Comparative Adsorptive Removal of Phosphate and Nitrate from Wastewater Using Biochar-MgAl LDH Nanocomposites: Coexisting Anions Effect and Mechanistic Studies

In this study, date-palm biochar MgAl-augmented double-layered hydroxide (biochar–MgAl–LDH) nanocomposite was synthesized, characterized, and used for enhancing the removal of phosphate and nitrate pollutants from wastewater. The biochar–MgAl–LDH had higher selectivity and adsorption affinity towards phosphate compared to nitrate. The adsorption kinetics of both anions were better explained by the pseudo-first-order model with a faster removal rate to attain equilibrium in a shorter time, especially at lower initial phosphate-nitrate concentration. The maximum monolayer adsorption capacities of phosphate and nitrate by the non-linear Langmuir model were 177.97 mg/g and 28.06 mg/g, respectively. The coexistence of anions (Cl−, SO42−, NO3−, CO32− and HCO3−) negligibly affected the removal of phosphate due to its stronger bond on the nano-composites, while the presence of Cl− and PO43− reduced the nitrate removal attributed to the ions’ participation in the active adsorption sites on the surface of biochar–MgAl–LDH. The excellent adsorptive performance is the main synergetic influence of the MgAl–LDH incorporation into the biochar. The regeneration tests confirmed that the biochar–MgAl composite can be restored effortlessly and has the prospective to be reused after several subsequent adsorption-desorption cycles. The biochar-LDH further demonstrated capabilities for higher removal of phosphate and nitrate from real wastewater.


Introduction
Nitrate and phosphate are necessary nutrients for the growth of plants, wildlife, and humans. However, elevated concentrations of nitrate and phosphate in surface and groundwater is a severe global concern leading to devastating impacts on ecosystems [1,2]. General sources of nitrate and phosphate contaminants in water bodies usually arise from the waste products of human activities such as discharges from industrialized practices, agricultural uses like inorganic fertilizers, compost, and wastewater treatment effluents [3]. A phosphate generally arises from the element named phosphorous, and affects water quality by the disproportionate development of algae. Phosphates ions present in the water nourish algae, which further destroy other forms of life and yield unsafe contaminants. Excessive concentration of phosphorous results in the eutrophication process, which decreases the amount of dissolved oxygen in the aquatic streams of watercourses and the adsorption performance of date palm derived biochar. In addition, the aforementioned studies produced a biochar-LDHs composite via a hydrothermal technique and investigated the removal of phosphate ions only. Therefore, this study investigated the production of biochar derived from date palm from waste intercalated with a MgAl-layered double hydroxides composite via simple co-precipitation technique and explored it for the recovery of both phosphate and nitrate from wastewater streams.
The main objective of this work is to enhance phosphate and nitrate recovery from the water phase using date palm waste-derived biochar augmented with MgAl layered double hydroxides. The synthesized nanocomposites characterization using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) measurement techniques to examine the surface and structure properties after the inclusion of LDH layers into biochar. The study focused on the effects of several influencing parameters such as contact time, adsorbent dosage, pH, anions concentration, and temperature on the percentage removal of phosphate and nitrate ions. The adsorption capacity and interaction mechanism of the synthesized composite with phosphate and nitrate anions were evaluated in depth by considering isotherms, kinetics, and thermodynamic models.

Chemicals
The production of the biochars used in this study was done at slow pyrolysis (700 • C and four h at a heating rate of 3 • C/min) using date palm waste fronds collected from Al-Hasa city, Saudi Arabia. The pure salts of aluminum (III) nitrate nonahydrate and magnesium (II) nitrate hexahydrate, were obtained from Sigma Aldrich (Darmstadt, Germany) and used without any pretreatment. Potassium dihydrogen phosphate (KH 2 PO 4 ), sodium nitrate (NaNO 3 ), and potassium sulfate (K 2 SO 4 ) were from Sigma Aldrich (Darmstadt, Germany). Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were from Merck (Darmstadt, Germany). The salts of KH 2 PO 4 and NaNO 3 were the sources of phosphate and nitrate solution used in the study. Preparation of a stock solution of 1000 mg/L for the experimental work used deionized water. Phosphate and nitrate concentrations were measured using an ultraviolet-visible (UV-visible) spectrophotometer (DR 6000, Hach Lange, Mena, Dubai, UAE). The initial desired value of pH was adjusted using HCl and NaOH.

Synthesis of Date Palm-Derived Biochar-Decorated MgAl Composites
The production of biochar-modified MgAl-layered double hydroxide composites were from different masses of biochar (5 and 10 g) with a 3:1 mole ratio of magnesium (7.69 g) and aluminum (3.75 g) nitrate salts (M 2+ :M 3+ ) via co-precipitation method. In detail, first, we dissolved a measured amount of salts in a reaction vessel containing 100 mL of double-distilled water. Simultaneously, a precisely known quantity of biochar derived date palm fronds (5 and 10 g) was also added in 100 mL deionized water, sonicated at 60 amplitude for 30-60 minutes, respectively and transmitted to the reaction flask. The mixture of salts and biochar was stirred vigorously at 800-1000 rpm for 10 min at 90 • C. Afterward, the pH of the reaction mixture was maintained at 10 ± 0.5 by adding 1 M of NaOH dropwise. When the required pH value was achieved, the reactor was kept for nearly 24 h reflux at 90 • C. After the reaction, the resultant precipitates were centrifuged and washed 3-4 times with double-distilled water, and finally by ethanol to remove all unreacted salts and impurities. The final product biochar decorated MgAl composite was kept at 60 • C in the oven for 48 h. The resultant composites were then stored in a desiccator for future experiments. The actual compositions of biochar-MgAl composites are listed in Table 1.

Adsorption of Phosphate and Nitrate onto Biochar MgAl Composites
A preliminary adsorption experiment was performed using biochar and biochar-decorated MgAl composites to evaluate the removal performance of nitrate and phosphate and select the adsorbent showing better adsorptive performance for detailed adsorption studies. Batch mode adsorption experiments were performed on the adsorbent chosen to investigate the effect of adsorption parameters including initial solution pH 2-12, adsorbent amounts of 5-100 mg, anion concentration between 10-50 mg/L, contact time within 10-840 min, and temperature (298-318 K) in temperature-controlled agitator at fixed rpm of 275 containing 40 mL of solution in 50 mL plastic vials. All the experiments were performed in duplicate and the average values reported herein.
The removal efficiencies and adsorption capacities were determined by using the following Equations (1) and (2): where C o and C e are the initial and the equilibrium concentrations (mg/L) of PO 4 3− and NO 3 − , respectively, q e is the equilibrium amount of PO 4 3− and NO 3 − adsorbed (mg) per unit mass of adsorbent (mg/g), V is the volume of the solution (L), and m is the mass of the adsorbent in the solution (g).

Co-Existing Ions Experiment
There are a lot of anions present in the water or groundwater such as carbonate, bicarbonate, nitrate, sulfates, chlorides, and phosphates, etc. which might prevent the removal of nitrate or phosphate during a sorption process. Therefore, the influence of these anions (Na 2 CO 3 , NaCl, Na 2 SO 4 , NaHCO 3 , and NaNO 3 ) was investigated at varied concentration from 0.001 to 0.1 M. The co-existing anions sorption experiment was conducted by using 40 mL of reaction mixture containing specified concentration of co-existing anions and 10 mg/L of phosphate and nitrate ions agitated with 5 mg of B 5 MgAl composite at 200 rpm for 24 h.

Adsorption Isotherm and Kinetic Modeling
The isotherm and kinetic experiments were performed at various phosphate and nitrate concentrations ranging between 10-50 mg/L, contact time between 10-840 minutes, and variable temperature values (298-318 K) at constant parameters including composite dosage of 5 mg, pH 6, and agitation speed of 275 rpm. The final concentrations of the samples were estimated using a barcoded test kits on the UV-visible spectrophotometer. The adsorption mechanism of phosphate and nitrate onto biochar MgAl composite was demonstrated by utilizing kinetics, isotherm models as well as characterization of the spent composites. Thus, in this study, three kinetic models (pseudo-first, second-order) were used while the linear and non-linear form of Langmuir and Freundlich's models were employed for the equilibrium study. These models equations and the respective estimated models' parameters are listed in Section 3.5 "Adsorption Rate and Kinetic Modeling" and Section 3.6 "Isotherm Modeling", respectively.

Sequential Adsorption-Regeneration Cycles
The adsorption-regeneration experiment was conducted by shaking 200 mL of known concentration of phosphate and nitrate (10.0 mg/L) with 50 mg B 5 MgAl composite at 200 rpm and 24 h. After the adsorption experiment, the supernatant was separated from the spent sorbent with the help of filtration followed by centrifugation (5000 rpm and 5 min). The final concentration of the supernatant was measured, as indicated in Section 2.1. The spent B 5 MgAl composite was regenerated with two different levels of NaOH (0.1 and 1.0 M). The recyclability of the B 5 MgAl composites was assessed by accomplishing the five consecutive adsorption-regeneration sequences. The successive adsorption/regeneration experiments repeated, and the average values indicated in results. The adsorption-regeneration experiment was conducted by shaking 200 mL of known concentration of phosphate and nitrate (10.0 mg/L) with 50 mg B5MgAl composite at 200 rpm and 24 h. After the adsorption experiment, the supernatant was separated from the spent sorbent with the help of filtration followed by centrifugation (5000 rpm and 5 min). The final concentration of the supernatant was measured, as indicated in Section 2.1. The spent B5MgAl composite was regenerated with two different levels of NaOH (0.1 and 1.0 M). The recyclability of the B5MgAl composites was assessed by accomplishing the five consecutive adsorption-regeneration sequences. The successive adsorption/regeneration experiments repeated, and the average values indicated in results. Figure 1a,b displays the FTIR and XRD spectrums of B5MgAl and B10MgAl composites, respectively. The FTIR spectrum of both composites showed the presence of surface functionalities associated with biochar and MgAl LDH. For instance, the sharp broadband at 1371 cm −1 was attributed to the stretching vibration of nitrate in the interlayer space of the LDH [34]. Similarly, the characteristic peaks of C-O and mixed metal oxides (MM-O) were observed at 980 and 870 cm −1 , respectively. The sharp peak at 617 cm −1 was ascribed to lattice vibrations of the MM-O bonds [35]. Also, the crystallographic structure results of the biochar-MgAl composites assessed by XRD patterns are shown in Figure 1b. The sharp and broad diffraction peaks located at 2θ = 23.00°, 29.61° and 60.36° correspond to quartz (SiO2), potassium aluminate (KAlO2), and dolomite (CaMg(CO₃)₂), respectively, indicating excellent intercalation of biochar into LDH layers without damaging the crystalline structure of the composite with slightly deviation at increased biochar content. The results clearly demonstrated that the synthesized composite is functionalized with different oxygen functionalities (OH, C-O-C, NO3 − , MM-O) and excellent crystallinity which may expected to provide improved uptake of phosphate and nitrate from water phase.   The dispersion of biochar into layers of MgAl LDH further demonstrated by analyzing surface morphology of composites using SEM and transmission electron microscopy (TEM, Morgagni 268, FEI Company, Brno, Czech Republic) techniques ( Figure 2). The SEM images indicated rough heterogeneous structure comprised of tiny particles surrounded in the entire surface. Moreover, as the biochar amount increased from 5 to 10 g, the proportion of small particles associated with biochar increased, leading to smooth and homogenous surface morphology. TEM analysis carried out to explain the dispersion of biochar at various mass ratios indicates that in B 5 MgAl composite, the particles of biochar homogeneously intercalated throughout the layers of MgAl LDH, which suggests substantial enhancement in surface and structure properties and favorable uptake of contaminants. Moreover, at a higher biochar amount (see B 10 MgAl), biochar particles seem to agglomerate within the layers of LDH reflecting breakage of the LDH structure and this is expected to reduce the anion removal efficiency.

Characterization of Biochar-MgAl Composites
Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 19 substantial enhancement in surface and structure properties and favorable uptake of contaminants. Moreover, at a higher biochar amount (see B10MgAl), biochar particles seem to agglomerate within the layers of LDH reflecting breakage of the LDH structure and this is expected to reduce the anion removal efficiency. The N2 adsorption-desorption isotherm ( Figure 3) used to calculate the specific surface area, average pore radius, and volume of B5MgAl and B10MgAl composites and their values are listed in Table 2. Results show that the B5MgAl composite exhibited better textural characteristics (surface area and pore volume) compared to B10MgAl and other reported biochar and LDH composites. Improved surface area and pore volume of B5MgAl indicated better interaction of biochar with MgAl surface whereas lower textural properties of the high loading of biochar (B10MgAl) could lead to agglomeration of biochar within the layers of MgAl LDH (as confirmed from TEM results) and this is expected to reduce the sorption performance [36].  Table 2. Results show that the B 5 MgAl composite exhibited better textural characteristics (surface area and pore volume) compared to B 10 MgAl and other reported biochar and LDH composites. Improved surface area and pore volume of B 5 MgAl indicated better interaction of biochar with MgAl surface whereas lower textural properties of the high loading of biochar (B 10 MgAl) could lead to agglomeration of biochar within the layers of MgAl LDH (as confirmed from TEM results) and this is expected to reduce the sorption performance [36].

Phosphate and Nitrate Adsorption Performance of Biochar and Biochar-MgAl Nanocomposite
Results of the preliminary batch-test performed for the percentage removal performance of biochar and biochar-decorated MgAl composites (B10MgAl and B5MgAl) are displayed in Figure 4a. It was found that the percent removal of virgin biochar significantly increased by incorporating with MgAl LDH. B5MgAl composite yielded better performance for phosphate and nitrate removal compared to the B10MgAl composite. The high performance was attributed to better intercalation of biochar into MgAl LDHs interlayers leading to significant improvement in surface functionalities, improved textural properties, and heterogeneous surface morphology with excellent crystallinity as confirmed by the characterization results (Section 3.1). Therefore, B5MgAl composite was selected for further adsorption experiments whose results are discussed below.

Phosphate and Nitrate Adsorption Performance of Biochar and Biochar-MgAl Nanocomposite
Results of the preliminary batch-test performed for the percentage removal performance of biochar and biochar-decorated MgAl composites (B 10 MgAl and B 5 MgAl) are displayed in Figure 4a. It was found that the percent removal of virgin biochar significantly increased by incorporating with MgAl LDH. B 5 MgAl composite yielded better performance for phosphate and nitrate removal compared to the B 10 MgAl composite. The high performance was attributed to better intercalation of biochar into MgAl LDHs interlayers leading to significant improvement in surface functionalities, improved textural properties, and heterogeneous surface morphology with excellent crystallinity as confirmed by the characterization results (Section 3.1). Therefore, B 5 MgAl composite was selected for further adsorption experiments whose results are discussed below.

Influential of Adsorption Parameters on Percentage Removal of Nitrate and Phosphate
The effect of pH on the removal of nitrate and phosphate onto B5MgAl composite is displayed in the ( Figure 4b). As shown, the removal efficiency of phosphate increased as the initial pH increased from 2 to 4. Further increase in pH value from 4 to 12 showed almost minimal change in percent removal of phosphate. The highest removal of phosphate was observed between pH 4 to 6. Similarly, in the case of nitrate, there was a substantial improvement in the removal efficacy (10% to 40%) when the pH was increased from 2 to 6, respectively. However, a significant reduction was observed with further change in pH above 6. To further illustrate the pH effect on removal performance, point of zero charges (pHPZC) of B5MgAl were estimated using a pH drift method, and its value found as 10.02. Therefore, the surface charge of B5MgAl was entirely protonated at pH < pHPZC = 10.02. In acidic medium, phosphate mainly exists in two forms individually as HPO4 2− and H2PO4 − , thus at weakly acidic conditions (4 < pH < 6), the positively charged surface of B5MgAl strongly interacted with negative phosphate anions via electrostatic attractions.
On the other hand, at solution pH > pHPZC, the surface will become deprotonated, causing electrostatic repulsion with negatively charged anions, resulting in a decrease in percentage removal. The results further indicated that the synthesized biochar-MgAl-LDH composite is highly efficient for the removal of phosphate and nitrate at pH ranges (4-10) and (neutral), respectively. Therefore, based on pH results and considering the chemical stability of LDH due to dissolution at powerful acidic medium, pH values of 4 and 6 were selected for further phosphate and nitrate adsorption experiments.

Influential of Adsorption Parameters on Percentage Removal of Nitrate and Phosphate
The effect of pH on the removal of nitrate and phosphate onto B 5 MgAl composite is displayed in the ( Figure 4b). As shown, the removal efficiency of phosphate increased as the initial pH increased from 2 to 4. Further increase in pH value from 4 to 12 showed almost minimal change in percent removal of phosphate. The highest removal of phosphate was observed between pH 4 to 6. Similarly, in the case of nitrate, there was a substantial improvement in the removal efficacy (10% to 40%) when the pH was increased from 2 to 6, respectively. However, a significant reduction was observed with further change in pH above 6. To further illustrate the pH effect on removal performance, point of zero charges (pH PZC ) of B 5 MgAl were estimated using a pH drift method, and its value found as 10.02. Therefore, the surface charge of B 5 MgAl was entirely protonated at pH < pH PZC = 10.02. In acidic medium, phosphate mainly exists in two forms individually as HPO 4 2− and H 2 PO 4 − , thus at weakly acidic conditions (4 < pH < 6), the positively charged surface of B 5 MgAl strongly interacted with negative phosphate anions via electrostatic attractions.
On the other hand, at solution pH > pH PZC , the surface will become deprotonated, causing electrostatic repulsion with negatively charged anions, resulting in a decrease in percentage removal. The results further indicated that the synthesized biochar-MgAl-LDH composite is highly efficient for the removal of phosphate and nitrate at pH ranges (4-10) and (neutral), respectively. Therefore, based on pH results and considering the chemical stability of LDH due to dissolution at powerful acidic medium, pH values of 4 and 6 were selected for further phosphate and nitrate adsorption experiments. To evaluate the effect of adsorbent dose as it is a known vital factor in the adsorption of pollutants from the aqueous phase, adsorption experiments were conducted at varying B 5 MgAl composite dosage between 5-100 mg at pH 6, initial concentration of 10 mg/L, and temperature of 298 K (Figure 4c). As can be observed, an increase in dosage of B 5 MgAl from 5 mg to 100 mg significantly improved the nitrate removal from 39.4% to 70.9%. Besides, compared to nitrate, B 5 MgAl showed strong interaction with phosphate with almost 84.5% removal at only 5 mg. The higher removal efficiency of phosphate at low B 5 MgAl amount was attributed to the synergetic effect of biochar, and MgAl-LDH was leading to stronger interaction of phosphate molecules with surface functionalities of composite via electrostatic and ion-exchange mechanisms during the adsorption process.

Adsorption Rate and Kinetic Modeling
The evaluation of the adsorption rate of phosphate and nitrate onto B 5 MgAl composite was conducted at three different concentrations (10, 30, and 50 mg/L) with different contact times (0-840 min) while other parameters such as pH, temperature, dosage were kept constant. As shown in Figure 5a,b, for both nitrate and phosphate, the rate of adsorption was fast at the first 120 min followed by slow adsorption and reached equilibrium within 180-600 minutes for 10-50 mg/L of anions concentrations. The rapid adsorption rate was attributed to the availability of active binding sites on the B 5 MgAl surface leading to quick uptake and intense interaction with anion molecules, which is comparatively faster than previously reported biochars and their derivatives for phosphate and nitrate removal.
Similarly, Zhanghong Wang et al. [39] calculated rate constants k1 for the first-order kinetics model 9.47 min −1 that were still higher than the present study. Thus, it is concluded that B5MgAl composite holds a quicker removal rate at a lower initial phosphate-nitrate concentration and probably will attain equilibrium in a shorter period. These outcomes demonstrated that a shorter time is required for the B5MgAl composite to remove phosphate and nitrate effectively from the aqueous medium. Furthermore, the experimental value qe is almost similar to the qe value obtained in the pseudo-first-order kinetic model.   To further examine the sorption behavior of nitrate and phosphate, pseudo-first-order, and pseudo-second-order models were applied to the experimental data. The significant factors and correlation coefficients (R 2 ) of kinetics models are listed in the above Table 3 and their respective plots depicted in Figure 5c,d. The regression coefficients R 2 of the pseudo-first-order model varied between 0.951-0.988 in 10 mg L −1 of nitrate and phosphate ions at 303 K, which is higher than pseudo-second-order model R 2 ranging from 0.802-0.938 [37]. The high value indicates that the pseudo-first-order model better explained the adsorption process. The adsorption process of B 5 MgAl on the removal of nitrate and phosphate relates to the physic-sorption method between the dynamic sorbent sites and phosphate-nitrate ions. As compared to other studies, the rate constants of the pseudo-first-order model (k 1 ) were 0.0057 and 0.0108 g/(mg min) at the initial concentrations of 10 mg/L phosphate and nitrate respectively was lower. For example, Xiaolan Hu et al. [38] calculated the rate constants for the first-order model k 1 0.381 min −1 .
Similarly, Zhanghong Wang et al. [39] calculated rate constants k 1 for the first-order kinetics model 9.47 min −1 that were still higher than the present study. Thus, it is concluded that B 5 MgAl composite holds a quicker removal rate at a lower initial phosphate-nitrate concentration and probably will attain equilibrium in a shorter period. These outcomes demonstrated that a shorter time is required for the B 5 MgAl composite to remove phosphate and nitrate effectively from the aqueous medium. Furthermore, the experimental value q e is almost similar to the q e value obtained in the pseudo-first-order kinetic model.

Isotherm Modeling
The interaction of adsorbate (phosphate and nitrate) on the surface of B 5 MgAl composite was further investigated via commonly used isotherm models, namely the Langmuir and Freundlich models. The equilibrium data fitted on linear and non-linear models equations at three different temperatures. The calculated parameters from linear and non-linear equations and non-linear fitted curves are displayed in Tables 4 and 5 and Figure 6, respectively. Considering the linear and non-linear correlation coefficient values, the Langmuir isotherm model (R 2 > 0.982) revealed better fitting than the Freundlich model (R 2 > 0.930) for phosphate adsorption. However, for nitrate, both Langmuir and Freundlich showed better fits with R 2 > 0.986 at all studied temperatures. The estimated maximum adsorption capacities of phosphate and nitrate by the non-linear Langmuir model were 41.75, 176.09, and 177.97 mg/g and 28.06, 20.78 and 19.00 mg/g at 298, 308 and 318 K, respectively. The values of K L (indicates the level of interaction between sorbate and sorbent) are higher for phosphate than nitrate which suggests that phosphate species more strongly interacted with surface functionalities on to biochar-MgAl composite. The value of 1/n in the Freundlich isotherm model is the measure of concentration effect on adsorption performance, ranging from 0 to 1, and this was favorable adsorption. In our study, the 1/n value of B 5 MgAl for phosphate adsorption was between 0.22 and 0.28 demonstrating better adsorption than that of nitrate (1/n = 0.183 − 0.425). The excellent adsorption behavior of as B 5 MgAl confirmed that the coupling of MgAl with date palm-derived biochar via the co-precipitation technique is a promising approach to produce adsorbent for the recovery of phosphate and nitrate from wastewater streams. Also, the improved adsorption capacity at elevated temperature suggests that higher temperature is favorable for phosphate removal onto B 5 MgAl resulting in better interaction forces between adsorbent and phosphate anions. The enhanced adsorption further confirms that chemical interactions were mainly associated with the sorption of phosphate. Consequently, this corroborates results reported by Hui He et al. [40] that investigated MgAl-decorated tobacco-stalk biochar for phosphate recovery. In contrast, nitrate adsorption performance showed a reduction in removal efficiency with increased adsorption temperatures. The previous reduction demonstrates the exothermic nature and physisorption mechanism of nitrate onto B 5 MgAl composite.

Thermodynamic Modeling
During the adsorption process, changes in the thermodynamic parameters including standard free Gibbs energy (ΔG), the standard enthalpy change (ΔH), and the standard entropy change (ΔS) control the mechanism and feasibility of the adsorption. The Van't Hoff equations in Equations (3) and (4) were used to calculate these thermodynamic parameters [41,42].
where Kd is the thermodynamic equilibrium constant calculated by plotting ln(qe/Ce) against qe where qe extrapolated to zero [34]; R is the universal gas constant (8.314 J/mol K); T is the absolute temperature (K). The thermodynamic parameters ΔH and ΔS are calculated by plotting lnKd against

Thermodynamic Modeling
During the adsorption process, changes in the thermodynamic parameters including standard free Gibbs energy (∆G), the standard enthalpy change (∆H), and the standard entropy change (∆S) control the mechanism and feasibility of the adsorption. The Van't Hoff equations in Equations (3) and (4) were used to calculate these thermodynamic parameters [41,42].
where K d is the thermodynamic equilibrium constant calculated by plotting ln(q e /C e ) against q e where q e extrapolated to zero [34]; R is the universal gas constant (8.314 J/mol K); T is the absolute temperature (K). The thermodynamic parameters ∆H and ∆S are calculated by plotting lnK d against 1/T. The thermodynamic parameters, including K d , ∆H, and ∆S, and ∆G, computed for both phosphate and nitrate adsorption at three different temperatures of 298, 308, and 318 K are given in Table 6. Accordingly, the thermodynamic parameters were assessed to reveal the effect of temperature on phosphate and nitrate adsorption onto B 5 MgAl composite. The negative ∆G values found for both pollutants suggest spontaneous adsorption processes. Values of ∆G ranging between 0 and −20 kJ/mol ascribed to physical adsorption [43,44]. Accordingly, the phosphate adsorption processes on B 5 MgAl composite was endothermic due to the positive value of ∆H, whereas negative ∆H for nitrate indicates an exothermic reaction. The ∆S values specify the increase in the randomness of the solid/liquid interface during the adsorption process, exhibiting that the uptake of phosphate and nitrate onto B 5 MgAl composite were favorable processes. These thermodynamic modeling results are in good agreement with the findings of previously reported studies using various other MgAl LDHs for phosphate and nitrate adsorption [41,45,46]. FTIR, XRD, and SEM analyses were performed for B 5 MgAl composite after anions adsorption to provide further insight into possible adsorption mechanisms. The FTIR spectra of studied B 5 MgAl composite before and after phosphate and nitrate adsorption are compared in Figure 7a. The involvement of MM-O bonds in phosphate and nitrate adsorption slightly weakened the strength of MM-O stretching vibrations observed between 540 and 690 cm −1 . A broad absorption band around 3370 cm −1 appears after phosphate and nitrate adsorption, which was attributed to the O-H stretching vibrations of interlayer water molecules [41,47]. The appearance of this band after adsorption is a result of the rehydration of M-M-Os. The intensity of this appeared band after phosphate adsorption was more than that of nitrate adsorption, clarifying the higher adsorption capacity of B 5 MgAl composite toward phosphate than for nitrate. Previous studies reported that the intercalation of phosphate into the interlayer space of the MgAl LDH composite after the adsorption process results in a P-O stretching vibration leading to a distinct band noticed between 1030 and 1100 cm −1 [27,40,42,[47][48][49][50][51]. Accordingly, a shift of the band from 980 to 1000 cm −1 and the relatively stronger right shoulder of the peak at 1000 cm −1 after phosphate adsorption is due to the formation of mostly outer-sphere surface complexes and partially inner-sphere surface complexes on the B 5 MgAl composite [47].
intercalated into the interlayer of B5MgAl composite.
SEM images of B5MgAl composite after anions adsorption shown in Figure 7d. The rough and porous morphology of the pre-adsorption B5MgAl composite (see Figure 2) became smooth and nonporous after anions adsorption. The surface of B5MgAl composite after phosphate adsorption was softer than that of the composite after nitrate adsorption due to the uniform surface coverage by phosphate ions and higher affinity of the B5MgAl composite toward phosphate than for nitrate.   Figure 7b depicts the variation in equilibrium solution pH after phosphate and nitrate adsorption onto B 5 MgAl composite. The equilibrium solution pH is higher than the initial pH values; 4-6 for phosphate and 6-8 for nitrate, which could be due to the ligand exchange of phosphate and nitrate with OH − on the surface of the B 5 MgAl LDH composite [27,40,48,50,51]. The structure of B 5 MgAl was partially destroyed at pH = 2 and, the released OH − from its structure slightly increased the equilibrium solution pH. The effect of initial pH on phosphate removal efficiency was not significant (< 10%), as indicated in Figure 4b. These results imply that the phosphate adsorption of B 5 MgAl composite is probably due to the formation of outer-sphere and inner-sphere surface complexes on the B 5 MgAl composite. At pH 6, the maximum difference between the initial solution pH and the equilibrium solution pH was accompanied by the highest nitrate adsorption. The nitrate adsorption capacity decreased remarkably against further increases or decreases in pH. The affinity of LDHs for anions follows the order PO 4 3− > OH − > NO 3 − according to the Hofmeister series [46], which could explain the decrease in nitrate adsorption at high pH values. The XRD patterns of B 5 MgAl composites before and after anions adsorption are given in Figure 7c. The XRD spectrum of B 5 MgAl composite before and after nitrate adsorption indicated a similar crystallographic structure. However, the peak intensities after nitrate adsorption significantly reduced compared to the pre-adsorption B 5 MgAl composite, which is most probably due to the ion exchange resulting from nitrate adsorption [47]. After phosphate adsorption, peak positions slightly shifted to the right, peak intensities significantly increased, and a new peak formed at 30.44 • (2θ). Yao, et al. [52] reported the formation of new Mg-P crystals in the forms of MgHPO 4 and Mg(H 2 PO 4 ) 2 (around 30 • (2θ)) after phosphate adsorption by Mg-biochar-LDH nanocomposite. Liao, Liu, Li and Yang [46] showed that Mg atoms played a more prominent role in phosphate and nitrate adsorption compared to Al atoms. As a consequence of XRD data assessment, the variations in layer charge density, phosphate intercalation into the interlayer, and formation of inner-sphere complexation contributed to the selectivity and phosphate adsorption capacity of the B 5 MgAl composite. It is worth mentioning that nitrate mostly adsorbed on the surface and the near-edge interlayer but was not intercalated into the interlayer of B 5 MgAl composite. SEM images of B 5 MgAl composite after anions adsorption shown in Figure 7d. The rough and porous morphology of the pre-adsorption B 5 MgAl composite (see Figure 2) became smooth and non-porous after anions adsorption. The surface of B 5 MgAl composite after phosphate adsorption was softer than that of the composite after nitrate adsorption due to the uniform surface coverage by phosphate ions and higher affinity of the B 5 MgAl composite toward phosphate than for nitrate.

Reusability Performance of Spent Biochar-MgAl Composite
Regeneration was undertaken to find the re-usability of the adsorbent after successive cycles of adsorption-desorption tests. The used B 5 MgAl composite was soaked in the NaOH solutions (0.1 and 1 M) to regenerate it for the next consecutive adsorption-desorption cycles. Typically in basic solutions, the electrostatic interaction between the sorbent and the anionic species weakens because of the removal of H + ions from the solution, and hence desorption of the anions occurs from the adsorbent sites [53]. Figure 8a,b displays the percentage removal of nitrate and phosphate after each regeneration cycle with 0.1 and 1 M NaOH, respectively. As shown in Figure 8a, the adsorption capacities of nitrate and phosphate ions regenerated adsorbents found to be about 20.0 mg/g and 74.5 mg/g respectively even after fifth adsorption-desorption sequence. For the given adsorbent, the percentage removal decreased from 57.9% to 49.5% in the first two sequences and then concluded at 11% in the fifth sequence. Likewise, for phosphate ion, the removal efficiency reduced by about 50% after five sequences (Figure 8a). Moreover, regeneration using 1 M NaOH, resulted in better reusability performance of the biochar-MgAl composite. In detail, the percent removal of nitrate and phosphate (C o = 10 mg/L) reduced from 57% to 27% and from 92% to 67%, respectively, after five consecutive adsorption-regeneration cycles (Figure 8b). The achieved removal performances of the composite regenerated at 1 M NaOH were higher than those of 0.1 M NaOH Nanomaterials 2020, 10, x FOR PEER REVIEW 15 of 19

Reusability Performance of Spent Biochar-MgAl Composite
Regeneration was undertaken to find the re-usability of the adsorbent after successive cycles of adsorption-desorption tests. The used B5MgAl composite was soaked in the NaOH solutions (0.1 and 1 M) to regenerate it for the next consecutive adsorption-desorption cycles. Typically in basic solutions, the electrostatic interaction between the sorbent and the anionic species weakens because of the removal of H + ions from the solution, and hence desorption of the anions occurs from the adsorbent sites [53]. Figure 8a,b displays the percentage removal of nitrate and phosphate after each regeneration cycle with 0.1 and 1 M NaOH, respectively. As shown in Figure 8a, the adsorption capacities of nitrate and phosphate ions regenerated adsorbents found to be about 20.0 mg/g and 74.5 mg/g respectively even after fifth adsorption-desorption sequence. For the given adsorbent, the percentage removal decreased from 57.9% to 49.5% in the first two sequences and then concluded at 11% in the fifth sequence. Likewise, for phosphate ion, the removal efficiency reduced by about 50% after five sequences (Figure 8a). Moreover, regeneration using 1 M NaOH, resulted in better reusability performance of the biochar-MgAl composite. In detail, the percent removal of nitrate and phosphate (Co = 10 mg/L) reduced from 57% to 27% and from 92% to 67%, respectively, after five consecutive adsorption-regeneration cycles (Figure 8b). The achieved removal performances of the composite regenerated at 1 M NaOH were higher than those of 0.1 M NaOH The regeneration revealed that there was a decline in the percentage removal efficiency after five sequences, indicating that the regeneration cycle was approximately useful for the retrieval of sorption sites on biochar composites. The regeneration test confirmed that the biochar-MgAl composite can be restored effortlessly and has the potential to be recycled after several treatment sequences. The potential of real application of the new date-palm biochar-MgAl-LDH composite was evaluated for the removal of phosphate and nitrate ions from wastewater influent collected from a wastewater treatment plant (Imam Abdurrahman Bin Faisal University campus). The primary influent sample contained 3.68 and 3.65 mg/L concentration of phosphate and nitrate ions, The regeneration revealed that there was a decline in the percentage removal efficiency after five sequences, indicating that the regeneration cycle was approximately useful for the retrieval of sorption sites on biochar composites. The regeneration test confirmed that the biochar-MgAl composite can be restored effortlessly and has the potential to be recycled after several treatment sequences.

Treatment Performance of Biochar/MgAl for Wastewater Influent
The potential of real application of the new date-palm biochar-MgAl-LDH composite was evaluated for the removal of phosphate and nitrate ions from wastewater influent collected from a wastewater treatment plant (Imam Abdurrahman Bin Faisal University campus). The primary influent sample contained 3.68 and 3.65 mg/L concentration of phosphate and nitrate ions, respectively. The removal of anions by biochar/MgAl (B 5 MgAl) composite were carried out at adsorption conditions (pH 6, dosage 0.125 g/L, contact time 600 min and T 25 • C). The percentage removal of phosphate and nitrate in real wastewater by date-palm biochar-MgAl composite obtained 84.80% and 41.90% which is nearly similar to the removal performance for 10 mg/L of anions in synthetic water. These results confirm the synthesized date-palm biochar-decorated MgAl-LDH as a highly promising and sustainable adsorbent material for efficient recovery of phosphate and nitrate ions from real wastewater streams.

Conclusions
This paper reports the enhanced phosphate and nitrate recovery from the wastewater phase using date palm waste-derived biochar decorated with MgAl-layered double hydroxides. The synthesized composites were characterized by FTIR, SEM, TEM, XRD, and BET techniques to examine the surface and structure properties after the inclusion of LDH layers into biochar. The performance of the new nanocomposite was investigated under several influencing parameters such as contact time, dosage, pH, anions concentration, and temperature. The adsorption capacity and interaction mechanism of the synthesized composite with phosphate and nitrate anions were evaluated in depth by considering isotherms, kinetics, and thermodynamic models. The presence of coexisting anions (Cl − , SO 4 2− , NO 3 − , CO 3 2− , and HCO 3 − ) negligibly affected the removal of phosphate attributed to its stronger bond on the composites, while the presence of Cl − and PO 4 3− reduced the nitrate removal as a result of these ions participating in the active adsorption sites on the surface of biochar-MgAl with nitrate ions. The composite had a superior selectivity and adsorption affinity towards SO 4 2− and CO 3 2− as compared to Cl − and PO 4 3− ions. The pseudo-first-order model explained better the adsorption of both phosphate and nitrate onto the biochar-MgAl-LDH with a faster removal rate probably with equilibrium attainable quickly, especially at lower initial phosphate-nitrate concentration. The maximum monolayer adsorption capacities of phosphate and nitrate by the non-linear Langmuir model were 177.97 mg/g and 28.06 mg/g at 308 and 298 K, respectively. The regeneration test confirmed that the biochar-MgAl composite can be restored effortlessly and has the potential to be recycled after several treatment sequences. The biochar-MgAl-LDH demonstrated capabilities for higher removal of phosphate and nitrate from real wastewater.