Adsorption of Phosphates onto Mg/Al-Oxide/Hydroxide/Sulfate-Impregnated Douglas Fir Biochar

Abstract

Nitrates and phosphates, found in fertilizers, are the most common eutrophication-causing agents. Douglas fir biochar (BC), a syngas byproduct, was treated with different Al/Mg ratios of sulfate (5% w/w metal loading) followed by an NaOH treatment. The greatest phosphate uptake at 25 °C and pH 7 was attributed to the composite with a Mg/Al 2:1 ratio prepared at pH 13 (AMBC). Batch AMBC phosphate uptake was optimized for initial pH, equilibrium time, temperature, and initial phosphate concentration. Phosphate removal following pseudo-2nd-order kinetics and increases gradually before reaching a max at pH 11, with 95% phosphate uptake in 15 mins. The Sips isotherm model provided the best sorption data fit resulting in a 42.1 mg/g capacity at 25 °C and pH 11. Endothermic and spontaneous adsorption were determined using van ’t Hoff’s plots. BET, XRD, XPS, SEM, TEM, and EDS were used to characterize the biochar before and after phosphate sorption. Used AMBC has the potential to be exploited as a phosphate fertilizer as a key part of an environmentally friendly agricultural management plan.

1. Introduction

Phosphate (PO₄³⁻) is a plant macronutrient found in detergents, soaps, and soil fertilizers. However, due to excessive amounts, phosphorus pollution is a growing environmental concern [1]. A strong correlation exists between eutrophication and phosphate pollution [2]. Agricultural chemical runoffs containing phophates and nitrates are increasing, bringing surges in eutrophication and water treatment costs [3]. In freshwater systems, phosphorus levels usually range from 0.005 mg/L to 0.5 mg/L [4]. Long-term eutrophication is prevented if phosphorus levels are below 0.5 mg/L [4]. Humans accelerate the rate of cultural eutrophication with fertilizers and detergents.

The effect of cultural eutrophication leads to the increased production of phytoplankton and algal blooms covering the top layer of lakes, reducing sunlight needed to sustain other forms of marine life [5]. Algal bloom species produce toxins that accumulate in shellfish at dangerous levels for humans and animal species [6]. Shellfish (mussels) are responsible for amnestic shellfish poisoning, resulting in memory loss and possibly coma [6]. Clams and other crustaceans can cause diseases such as neurotoxic shellfish poisoning, resulting in muscular paralysis or even death. High rates of photosynthesis cause a decrease in inorganic carbon, which raises the pH of water which impairs the chemosensory skills of the organisms [6]. Oxygen levels decrease because of the further decomposition of matter, creating a hypoxic environment or dead zone. In Europe, the Baltic Sea and the Mediterranean are impacted by treated and untreated phosphorus runoff into the marine waters [7]. In US waters, hypoxic dead zones can be found along the East Coast and span into the Gulf of Mexico resulting from the washing of fertilizers into rivers and streams [8].

There are many methods for removing phosphorus in water, including physical and biological treatment, with mixed success [9,10]. Physical methods are primarily filtration and membrane technology. Membrane bioreactors, tertiary membrane filtration devices, and reverse osmosis methods are used [10]. The biological processes for removing phosphates include an anaerobic tank and an activated sludge tank, where phosphorus-accumulating organisms are removed due to sludge wasting. Current methods of removing phosphorus can be costly and inefficient for eutrophication-affected areas. Adsorptive removal of phosphates has been reported for metal oxides [MgO, Fe₂O₃, Fe₃O₄, Al₂O₃], metal hydroxides [(Fe(OH)₃, Fe(OH)₂, Mg(OH)₂], layered double hydroxides, a class is known as anionic clays [11]. Since the solubility products for their stoichiometric compounds are very low, these materials exhibit a rich chemistry with phosphate [12,13].

Layered double hydroxides can exchange oxyanions with their flexible interlayer region. For example, positively charged brucite stacking sheets are balanced by anions in the interlayer region by hydrogen-bonded water molecules [14,15,16]. Additionally, oxyanions like phosphate can form insoluble surface complexes on their surfaces. All of these may result in substantial phosphate absorption.

However, using these LDH, oxide, or hydroxide materials in a sorption system has limitations, such as particle clustering, poor mechanical strength, and aggregation [17]. Adsorption speed and capacities suffer from the aforementioned technical deficiencies and limit its widespread application. Activated carbon, bentonite, diatomite, resin, sand, tea-waste, and zeolites have been used to stabilize the agglomeration [12]. Correspondingly, biochar has been engaged in developing composites with improved sorption features. Coating biochar with mineral oxides has recently been used to improve phosphate sorption capacity under variable conditions: equilibrium time and concentration, ionic strength, pH, and competing contaminants [18,19].

Biochar is a charcoal-like material saturated in carbon. It has unique properties, such as a high surface area, a stable carbon matrix, and a sizeable porous layer that creates an excellent adsorbent or absorbent. Biochar is typically made by fast/slow pyrolysis of wood biomass, animal debris, and plant wastes. It is formed through a thermochemical conversion of biomass under starved O₂ conditions into charcoal-like material [20]. Inexpensive biochar adsorbents are readily available and can be used in large-scale agricultural roles where activated carbons are prohibitively expensive. Agricultural byproducts, energy crops, and waste wood and brush provide a plentiful and renewable source of lignocellulosic feeds for biorefineries that produce biochar as a byproduct [21].

This work optimized phosphate uptake by 28 different commercial Douglas fir biochar (BC)/Al-Mg/oxide/hydroxide/sulfate composites prepared with different ratios of NaOH treated in Mg/Al sulfates. Adsorption benchmarks such as pH effect, equilibrium time, and initial concentration of phosphate concentration were performed. The virgin and phosphate-laden material was extensively characterized by imaging and spectroscopy techniques to unveil the adsorption interactions and mechanism.

2. Materials and Methods

Most of the chemicals and reagents used in this program were analytical-grade and purchased from Sigma-Aldrich, MO (USA). MgSO₄·7H₂O and Al₂(SO₄)₃·18H₂O were used to prepare Mg/Al biochar composites. A 1000 mg/L phosphate solution was prepared by dissolving NaH₂PO₄.H₂O in deionized (DI) water. Douglas fir biochar (BC), provided by Black Owl Biochar Supreme, has a surface area ~700 m²/g, porosity ~0.25 cm³/g, and a contact angle ~122.3 ± 4.2°. Elemental analysis gave C = 81.5%, H = 2.1%, N = 1.2%, O = 13.0%, Fe = 0.07% while ash content = 2.3%. BC was produced from wet gasification of timber industry waste Douglas fir as a byproduct. Three-inch chipped wood (12–15% moisture content) was auger fed into an air-fed updraft gasifier at 800–1000 °C, with a hot zone residence time of 1–10 s. Biochar particles of ∼2 cm were collected, washed thoroughly with water, dried at ambient temperature for 2 days then ground, and sieved to a 0.1–0.3 mm particle.

2.1. Preparation of AMBC

AMBC preparation was carried out using a chemical co-precipitation method described by Li, Ronghua, et al. with slight modifications [18]. First, a 25.0 g portion of raw BC was soaked in Mg/Al-SO₄ solutions for 1 h to incorporate Mg/Al salts into the BC. Next, the obtained slurry was treated with 10 M NaOH after drying at room temperature for 6 hours. Well-stirred conditions were maintained at the corresponding pH (10, 11, 12, or 13). The resulting slurry was aged for 24 hours, filtered, thoroughly washed with methanol and de-ionized water, and dried at 70 °C for 12 hours to obtain Mg/Al-SO₄/OH-impregnated BC (AMBC) (Figure 1). Finally, the different ratios (at different pH = 10, 11, 12, 13) of Mg/Al-biochar composites prepared were subjected to preliminary phosphate sorption screening tests in triplicate (Figure 2).

2.2. Sorption Kinetics and Isotherms

Preliminary sorption experiments were conducted using 100 mg/L phosphate solution (20 mL). After being shaken in an orbital shaker at 200 rpm for 1 hour (at pH = 7), the vessels were withdrawn and filtered through 0.22 micron filter paper. Next, phosphate concentration in the filtrate was determined using UV-spectrometry at 820 nm wavelength, followed by the ammonium molybdate ascorbic acid method [22]. Finally, the composite with the highest phosphate adsorption was selected for batch adsorption experiments. All the batch sorption experiments were conducted in triplicate, and Grubbs test was used to exclude any outliers.

The effect of the solution pH on phosphate adsorption on AMBC was studied using 100 mg/L of phosphate solutions at 25 °C within pH 1 to 13 (at an interval of pH 2). Different molarities of HCl and NaOH are used to adjust the pH of the solutions.

Adsorption kinetics of AMBC were examined by mixing 50 mg of AMBC with 20 mL of 12.5, 25 ppm, and 50 mg/L phosphate solutions at pH = 11, and a controlled study was carried out at pH 7 using 50 mg/L phosphate solution following the same agitation and filtering procedures used in preliminary screening experiments. For the isotherm experiments, 5–1000 mg/L concentrations of phosphate solutions were equilibrated with AMBC at pH 11 and room temperature (25 °C). The final pH was recorded after each experiment. In addition, the adsorption thermodynamics was assessed by repeating the same experiment at lower (10 °C) and higher (40 °C) temperatures.

The mass (mg) of analyte removed per gram of char (q_e) was calculated as follows,

$${\mathrm{q}}_{\mathrm{e}}=\frac{\mathrm{V}\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)}{\mathrm{M}}$$

(1)

where C_o and C_e are initial and equilibrium analyte concentrations (mg/L), V is the solution volume (L), and M is the total mass of adsorbent added (g).

Adsorbent Characterization

Multiple analytical techniques were used to characterize BC, AMBC, and phosphate-laden AMBC including BET (Brunauer–Emmet–Teller), SEM and TEM (Scanning and Transmission Electron Microscopy), EDX (Energy Dispersive X-ray), XRD (X-ray diffraction), and XPS (X-ray photoelectron spectroscopy). The specific surface area (BET) was determined using a nitrogen adsorption isotherm at 273.15 K (Micromeritics Tristar II Plus) and the Dubinin–Astakhov equation $(\log \mathrm{a}={\log \mathrm{a}}_0-{\mathrm{Dlog}}^{\mathrm{n}}\left(\frac{{\mathrm{P}}_0}{\mathrm{P}}\right)$, where a indicates the quantity of gas adsorbed per unit mass of adsorbent (mol/g), a₀ is the micropore capacity (mol/g), D is constant, P is the equilibrium pressure, and P₀ is the saturation vapor pressure of adsorbate at temperature T (K)). Density Functional Theory (DFT) was used to calculate micropore volume (${\mathrm{W}}_0=\frac{\mathrm{44,000}{\mathrm{a}}_0}{\mathsf{\rho }}$, where W₀ is the limiting micropore volume (cm³/g), a₀ is the micropore capacity (mol/g), and ρ is the density of adsorbed gas (g/cm³)) [23]. Biochar surface textures were observed by SEM using a JEOL JSM-6500F FE instrument at 5 kV coupled with a Zeiss, EVO 40 SEM containing a BRUKER EDX system. TEM of AMBC and P-laden AMBC were obtained using a JEOL model 2100 electron microscope operated at 200 kV. TEM/EDX analysis was done with an Oxford X-max-80 detector. XRD analysis was used to determine the Al/Mg-specific crystallographic structure phase after precipitation to make AMBC and to detect any alterations in the AMBC structure after phosphate sorption using a Rigaku ultima III (using Cu-K ($\mathsf{\alpha }$ (λ = 1.54 Å)). XPS analysis was done with a Thermo Scientific K-Alpha system outfitted with a monochromatic X-ray source at 1486.6 eV, corresponding to the Al K_α line, with a 400 µm spot size. AMBC samples equilibrated with 100 mg/L phosphate solutions at pH 11 were used for XPS analysis. In addition, the prepared composite’s PZC (point of zero charges) was measured using the pH drift method, where 20 mL of a series of pH solutions was prepared in a 0.01 NaCl matrix. Solutions were equilibrated for 24 h with 50 mg of biochar before measuring the final pH. PZC values were determined using a graphical approach [24].

3. Results

3.1. Preliminary Screening, pH Optimization, and Point of Zero Charge

The Mg/Al 2:1 ratio prepared at pH 13 showed the best phosphate uptake performance on preliminary screening tests (Figure 2). H₃PO₄ dissociation constants are pK_a1 = 2.12, pK_a2 = 7.21, and pK_a3 = 12.67 (Figure 3A). BC has no significant phosphate uptake due to the lack of functional groups. pH influence on AMBC’s phosphate sorption remained constant (40–50%) except for the pH range 9–11. At pH 11, the highest uptake (~70% removal) was observed (Figure 3A). The PZCs for BC and AMBC are 9.2 and 6.9, respectively (Figure 3B). The basic BC PZC is due to the presence of carbonates and oxides of Mg, Ca, and K formed in the biochar during the pyrolysis, and they are hydrolyzed with the contact with water [25]. The AMBC PZC is due to the contributions of the BC (~9.2), Al₂O₃ (~7), MgO (~11.6), and MgAl₂O₄~(8.3) phases [26,27].

3.2. The Surface Area and Morphological Architecture of the Biochar

The BC utilized was a byproduct of the gasification of wet waste wood (Douglas fir), which was done in an updraft gasifier and produced at a residence time of 1–30 s at 900 °C. This process yields biochar with a relatively large surface area (695 m²/g) and a large pore volume (0.264 cm³/g). When Mg/Al oxides precipitate on BC to create AMBC, a portion of the char’s micropores and ultramicropores cross the char surface and are blocked from adsorbing nitrogen because of the deposited nanoparticles and their aggregates. As a result, it loses ~90% of its pore volume and ~88% of its original surface area (0.264 to 0.023 cm³/g and 695 to 61 m²/g, respectively). Scanning electron micrographs (SEM) for the BC and AMBC are shown in Figure 4. Micrographs (A–D) show the morphological architectural changes due to the precipitation of Mg/Al sulfates, oxides, and hydroxides. The particles were dispersed on the biochar either as primary spherical nano (~200 nm) particles or their aggregates [28]. Elemental mapped TEM-EDX images of phosphate-laden AMBC show the presence of phosphorus (~8.4%) (Figure 5A,B). Furthermore, it was preferentially adsorbed to Al/Mg-sulfate/hydroxide-rich regions on the BC surface rather than on the carbonaceous surface (Figure 5B), which illustrates reasonable agreement that the adsorption of phosphate is mainly due to the ion-exchange or chemisorptive interactions with the Al and Mg oxide/hydroxide phases or formation of insoluble AlPO₄ or Mg₃(PO₄)₂ stoichiometric phosphate compounds [12,13].

3.3. X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy

BC XRD has a broad peak centered at 22.2° due to deformed (amorphous) cellulose, resulting during biomass pyrolysis (Figure 5C) [13]. In addition, the AMBC and phosphate-laden AMBC displayed new peaks corresponding to Al/Mg oxides/hydroxides and sulfates: Mg(OH)₂ (18.2°), MgSO₄ (20.9°), Al(OH)₃ (24.3°), Al₂(SO₄)₃ (25.6°), Al₂(SO₄)₃/Al₂O₃ (30.1°), Al(OH)₃ (40.3°), MgAl₂O₄ (48.1°), and Mg(OH)₂ (52.4°) (Figure 5C) [29,30,31,32]. Moreover, the peak intensities decreased upon phosphate adsorption, indicating a possible formation of amorphous Al/Mg-phosphate surface complexes/stoichiometric compounds [33,34]. Typically, the phosphate surface complexes with Al/Mg oxides/hydroxides result in amorphous phases and will not yield new XRD peaks [13].

XPS further investigated the elemental composition of the near surface (10–100 Å) region of phosphate-laden BC and AMBC. Both LR and HR P-laden XPS spectra are given in the supporting material (Figures S1 and S2). The BC contains trace levels of Al and S as well as undetectable Mg concentrations. Additionally, the P2p signal was noisy due to the extremely low phosphate uptake, therefore the XPS data could not be interpreted as P-loaded BC.

The XPS survey spectra clearly show phosphorus’s presence in phosphate-laden AMBC (Figure 6a). The P2p signal has a relatively low signal-to-noise ratio and was deconvoluted into H₃PO₄ (137.2 eV) (physisorbed phosphates), Mg-O-P (135.8 eV), and Al-O-P/Mg-Al-O-P (134.3 eV) (Figure 6b) [35,36]. O1s high resolution XPS spectra were resolved into four peaks (Figure 6c). These regions are assigned, respectively, adsorbed H₂O (535.6 eV), MgAl₂O₄ (534.2 eV), CO₃²⁻/O-C=O (532.7 eV), and Mg-Al-P, Al₂O₃, O-C/C=O, P-OH, P=O, Al-O-P (531.3 eV) [37,38]. Peak binding energies for C1s at 287.7, 286.1, 284.6, 283.7, and 282.9 eV correspond to -CO₂R(H) and CO₃²⁻, C=O, C-O-C/C-OH, C-C, and C-H (Figure 6d). Al2p was deconvoluted to Al-O-P (78.3 eV), -Al-OH (76.9 eV), and Al₂O₃/Mg-Al-O-P (75.7 eV) (Figure 6e) [39], and Mg1s was deconvoluted to MgHPO₄ (1307.6 eV), Mg(H₂PO₄)₂ (1306.3 eV), MgO/MgAl₂O₄ (1304.9 eV), and Mg/Mg(OH)₂ (1303.3 eV) (Figure 5F) [38,39,40].

3.4. Adsorption Mechanism/Interactions

Multiple interactions are sought to govern the uptake process. Biochar has negligible adsorption, but some electrostatic attractions may be present (via hydrogen bonding, charged metal hydroxide surface, etc.) [13]. The adsorption-onto-Mg/Al-oxides phases are driven by physisorption and chemisorption interactions. Physisorption interactions can be governed by charged metal hydroxide surfaces and hydrogen bonding (Equation (2)).

(MgAl₂O₄)-M-OH + H₊ ⟷ M-OH₂⁺

(2)

where M = Al or Mg. Chemisorption is thought to occur via the formation of M-O-P (M = Al/Mg) bonds associated with H₂O/H₃O⁺ or OH⁻ exchange. Temperature, pH, and initial phosphate concentration all influence the formation of monodentate or bidentate forms (Figure 7A) [12,13,41]. In addition, stoichiometric insoluble phosphate compounds with leached Al³⁺ [Al³⁺(aq) + PO₄³⁻(aq) ⇌ AlPO₄(s) (K_sp = 9.84 × 10⁻²¹ mol²L⁻²)] and Mg²⁺ [3Mg²⁺(aq) + 2PO₄³⁻(aq) ⇌ Mg₃(PO₄)₂(s) K_sp = 1.04 × 10⁻²⁴ mol⁵L⁻⁵] [13] can be formed. They will be precipitated back onto the biochar surface, resulting in a higher uptake. This is more likely to occur above 35 °C with the initial phosphate concentration >100 mg/L.

3.5. Adsorption Kinetics

At pH 11, 85–95% phosphate removal was achieved within 15 min (Figure 8A–C), and for pH 7, 50–55% removal was achieved after 240 min (Figure 8D). Both data sets were fitted well onto a pseudo-2nd-order (PSO) [42] kinetic model with high correlation coefficients (0.96–0.99).

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

(3)

Here, t corresponds to contact time, q_e is the phosphate equilibrium capacity (mg/g), q_t is the phosphate capacity (mg/g) at time t (min), and k₂ (g/mg·min) is second-order rate constant.

The PSO rate constant (k₂) increased from 0.25 to 0.77 g/mg·min when increasing initial phosphate concentrations from 25 to 50 mg/L (

Table 1. Pseudo-2nd-order model fit data for phosphate uptake ^a.

Phosphate Concentration (mg/L)	R2	qcal (mg/g)	qexp (mg/g)	k2 (g/mg·min)
pH 11, 12.5	0.99	5.05	4.94	0.2548
25	0.99	4.66	4.57	0.7797
50	0.96	16.88	16.44	0.0319
pH 7, 100	0.98	18.85	21.91	0.0238

^a Rate constants and regression coefficients significant figures on the table are related to the model fittings and may not reflect the actual uncertainties of experimental data.

). However, a further increase in [P] decreased the rate constant (0.02 g/mg·min). This is perhaps due to switching from a mono dentate to bidentate complex formation or to a stoichiometric compound precipitation. Monodentate and bidentate surface complexes or stoichiometric compounds are produced with distinct kinetics while all adsorption paths combine to produce the observed adsorption rate.

3.6. Adsorption Isotherms and Thermodynamics

The isotherm data were well fitted (R² = 0.99) to the Sips [43] isotherm model, and capacities spanned from 36.9 to 47.2 mg/g (Figure 9) and showed an endothermic nature in its adsorption (Figure 9B and

Table 2. Sips isotherm model constants and van’t Hoff’s data for phosphate uptake.

Temperature (°C)	Temperature (K)	1/T (K−1)	Ks (L/mg)	Ks (unitless)	ln Ks	ΔG (kJ/mol)	ΔH (kJ/mol)	ΔS (kJ/mol·K)
10	283.15	0.003532	0.28	280,000	12.54	−29.52	39.3	0.24
25	298.15	0.003354	0.4	400,000	12.89	−31.97
40	313.15	0.003193	1.41	1,410,000	14.15	−36.86

). Sips isotherm (Equation (4)) is a hybrid of the Langmuir and Freundlich expressions used for heterogeneous adsorption systems—it circumvents the Freundlich isotherm model limitation of the rising adsorbate concentration. It reduces to the Freundlich isotherm at low adsorbate concentrations, while at high concentrations, it provides a prediction of a monolayer adsorption capacity characteristic of the Langmuir isotherm [43].

$${\mathrm{Q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{\mathrm{o}}{({\mathrm{K}}_{\mathrm{S}}{\mathrm{C}}_{\mathrm{e}})}^{\mathrm{n}}}{\left(1+{\left({\mathrm{K}}_{\mathrm{S}}{\mathrm{C}}_{\mathrm{e}}\right)}^{\mathrm{n}}\right)}$$

(4)

Here, q_o is the theoretical isotherm saturation capacity (µg/g), q_e is the quantity of adsorbate in the adsorbent at equilibrium (mg/g), C_e is concentration at equilibrium (mg/g), n is adsorption intensity, and K_s is the Sips isotherm constant (L/mg).

Gibbs free energy (ΔG), entropy (ΔS) and enthalpy (ΔH) were calculated utilizing van’t Hoff’s calculations. The Sips constant (K_s) was converted to a dimensionless constant after multiplying by the density of the liquid phase (~1 × 10⁶ mg/L). The calculated negative ΔG values indicated spontaneous adsorption, and its magnitude increased with temperature (from −29.52 to 36.86 kJ/mol). The positive ΔH (39.3 kJ/mol) confirmed endothermic adsorption. Physisorptions take place when ΔH < 20 kJ/mol, and chemisorptions happen when ΔH > 40 kJ/mol. This ΔH indicates that phosphate uptake is both physisorption and chemisorption. The positive value of ΔS^ο equates to a slightly increased randomness during phosphate uptake onto the Mg/Al oxide/hydroxide phases.

Isotherm fitting curves, isotherm capacities, and constant data were produced after the refinement of empirical isotherm equations using the inbuilt Levenberg–Marquardt nonlinear regression algorithm in Origin2020b. An average of three replicates were used for each data point. Isotherm capacities, significant figures, regression coefficients, and constants are based on the model-reported data and may not reflect the actual experimental uncertainties.

3.7. Comparison with Other Adsorbents

AMBC displayed reasonably fast uptake kinetics (15 min) and comparable capacities (up to ~47 mg/g). Previous results indicate that adsorption at acidic pH is not just the result of adsorption but also stoichiometric precipitation, which is irreversible (

Table 3. Comparison of uptake data with similar adsorbents.

Adsorbent	Temp (°C)	Equilibrium Time	pH	BET Surface Area (m2/g)	Adsorption Capacity (mg/g)	Ref.
Marine macroalgae BC	20	48 h		2.4	3.3	[44]
Waste-derived fungal biomass magnetite BC	25	24 h		53.0	23.9	[45]
2:1 Mg/Al-LDHs sugar cane leaf BC composite	25	1 h	3	10.17	53.4	[18]
3:1 Mg/Al-LDHs sugar cane leaf BC composite				11.41	72.1
4:1 Mg/Al-LDHs sugar cane leaf BC composite				12.25	81.8
Magnetic biochar (MBC)	25	2 min	3	312.6	91.3	[12]
	35				91.0
	45				90.0
Zn-Al LDH	25	72 h			35.9	[46]
	30				58.2
	40				79.1
	50				92.6
Fe3O4 Zn/Al-LDH	25	1 h		133	36.9	[47]
Fe3O4 Mg/Al-LDH				71.9	31.7
Fe3O4 Ni/Al-LDH				50.9	26.5
Mg/Mn Layered double hydroxides	10	3 d			6.2	[48]
	25				7.3
	40				7.5
Magnetite based nanoparticles	24		3	31	5.2	[49]
Al/Mg oxide/hydroxide/sulfate impregnated Douglas fir biochar	10	15 mins	11, 7	61	36.9	This work
	25				42.1 (21.9 @ pH 7)
	40				47.2

). Adsorption also occurs at near neutral pH. In addition, the metal-loading percentages are above 20% in most adsorbents. AMBC only has 5% metal loading, and its preparation is cheaper and more sustainable since the biochar support is a byproduct and is also available on a commercial scale.

4. Conclusions

Douglas fir biochar engineered with Al/Mg (AMBC (Mg/Al 2:1 pH = 13)) demonstrates fast phosphate removal kinetics and the potential ability to remediate phosphate-contaminated water. Optimal sorption pH was 11, requiring 15 min to reach 95% phosphate saturation following pseudo-2nd-order kinetics. AMBC also perform satisfactorily at neutral pH solutions allowing for real-world applications. Sorption data were best fitted into the Sips isotherm model giving a 42.1 mg/g capacity at 25 °C and pH 11. The process was endothermic with increased phosphate adsorption with increasing temperature. Uptake is thought to be governed by a combination of physisorption, chemisorption, and stoichiometric compound formation pathways. Used biochar composites could be used as a soil-enriching phosphate fertilizer, becoming part of an environmentally friendly agricultural management plan.