potassium dihydrogen phosphate (KH 2 PO 4 ) was purchased from Liaoning Quanrui Reagent Co., Ltd. A 100 mg·L −1 phosphorus standard solution (GSB04-1741-2004a) was supplied by China's national center for analysis and testing of nonferrous metals and electronic materials. Ascorbic acid was purchased from Liaoning Quanrui Reagent Co., Ltd, China. The other chemicals were purchased from Shenyang Huadong Reagent Factory, China. All chemicals were of analytic grade (or higher) and were used without further purification.

Preparation of CR-LDH
The CR-LDH was prepared as the following steps: firstly, the raw material of calcium carbide slag and red mud were roasted at 600°C for 3 h in muffle furnace, respectively. After cooling to room temperature, 0.5 g of the obtained calcium carbide slag and 0.5 g of red mud were fully mixed and then added into 300 mL deionized water. The resulting slurry was stirred at 45°C for 3 h under mechanical stirring of 700 r·min −1 . Subsequently, the above mixture was crystallized in 70°C water bath for 24 h. The obtained residue was washed thoroughly with distilled water, dried at 105°C to constant weight, and then grounded to pass through 200-mesh sieve. Finally, carbide slag red mud composite powder was obtained, denoted as CR-LDH. The picture of the prepared of CR-LDH was shown in Fig. S2.

Physicochemical characteristic of CR-LDH
The morphology and element distribution of CR-LDH were determined by a S-4800 scanning electron microscopy (SEM) (Hitachi, Japan) and energy-dispersive spectroscopy (EDS) mapping analysis. To determine the surface area of CR-LDH, a ASAP2460 Brunauer-Emmett-Teller N 2 adsorption analysis (BET) (Micromeritics, USA) was used. The functional groups and chemical bond types of CR-LDH from 4000 to 400 cm −1 were measured by a NICOLET iS5 Fourier transform infrared spectroscopy (FT-IR) (Thermo Nicolet Corporation, USA). A x′pert pro X-ray diffraction (XRD) (Spectris, Holland) with Cu K α radiation (40 kV, 40 mA) at a scan speed of 4°·min −1 was used to detect the crystallographic structure of CR-LDH.

Batch adsorption experiments of CR-LDH
A series of batch adsorption tests were conducted to evaluate the adsorption performance of CR-LDH with respect to phosphate. The adsorption experiments were carried out in 250-mL conical flasks with a certain amount of CR-LDH and 100 mL of phosphate at 25°C. After adsorption equilibrium, the supernatants were collected and then filtered through 0.45-μm membrane filter and diluted for further phosphate analysis by the UV spectrophotometer at a wavelength of 700 nm using the molybdate colorimetric method . The initial pH values of the phosphate solutions were adjusted by 0.1 mol·L −1 HNO 3 or NaOH. The adsorption amount and removal efficiency of CR-LDH towards phosphate were calculated by the following equations: Removal efficiency where Q t is adsorption amount of phosphate onto CR-LDH at time of t, mg·g −1 ; C 0 is the initial concentration of phosphate, mg·L −1 ; C t is the concentration of phosphate at time of t, mg·L −1 ; m is the mass of CR-LDH, mg; and V is the volume of solution, mL.

Influence of co-existing anions
To investigate the effect of co-existing inorganic anions on phosphate adsorption, 0.5 g CR-LDH was immersed into 100 mL of 30 mg·L −1 phosphate solutions within the concentration of co-existing anions ranging from 20 to 200 mg·L −1 . The co-existing anions used in this study were CO 3 2− , SO 4 2− , NO 3 − , and Cl − by adding Na 2 CO 3 , Na 2 SO 4 , NaNO 3 and NaCl, respectively. The adsorption experiment was carried out under pH of 7.0 with stirring speed of 180 r·min −1 for 8 h, 25°C. Afterwards, the supernatants were filtered and measured.

Adsorption kinetics
The adsorption kinetics of phosphate was carried out by immersing 0.5 g of CR-LDH into 100 mL phosphate solution with concentration of 30 mg·L −1 (25°C, 10 h). After the adsorption experiments, the supernatants were collected, filtered, and measured. The adsorption kinetics of phosphate adsorbed by CR-LDH was studied by pseudo-first-order, pseudo-second-order, and intra-particle diffusion models.
where t (min) represents the adsorption time; Q t is the adsorption amount of phosphate adsorbed by CR-LDH at time t, mg·g −1 ; Q e represents the equilibrium adsorption capacity, mg·g −1 ; k 1 (min −1 ), and k 2 (g·mg −1 ·min −1 ) donate the pseudo-first-order and pseudo-second-order rate constants, respectively. The intra-particle diffusion model can be expressed as follows: where k 3 represents the intra-particle diffusion rate constant (mg·(g min 1/2 ) −1 ) and C is the constant related to the thickness of the boundary layer (mg·g −1 ).

Adsorption isotherms
For the adsorption isotherms study, 0.5 g of CR-LDH was added into 100 mL of phosphate solution with initial concentration ranging from 5 to 210 mg·L −1 by maintaining reaction temperature at 15, 25, 35°C, respectively. After adsorption equilibrium, the supernatants were collected, filtered, and analyzed. The adsorption isotherms of phosphate adsorbed by CR-LDH were studied by Langmuir and Freundlich models.
The equation of Langmuir (chemical adsorption model of single molecular layer) and Freundlich (empirical model) isotherm models can be expressed as (Moradi 2016;Rajabi et al. 2017;Robati et al. 2016) follows: where C e represents equilibrium concentration of phosphate, mg·L −1 ; Q max is the maximum adsorption capacity, mg·g −1 ; b is the affinity coefficient, L·mg −1 ; K F and n represent the constants of Freundlich isotherm model.

Adsorption thermodynamics
The adsorption thermodynamics formula can be expressed as follows (Moradi and Sharma 2021; Moradi and Zare 2013): where ΔG 0 is the Gibbs free energy, KJ·mol −1 ; R represents the constant of perfect gas and R=8.314 J·mol −1 ·K −1 ; T is the absolute temperature of solution, K; ΔS 0 represents the entropy change, J·mol −1 ·K −1 ; ΔH 0 is the enthalpy change, KJ·mol −1 ; and K 0 is the standard equilibrium constant.

Characterization of CR-LDH composites
The structural characteristics of calcium carbide slag, red mud, and CR-LDH are investigated by XRD patterns and the results are shown in Fig. 1 (002) and (004) crystal planes of hydrocalumite were appeared, which were consistent with the PDF card of the International Diffraction Data Center (PDF#16-0333). The CR-LDH composite exhibited hydrocalumite structure. Hydrocalumite was an ordered layered inorganic compound of LDHs with excellent removal ability for phosphate. The diffraction peaks corresponding to the hydrocalumite (002) crystal plane had the characteristics of sharp peaks, good peak type, low and flat base line, high crystallinity, and a relatively complete monoclinic symmetrical layered structure. In addition, the characteristic peaks corresponding to the (003) crystal face of boehmite were appeared in the XRD patterns of CR-LDH. The peaks of CaCO 3 (2θ=43.14°) and Fe 2 O 3 (2θ=33.24°, 2θ=35.73°, and 2θ=54.21°) were also detected in the XRD patterns of CR-LDH. The reason for the existence of these metal oxides was that their raw materials were industrial waste slag, and the metal oxides were not fully reacted in the process of preparing the composites, and there were still some metal oxides in the CR-LDH composite. All these findings indicated that calcium carbide slag and red mud had been successfully compounded to form CR-LDH. SEM and EDS are used to investigate the surface morphology and element content distribution of CR-LDH, respectively. As shown in Fig. 2a, and b, CR-LDH presented a typical plate-like morphology of LDHs . The surface of CR-LDH was relatively rough and there were a large number of gaps, which was conducive to the adsorption process. The BET surface area of CR-LDH was 42.46 m 2 ·g −1 , providing a large number of surface active sites for the adsorption of phosphate in the aqueous solutions. According to the EDS results ( Fig. 2c and Table 1), the content ratio of main elements of CR LDHs mainly contained O, Al, and Ca, and the content of Ca was much higher than that of other metal elements, which were consistent with the XRD results that CR-LDH had the characteristic adsorption peaks of hydrocalumite and boehmite.

Effect of reaction time on phosphate adsorption
The process of phosphate adsorbed by CR-LDH is a timedependent reaction. The adsorption behavior of CR-LDH with respect to phosphate as a function of time was studied and the results were shown in Fig. 3a. It can be seen that the adsorption of phosphate by CR LDHs can be roughly divided into three stages, namely, the rapid adsorption reaction stage of 0-60 min, the slow adsorption reaction stage of 60-480 min, and the adsorption equilibrium stage after 480 min. At the initial stage of adsorption, the surface of CR-LDH possessed abundant adsorption sites, and a large number of phosphate were existed in the solution, leading to the faster adsorption rate of phosphate by CR-LDH. With the progress of the reaction, the concentration of phosphate was decreased in the solution, and numerous adsorption sites on the surface of CR-LDH were occupied by phosphate. As a result, the available adsorption sites of CR-LDH were less and the adsorption rate of phosphate was decreased accordingly. When the contact time was 480 min, the removal efficiency of phosphate was 98.53%. Meanwhile, the phosphate concentration in the effluent was 0.44 mg·L −1 , which met the first-level discharge  -1996). Thus, the reaction time is set as 8 h in the follow-up experiment.

Effect of CR-LDH dosage on phosphate adsorption
To investigate the effect of dosage on the adsorption performance of CR-LDH with respect to phosphate, 0.5, 1, 2, 3, 4, 5, and 6 g·L −1 CR-LDH were added into 100 mL phosphate ion solution with a concentration of 30 mg·L −1 . Adsorption experiments were conducted for 8 h at 25°C with pH of 7.0 and shaking speed of 180 r·min −1 . The results are shown in Fig.  3a. It can be seen from Fig. 3a that the removal rate of phosphate was first gradually increased and then basically approached 100%, while the adsorption ability of per unit mass CR-LDH was gradually decreased. The increase of phosphate removal rate was due to the increase of adsorbent dosage, which increased the number of available adsorption sites on the material surface and the contact probability with adsorption sites in the solution, providing favorable conditions for phosphate adsorption removal. Under the condition that the initial concentration of phosphate was 30 mg·L −1 , the amount of phosphate in solution was fixed, and more phosphates were removed with the increasing dosage of CR-LDH, while the per amount phosphate adsorbed by CR-LDH was decreased relatively. Considering the phosphate removal performance and meeting the first-level discharge standard (≤0.5 mg·L −1 ) in the comprehensive sewage discharge standard of China (GB8978-1996), the optimal dosage of 5 g·L −1 was selected in the following study when the phosphate concentration was 30 mg·L −1 . Under this condition, the phosphate removal rate was 98.6%, and the phosphate concentration in the effluent was 0.42 mg·L −1 .

Effect of initial pH values on phosphate adsorption
pH is deemed as an important factor influencing the physicochemical adsorption reaction process of adsorbent towards adsorbate (Cheng et al. 2009;Kong et al. 2020;Moradi et al. 2009;Zhang et al. 2019): where pK 1 =2.15, pK 2 =7.20, and pK 3 =12.33, respectively. In this study, the effect of initial pH values on phosphate adsorption by CR-LDH was investigated at pH ranging from 3.0 to 11.0 and the results are shown in Fig. 3b. As shown in Fig. 3b, with the increase of initial pH, the phosphate removal rate increased gradually, but the change range was relatively small. When the pH was in the range of 3.0-11.0 (the main species of phosphate were H 2 PO 4 − and HPO 4 2− ), CR-LDH maintained a high phosphate removal rate. Due to the adsorption free energy of H 2 PO 4 − was lower than that of HPO 4 2− (Yang et al. 2014), so CR-LDH was more likely to adsorb H 2 PO 4 − . At pH 3.0 and 11.0, the phosphate removal rates were 98.1% and 98.7%, respectively. At pH of 11.0, the main species in solution was HPO 4 2− and OH − would compete for active adsorption site with phosphate anions , causing the slow decline in removal rate of CR-LDH. Meanwhile, OH − was consumed in the transformation process of calcium phosphate precipitation into hydroxyapatite which would cause the competition between OH − and phosphate anions to weaken. The combination of the above two forces led to the removal efficiency of pH 11.0 was slightly higher than that of pH 3.0 and made CR-LDH had a certain regulating effect on the pH of the solution. The final pH values were gradually increased after reaction equilibrium and remained ranging from 8.1 to 8.9, indicating that OH − was released by CR-LDH in the adsorption process under lower pH. While at higher pH values, OH − was consumed by forming hydroxyapatite and might also be accompanied by a small amount of OH − release. Considering that the actual water samples are mostly in the neutral range, pH of 7.0 is selected in the follow-up experiments.

Effect of co-existing inorganic anions on phosphate adsorption
Phosphate is not existed alone in wastewater, but may be coexisted with CO 3  , and Cl − were con-existed in solution, the removal rate of phosphate was deceased in varying degrees. The inhibition effect of coexisting inorganic anions was increased with the increase of its concentration. Under the same initial concentration of CO 3 2− , SO 4 2− , NO 3 − , and Cl − , the inhibition order of the above four co-existing ions was as follows: CO 3 2− >SO 4 2 − >Cl − >NO 3 − . When the concentration of CO 3 2− was 200 mg·L −1 , the removal rate of CR-LDH towards phosphate was reduced to 96.77%, and the effluent phosphate concentration was 0.97 mg·L −1 . The inhibition effect of co-existing inorganic anions was mainly due to the fact that these anions would compete with phosphate to enter the interlayer structure of CR-LDH in adsorption process, resulting in decrease adsorption sites of the interlayer. The interaction between the inorganic anions with higher valence and the positively charged laminates was stronger, which made it easier to enter the interlayer channels of CR-LDH. As a result, the inhibitory effect of bivalent anion (CO 3 2− and SO 4 2− ) was stronger than that of monovalent anions (NO 3 − and Cl − ), which were consistent with the research results reported in previous publications (Seftel et al. 2018).

Adsorption kinetics
To better describe the adsorption process of CR-LDH towards phosphate, pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic models were adopted in this study. As illustrated in Fig. 4a and Table 2, the pseudo-second-order kinetic model (R 2 =0.989) better fitted than the pseudo-second-order kinetic model (R 2 =0.896) for phosphate adsorption on CR-LDH, suggesting that the adsorption of phosphate onto CR-LDH was mainly controlled by chemical adsorption through sharing or exchanging electrons, thus forming valence force for adsorption (Cao et al. 2020).
To determine the adsorption rate limiting step, intraparticle diffusion model was used to fitting the adsorption experimental results. According to intra-particle diffusion kinetic model, if the adsorption rate is only determined by the intra-particle diffusion, the Q t -t 1/2 curve will be a straight line with C value of zero. As shown in Fig. 4b and Table 2, the adsorption experimental results of CR-LDH towards phosphate presented a linear relationship in three stages and value of C was not zero, suggesting that intra-particle diffusion was not the solely speed limiting step. The adsorption process of CR-LDH with respect to phosphate could be divided into three stages: film diffusion stage, intra-particle diffusion stage, and adsorption equilibrium stage. The film diffusion stage which occurred on the initial of the reaction of 0-60 min was a rapid adsorption stage. Phosphate could transfer to the abundant adsorption sites on the surface of CR-LDH through film diffusion to achieve rapid adsorption. The intra-particle diffusion stage happened on 60-480 min was a gradual adsorption stage. In this stage, phosphate reached the adsorption sites inside the pores of the CR-LDH with the happening of intraparticle diffusion. The third stage was adsorption equilibrium stage (>480 min), and the adsorption driving force was relatively weak due to the fact that there were fewer adsorption sites in the solution and the number of free phosphate radicals was very limited. The obtained C value followed the sequence of C 1 <C 2 <C 3 , indicating that the boundary layer thickness of CR-LDH was increased with the proceed of adsorption reaction of CR-LDH towards phosphate.

Adsorption isotherms
Adsorption isotherms of phosphate removed by CR-LDH were investigated within different concentrations of phosphate ranging from 0 to 150 mg·L 1 at three different temperatures (15, 25, and 35°C) and the results are shown in Fig. 5. As exhibited in Fig. 5, with the increasing of phosphate's initial concentration, the adsorption capacity of CR-LDH towards phosphate was increased. When the initial concentration of phosphate was at a relatively low level, the adsorption sites on CR-LDH could ensure that most of the phosphate in the solution was removed, and the corresponding phosphate adsorption capacity of CR-LDH showed a linear upward trend. With the further increase of the initial phosphate concentration, more and more adsorption sites were occupied by phosphate radicals, and the number of free adsorption sites was decreased, leading to the decrease in the tangent slope of the phosphate adsorption curve.
Langmuir and Freundlich isotherm models were conducted to simulate the adsorption experimental data of phosphate adsorbed by CR-LDH. The fitting results of experimental data of Langmuir and Freundlich isotherm models are exhibited in Fig. 5 and Table 3. Compared to Freundlich isotherm model, Langmuir isotherm model (R 2 >0.98) was more suitable to fit the adsorption process of phosphate adsorbed onto CR-LDH which indicated that the adsorbed phosphate had no interaction with each other and were distributed on the surface of CR-LDH as a molecular layer. The maximum adsorption capacity values calculated by Langmuir isotherm were close to the experimental data. When the temperature increased from 15 to 35°C, the maximum adsorption capacity of CR-LDH towards phosphate was enhanced from 14.41 to 16.59 mg·g −1 , indicating that high temperature had a beneficial effect on adsorption of phosphate. Comparison of the adsorption ability of different adsorbents towards phosphate is provided in Table 4. It can be seen that the maximum adsorption capacity of CR-LDH is in the middle level. In light of the cost of the adsorbent and the reuse of industrial waste-carbide slag, CR-LDH is still considered as a potential environmental friendly  . 4 Fitting results of pseudofirst-order, pseudo-second-order kinetic model (a), and intraparticle diffusion model (b) adsorbent. The n values of Freundlich isotherm model were all higher than 1, suggesting that the adsorption process of CR-LDH with respect to phosphate was easily implemented.

Adsorption thermodynamics
Adsorption thermodynamics analysis was also carried out to describe the adsorption process of phosphate adsorbed by CR-LDH. Because the adsorption process of CR-LDH with respect to phosphate fitted Langmuir isotherm model well with R 2 >0.98, so the Langmuir constant of b could be calculated as K 0 . As shown in Fig. 6 and Table 5, the values of ΔG 0 were all less than zero which suggested the adsorption process of phosphate by CR-LDH was spontaneous reaction. The absolute values of ΔG 0 became more positive with the increasing of temperature, suggesting that higher temperature was more conductive to the adsorption of CR-LDH towards phosphate. The positive ΔH 0 of 26.21 KJ·mol −1 implied that the adsorption of phosphate by CR-LDH was endothermic, and this finding was in agreement with the results that the adsorption capacities of CR-LDH were enhanced with the increasing of temperature. The positive ΔS 0 of 91.45 J·mol −1 ·K −1 indicated that the randomness at the solid-solution interface was increased which was due to the adsorption reaction of phosphate adsorbed by CR-LDH.

Application of CR-LDH to practical phosphate-containing wastewater
To investigate the applicability of CR-LDH for the removal of phosphate in practical phosphate-containing wastewater, the untreated wastewater in the sewage well of a machinery factory in Shenyang is selected and analyzed. Phosphating is an important process in the pretreatment process of coating surface in mechanical processing plants. A large number of phosphorus-containing agents are needed in the phosphating process, and the phosphate content of phosphating wastewater often exceeds the standard. The soluble orthophosphate forms in phosphating wastewater account for about 87.8% of total soluble phosphorus, which is comparable with the use of simulated phosphorus containing wastewater as experimental  water. The photos and water quality parameters of practical phosphate-containing wastewater before CR-LDH adsorption and after CR-LDH adsorption are given in Fig. S3 and Table 6, respectively. In the applicability of CR-LDH test, 0.5, 1, 1.5, 2, 3, 4, and 5 g/L of CR-LDH were added into the 100 mL practical phosphate-containing wastewater for the adsorption experiments, respectively. After adsorption for 8 h at 25°C, the samples were filtered through 0.45-μm cellulose filtered membrane and then diluted for further phosphate analysis. As exhibited in Fig. 7 and Table 5, the residual phosphate of practical wastewater was decreased gradually as the increase of CR-LDH dosage, while the phosphate removal rate was increased first and then stabilized. When the dosage of CR-LDH was 3 g·L −1 , the residual phosphate concentration was 0.48 mg·L −1 , and the corresponding removal rate was 95.1%, which met the first class discharge standard (≤0.5 mg·L −1 ) in the integrated wastewater discharge standard (GB8978-1996). As shown in Table 6, the amount of pH increase after adsorption process in practical wastewater was smaller than the result in Fig. 3c, which might be due to that the composition of practical phosphate-containing wastewater was very complex, and the contained coexisting ions as well as organic/inorganic substances might inhibit the release of OH − by CR-LDH. CR-LDHs with relatively rough surface and numerous gaps were conducive to the adsorption process. So when phosphate adsorbed by CR-LDH, it would also be accompanied by the removal of other pollutants. The effluent pH met the discharge standard, and the COD, turbidity, and NH 4 + -N were reduced by 42.39%, 77.20%, and 20.71%, respectively. The above results indicated that CR-LDH had great potential for adsorbing phosphate from practical phosphate-containing wastewater.

Adsorption mechanism
In order to clarify the adsorption mechanism, FT-IR was used to find out the functional groups' changes of CR-LDH before and after adsorption of phosphate. As shown in Fig. S4, the peaks at 3415.75 cm −1 and 1637.79 cm −1 were assigned to the stretching and bending vibration of the −OH in the CR-LDH and interstitial water molecules, respectively. After adsorbed phosphate, the peaks at 1003.28 cm −1 and 548 cm −1 appeared which were attributed to the stretching and bending vibration of phosphate. The band at 874 cm −1 belonged to the vibration of P-O in HPO 4 2− , indicating that effective adsorption of phosphate by CR-LDH was existed.
The XRD spectra of CR-LDH after adsorption of phosphate is shown in Fig. 8. Compared to XRD pattern of CR-LDH before adsorption of phosphate, the diffraction peak intensity of boehmite was decreased, and the characteristic diffraction peak of brucite was disappeared. New diffraction peaks of Ca 2 P 2 O 7 , Ca 2 P 2 , and Mg 3 (PO 4 ) 2 were appeared which might be due to that phosphate entered into the layered structure of CR-LDH after the adsorption process, and Ca 2+ dissolved on the surface combined with HPO 4 2− hydrolyzed to form CaHPO 4 . After that, CaHPO 4 was polymerized and dehydrated to form Ca 2 P 2 O 7 precipitation. The peaks at 2θ of 26.47 0 and 39.49 0 were attributed to CaO. The adsorption peak of CaCO 3 was appeared at 2θ of 43.14 0 . The appearance of peaks at 2θ of 33.24 0 , 35.73 0 , and 54.21 0 identified the existence of Fe 2 O 3 . The reason for the existence of the above metal oxides in the XRD pattern of CR-LDH after adsorption of phosphate was due to that the raw material of carbide slag was industrial waste, and the metal oxide did not fully react during the preparation process of CR-LDH.
According to the EDS results ( Fig. 9a and Table 1), nonmetallic P element was appeared in CR-LDH after adsorption of phosphate, indicating that the surface of the CR-LDH composite could adsorb phosphate by surface adsorption, surface complexation, electrostatic attraction, and chemical precipitation. The SEM images of CR-LDH after adsorption of phosphate (Fig. 9b, c) exhibited a relatively flat, irregular block morphology, and the surface gap of CR-LDH was filled, leading to the increase in the specific surface area which was conductive for the adsorption process. During the adsorption process, the free metal cations might be combined with PO 4 3− and hydroxyl groups to form phosphate or other metal oxides and then deposited on the surface of the adsorbent.
Based on the adsorption experimental results and characterization analysis, we could infer that the adsorption of phosphate by CR-LDH might be through the following four interactions: (1) physical adsorption: The strength of physical adsorption mainly depends on the specific surface area and pore size distribution of adsorbent. Generally, the larger the specific surface area of adsorbent, the higher the adsorption capacity of physical adsorption. CR-LDH with surface area of 42.46 m 2 ·g −1 had a lamellar structure due to the large number of gaps on the surface and the relatively rough surface (Fig. 2), providing a great quantity of active adsorption sites for the adsorption of phosphate. Thus, part of phosphate could be adsorbed on the surface of CR-LDH by van der Waals force of physical adsorption. (2) ion exchange: The exchangeability of interlayer anions was one of the important properties of LDHs. Some anions in the aqueous solution would enter the material ion layer, while the anions originally in the ion layer would transfer to the aqueous solution. The interlayer anions in the layered structure of CR-LDH in solution might be replaced by anions with stronger exchange performance to achieve partial adsorption of phosphate. (3) anion intercalation: Since CR-LDH laminates were positively charged, the anions in the solution would enter the layers of CR-LDH to balance the structure and electrical properties of the laminates, so anion intercalation played an important role in the process of phosphate removed by CR-LDH. This inference was consistent with XRD results that the intensity of the hydrocalumite diffraction peak was decreased after the adsorption of phosphate, the characteristic diffraction peak of magnesia was disappeared, and the diffraction peak of Ca 2 P 2 O 7 was appeared, indicating that part of phosphate entered into the interlayer structure of CR-LDH. The phosphate on the surface of CR-LDH would form phosphoruscontaining precipitate with dissolved Mg 2+ and Ca 2+ . (4) chemical precipitation: The free metal cations (Ca 2+ , Al 3+ , Fe 3+ , etc.) produced by CR-LDH would be combined with phosphate to form various precipitates and then deposited on  the surface of adsorbent, leading to the flat surface of CR-LDH after adsorption of phosphate (Fig. 9b). The appearance of Ca 2 P 2 O 7 in the XRD pattern of CR-LDH after adsorption of phosphate suggested that Ca 2+ dissolved from CR-LDH was combined with HPO 4 2− to form CaHPO 4 and then polymerize, dehydrated to obtain Ca 2 P 2 O 7 precipitate (Fig. 8). The detection of P element in the EDS of CR-LDH after adsorption of phosphate also indicated that chemical prchemical precipitation was occurred in the process of phosphate adsorbed by CR-LDH (Fig. 9a).

Conclusion
In this study, adsorbent of CR-LDH was successfully designed via a simple method by using calcium carbide slag and red mud as raw materials. Characterization results showed that the synthesized CR-LDH had typical properties of LDH. The experimental data of phosphate adsorbed by CR-LDH could be well fitted by pseudo-second-order model and Langmuir isotherm model with theoretical maximum adsorption capacity of 16.06 mg·g −1 at 25°C, suggesting that it is a Fig. 7 Effect of dosage on the removal performance of phosphate from practical phosphate-containing wastewater Fig. 8 XRD pattern of CR-LDH before and after adsorption of phosphate chemical and monolayer adsorption process. Based on the adsorption experimental results and characterization analysis, the adsorption of phosphate by CR-LDH was assumed to be through physical adsorption, ion exchange, anion intercalation, and chemical precipitation. In brief, the CR-LDH prepared from calcium carbide slag and red mud is an effective adsorbent for treating practical phosphate-containing wastewater, which is helpful for the reuse of industrial waste resources, the reduction of environmental pollutants of phosphate, and the protection of the environment.