Selective and Competitive Adsorption of Anions in Solution on Porous Adsorbent from Zea mays Steams: Kinetic and Equilibrium Study

Abstract

Surface modification can improve the adsorption capacity of biochar. Biochar was produced from corn stalks (Zea mays) by pyrolysis at 520 °C, activated with sulfuric acid (H₂SO₄) using impregnation ratios of biomass weight: volume 1:2 (B 1:2) and 1:3 (B 1:3). The kinetic study showed that the equilibrium is reached at 180 min; the maximum adsorption capacity of nitrate and sulphate was obtained with B 1:2 and for phosphate with B 1:3. The adsorption of nitrate and phosphate with the two biochars presented a good fitting to the pseudo-first order and pseudo-second-order model, while that of sulphate for B 1:2 is described by Elovich’s model. Freundlich’s model describes the equilibrium of adsorption of nitrate and phosphate using B 1:2 and B 1:3, while Dubinin–Radushkevich adjusts the removal of sulphate for C 1:2 and C 1:3; therefore it is suggested that adsorption occurs in multilayers. The multicomponent study evidences the preference of biocarbon for phosphate, without indicating competition for the active centers of the material among the anions studied.

1. Introduction

Significant use and excessive discharge of phosphorus, nitrogen, and sulphur compounds increase the concentration of nutrients in surface and groundwater bodies [1]. Common sources that generate contamination from the discharge of nitrate, sulphate, and phosphate into water bodies generally arise from the waste products of human activities, such as discharges from industrialized practices, domestic waters, agricultural uses such as inorganic fertilizers, compost, and wastewater treatment effluents [2]. The excessive concentration of nutrients in the water accelerates the growth of blue-green algae and causes the death of fish due to the reduction in available oxygen [3]. Therefore, the minimum concentration of phosphate, nitrate, and sulphate capable of inducing eutrophication was set at 0.02 mg/L [4,5].

On the other hand, the intake of these nutrients by humans can cause diarrhea, miscarriages, respiratory tract infections, vomiting, hypertension, and methemoglobinemia. Therefore, the US-EPA and the World Health Organization (WHO) establish in drinking water concentrations of 10 µg/L of phosphorus [6], 10 mg/L of nitrate [7], and 250 mg/L of sulphate [5]. For this reason, it is necessary to control the concentrations of nitrate, sulphate, and phosphate in effluents before discharging them into aquatic environments. Thus, several physicochemical methods have been used in the removal of these pollutants in water, including chemical precipitation [8], oxidation [9], artificial wetlands [10], electrodialysis [11], membrane processes [12], and adsorption [13,14]. Adsorption proved to be one of the most popular techniques among those mentioned, due to its low cost, ease of operation, higher disposal efficiency, and excellent reuse performance [15]. The selection of adsorbent material during the adsorption process is a key parameter; therefore, research has been conducted to develop sustainable and economical adsorbents to improve the remediation performance of water stream contaminants [16].

Among the bioadsorbents studied, biochar is one of the most popular due to its good performance; it is a solid carbon-rich by-product of biomass pyrolysis under the complete exclusion of oxygen at temperatures below 700 °C [17], can be derived from a wide range of low-cost biomass sources, including post-harvest agricultural residues, manure, bioenergy crops, residual sludge, and organic waste [1]. It has been reported that biochar has a high density of surface charge to increase ion exchange capacity, with a large specific surface area, internal porosity, and a polar and non-polar functional group at the surface to absorb nutrients [18]. Several studies have used biochar as an adsorbent to remove, individually or simultaneously, nutrients present in water [17,19,20,21], reporting that removal efficiencies vary depending on the types and properties of the adsorbents and the environmental conditions of the water phase.

Thus, this research aims to study the kinetics and equilibrium of adsorption of nitrate (NO₃), sulphate (SO₄), and phosphate (PO₄³) anions on porous carbon-type adsorbent from sulphuric acid-modified Zea mays (H₂SO₄). The effect of the impregnation ratio was investigated and the effect of competition between anions was determined.

2. Materials and Methods

2.1. Materials

To develop the kinetic and equilibrium study, synthetic solutions were prepared at 100 mg/L using distilled water and monopotassium phosphate (KH₂PO₄), sodium nitrate (NaNO₃), and potassium sulphate (K₂SO₄) of analytical grade, Merk Millipore with 95% purity. The biochar was activated with sulfuric acid (H₂SO4). The concentration of the anions was determined using a Biobase UV/VIS Spectrophotometer model BK-UV1900.

2.2. Preparation of Bioadsorbents

To prepare the biochar, post-harvest corn stalks were used as raw material, gathered in a village in Bolivar, Colombia. The raw material was salted with deionized water, dried for 12 h at 60 °C and the size reduction was done in an electric mill. The particle size was selected between 1–2 mm. The biomass was impregnated with a 50% solution of H₂SO₄ with impregnation ratios 1:2 (B 1:2) and 1:3 p/v (B 1:3); then it was carbonized in a muffle at 520 °C for 30 min with a 10 °C/min ramp. The biochar was cooled down to room temperature, washed with distilled water to neutral pH, dried at 100 °C to constant mass, and stored in airtight containers until its use in the adsorption tests [22]. The prepared materials were used previously in the evaluation of the effect of temperature [23].

2.3. Adsorption Kinetics

Adsorption kinetics provides information about the service time of the adsorbent and possible mechanisms of adsorption and to describe adsorption-adsorbent interactions. Kinetic experiments were conducted at 25 °C, at 100 mg/L using 0.2 g in 100 mL solution, 200 rpm sampling at 5, 10, 20, 30, 60, 180, 300, 420, and 1440 min for each experiment. The final concentration of phosphate nitrate, and sulphate in the solution was determined by UV-Vis spectrophotometry at 880 nm [24], 543 nm [25], and 420 nm, respectively [26]. The adsorption capacity was determined according to Equation (1), where q_t is the adsorption capacity at time t, C_i and C_f are the initial and final concentration of pollutant, respectively, in mg L⁻¹, V is the volume of the solution in L and m the mass of the adsorbent in g:

$$q_t\left(\frac{mg}{g}\right)=\frac{\left(C_i-C_f\right)\times V}{m}$$

(1)

The kinetic experiments indicate the duration needed to reach the equilibrium condition, therefore the models of pseudo-first order (Equation (2)), pseudo-second order (Equation (3)), and Elovich (Equation (4)) were adjusted to analyze the kinetic experimental data using non-linear regression due to a decrease in errors in OriginPro^® (Northampton, MA, USA), with the sum of errors (SS) as adjustment criteria.

The pseudo-first-order Lagergren model describes liquid phase adsorption processes, where it is assumed that the reaction rate is driven by only one of the reagents [27]. Where k₁ (min⁻¹) is the Lagergren’s constant, q_e is the amount of contaminant adsorbed per unit mass in the equilibrium, q_t is the amount of contaminant adsorbed per unit of mass at any time t, and t is the time.

$$q_t=q_e\left(1+e^{-kt}\right)$$

(2)

The pseudo-second equation assumes that the adsorption phenomenon occurs due to the ion exchange produced at the surface of the adsorbent and that the order of the reaction is two with respect to the number of adsorption sites available for exchange [28]. Where k₂ (g mmol⁻¹ min⁻¹) is the second-order reaction rate coefficient, t is the time in minutes, q_e the equilibrium adsorption capacity (mmol g⁻¹), and q_t is the adsorption capacity at time t.

$$q_t=\frac{t}{\frac{1}{k_2\times q_e^2}+\frac{t}{q_e}}$$

(3)

The Elovich equation is commonly used to describe the chemisorption of gases on solids; however, it is also used in the kinetics of adsorption of aqueous solutions on solid surfaces [29]. Where q_t is the adsorption capacity at time t, α (mg g⁻¹ min⁻¹) is the initial adsorption rate, and β (g mg⁻¹) is the desorption constant related to surface range and activation energy for chemisorption.

$$q_t=\frac{q}{\beta }\times \ln \left(\alpha \beta \right)+\frac{1}{\beta }\times \ln \left(t\right)$$

(4)

2.4. Adsorption Isotherms

Adsorption isotherms are the graphic representation of the amount of solute adsorbed per unit mass of adsorbent, describing the phenomena responsible for the process. Its models were developed based on the thermodynamic equilibrium of adsorption [30]. The experiments were performed using different concentrations of the anions: 20, 40, 60, 80, and 100 mg/L, for 24 h, with 100 mL of solution and 2 g/L of adsorbent dose at 25 °C. The models of Langmuir (Equation (5)), Freundlich (Equation (6)), and Dubinin–Radushkevich (Equation (7)) were used to adjust to the experimental data obtained at equilibrium time.

Langmuir’s isothermal model is widely used for the description of adsorption, and involves a process where the adsorbate creates a monolayer on the surface of the adsorbent until equilibrium is achieved between the liquid and solid [31]; it also considers that the surface of the adsorbent is homogeneous which has a certain number of available sites which have equal energy capable of adsorbing a single species.

$$q_e=\frac{q_{max}{K}_L{C}_e}{1+K_L{C}_e}$$

(5)

The Freundlich isotherm assumes that the adsorption process takes place in multilayers from the exposed surface into the pores of the adsorbent [32].

$$q_e=K_F{C}_e^{1/2}$$

(6)

Dubinin–Radushkevich’s model assumes heterogeneous surface and constant adsorption potential in the active centers [33].

$$q_e=q_{DR}\times e^{-K_{DR}{\epsilon }^2}$$

(7)

$$\epsilon =RT\times \ln \left(1+\frac{1}{C_e}\right)$$

(8)

$$E=\frac{1}{\sqrt{2K_{DR}}}$$

(9)

2.5. Multicomponent Adsorption

Competitive adsorption tests were performed by placing in contact 2 g/L of adsorbent dose with 50 mL of an equimolar pollutant solution, adding 50 mL of nitrate, sulphate, and phosphate solution at 100 mg/L, for 24 h at the best temperature condition found. The solutions were filtered with 0.45 nm microfilters and syringes and stored in plastic vials and phosphates in amber glass vials, then refrigerated until analysis.

3. Results

3.1. Adsorption Kinetics

To analyze the effect of time on the adsorption process, and to determine the service time of the adsorbent, the adsorption kinetics of the nutrients with the prepared adsorbents were evaluated. Elovich’s pseudo-first and pseudo-second-order models were used to adjust the experimental data and identify possible mechanisms involved in adsorption [34]. The adsorption kinetics are presented in Figure 1 and the model fitting parameters are summarized in

Table 1. Adsorption kinetic parameters of nitrate, sulphate, and phosphate.

Model	Parameter	Nitrate		Sulphate		Phosphate
		B 1:2	B 1:3	B 1:2	B 1:3	B 1:2	B 1:3
Pseudo-first order	qe	10.64	9.09	8.62	11.08	23.59	18.12
	k1	0.03	0.02	0.02	0.00039	0.01	0.008
	R2	0.96	0.96	0.87	0.79	0.99	0.97
Pseudo-second order	k2	11.53	10.05	9.90	10.83	26.83	20.95
	qe	0.004	0.003	0.001	8.78 × 10−4	5.26 × 10−4	4.82 × 10−4
	R2	0.93	0.94	0.94	0.87	0.99	0.97
Elovich	β	0.53	0.55	0.59	0.55	0.21	0.29
	α	1.33	0.64	0.53	0.38	1.08	0.75
	R2	0.81	0.88	0.97	0.94	0.94	0.93

.

From Figure 1, it can be determined that the maximum adsorption capacity of nitrate and sulphate was obtained with B 1:2, and for phosphate with the B 1:3, reaching the adsorption equilibrium close to 180 min in all cases, presenting a fast removal rate during the first minutes. This fast adsorption in the initial stages of the process is due to the influence of the diffusive phenomena from the solution towards the surface of the adsorbent and later to the interior of the pores [35]. Accordingly, with Balarak et al. [36], the high initial concentration of the pollutant at the onset of adsorption helps in creating a better mass transfer driving force, overcoming the resistance to external diffusion; hence, the accelerated adsorption rate.

The adsorption of nitrate and phosphate with the two bio-adsorbents presented a good fitting to the pseudo-first-order model which establishes that the rate of adsorption depends on a mechanism operating at an active site on the surface of the biomass [34]. It was found that the adsorption kinetics of nitrate and phosphate with all biochar follow the pseudo-second-order model, as well as the removal of sulphate with B 1:2, thus establishing that the rate-limiting step is chemisorption due to physicochemical interactions between the two phases [37]. The adjustment of sulphate kinetics for B 1:2 is also described by the Elovich model, thus it is assumed that the limiting step of the process is chemical adsorption [38].

3.2. Adsorption Equilibrium

Adsorption equilibrium was performed by studying the isotherms at 25 °C, initial concentrations of 20, 40, 60, 80, and 100 mg/L.

Table 2. Modelling adjustment parameters of the Cr (VI) adsorption isotherm.

Isotherm Model	Parameters	Nitrate		Sulphate		Phosphate
		B 1:2	B 1:3	B 1:2	B 1:3	B 1:2	B 1:3
Langmuir	qmax (mg/g)	9788.17	487.93	28.68	7625.07	8400.49	73834.8
	KL (L/mg)	3.68 × 10−5	2.73 × 10−4	0.01	2.28 × 10−5	3.77 × 10−6	2.99 × 10−6
	R2	0.99	0.95	0.76	0.89	0.59	0.61
Freundlich	kf (mg/g (L/mg)1/n)	0.13	0.12	0.45	0.11	7.21	1.4 × 10−4
	n	0.98	0.98	1.33	0.90	0.39	0.35
	R2	0.99	0.95	0.71	0.90	0.85	0.89
Dubinin–Radushkevich	qDR (mg/g)	11.93	12.03	12.64	15.03	44.558	47.75
	KDR (mol2/kJ2)	1.46 × 10−4	1.71 × 10−4	9.02 × 10−5	1.65 × 10−4	3.13 × 10−4	5.41
	E (KJ/mol)	58.54	54.01	74.45	55.01	39.99	30.39
	R2	0.82	0.89	0.86	0.99	0.79	0.79

summarizes the parameters of adjustment to Langmuir, Freundlich, and Dubinin–Radushkevich’s models by nonlinear regression of the nitrate (N), sulphate (S), and phosphate (P) adsorption isotherms on the synthesized biochar. The best-fitting was chosen according to the correlation coefficient determined (R²).

From Table 2 it is determined that the Freundlich model best adjusts the adsorption data of nitrate and phosphate using B 1:2 and B 1:3; this suggests that the adsorption of nitrate and phosphate is not limited by the monolayer coverage, meaning that multilayer adsorption occurs due to the heterogeneous surface [34]. Sulphate removal data using both adsorbents were described by Dubinin–Radushkevich’s model. The values recorded for E suggest that the process is mostly controlled by the ion exchange mechanism with strong interactions between the active centers and the anion; similarly, the calculated maximum adsorption capacity (q_DR) values are closest to the experimental ones. The fitting to Dubinin–Radushkevich’s model assumes that B 1:3 has a heterogeneous structure [39]. Halajnia et al. [40] reported that nitrate and sulphate adsorption was adjusted to Freundlich’s model and phosphate adsorption to Dubinin–Radushkevich’s model. On the other hand, Matusik [41] found that Freundlich’s model described the adsorption of phosphate, nitrate, and sulphate, and the calculated energies of adsorption (E) suggested that ion exchange was dominant for all anions, as in the present study, being all above 8 KJ/mol.

Table 3. Modelling adjustment parameters of the Cr (VI) adsorption isotherm.

Pollutant	Adsorbent	qmax (mg/g)	Reference
Nitrate	Zn–Al LDHs/activated carbon composite	73.742	[39]
	Amine crosslinked tea waste	136.43	[42]
	Polyurethane/sepiolite cellular nanocomposites	23.30	[43]
	Corn husk quaternized with N,N-dimethylformamide, ethylenediamine, and triethylamine	79.09	[44]
	Jackfruit peel quaternized with N,N-dimethylformamide, ethylenediamine, and triethylamine	62.91
	Cetylpyridinium bromide modified zeolite	28.06	[16]
	Unmodified activated carbon	3.86	[45]
	25% CTAC-modified activated carbon	7.1
	Activated carbon modified with CTAC at 50%.	10.5
	100% CTAC-modified activated carbon	14.3
Sulphate	Manure biochar	78.4	[46]
	Mangenite	16.4	[47]
	Clinoptilolite	74.63	[48]
	Magnetic nanotubes	94
	Zirconium oxide-modified biochar derived from pomelo peel	35.21	[49]
	Pomelo peel	1.02
	Barium-modified analcime	2.3	[50]
	Barium-modified acid-washed analcime	13.7
	Barium-modified zeolite	3.8
Phosphate	Strontium magnetic graphene oxide nanocomposite	238.09	[51]
	Zirconium hydroxide encapsulated in quaternized cellulose	83.6	[52]
	Lanthanum-coated biochar from urban dewatered sewage sludge	93.91	[1]
	Biochar from wheat straw modified with chitosan	77.61	[53]

summarizes the q_max values obtained in previous studies and the present research for the adsorption of nitrate (NO₃⁻), sulfate (SO₄²⁻), and phosphate (PO₄³⁻) onto adsorbents of different nature. It is observed that the results obtained in the present study for the removal of NO₃⁻ are in the interval reported from 5 mg/g up to 68.96 mg/g; for PO₄³ they are between 19.24 mg/g and 46.67 mg/g, having the adsorbents synthetized in the present study a great performance; finally, the capacities obtained when removing SO₄²⁻ are in the range of 8.38 and 78.10 mg/g. It is evidenced that the most effective modifications involve the quaternization of bioadsorbents with, epichlorohydrin, pyridine, and quaternary salts as N,N-dimethylformamide, trimethylamine, and diethylamine.

3.3. Multicomponent Adsorption Test Parameters

An experimental set-up was performed in a batch system using a ternary equimolar solution of phosphate, nitrate, and sulphate at 25 °C using 2 g/L g of adsorbent dose in 150 mL of solution (Figure 2).

Figure 2 shows a higher removal capacity of phosphate and sulphate by B 1:3, while nitrate was the least removed material from the solution with similar behavior in the two adsorbents. This behavior of nitrate in the presence of the other two anions may be due to its monovalent nature, and it would have a higher selectivity per specific active center of the bioadsorbent, thus decreasing its removal by competing for the adsorption sites with phosphate and sulphate [54]. It have been demonstrated that adsorbents have higher affinity for higher charge density and divalent anions than for monovalent anions [55]. The selectivity towards phosphate and sulphate over nitrate may be due to the fact that phosphate and sulphate adsorption included electrostatic interactions, surface complexation, and/or ligand exchange by direct bond with hydroxyl groups, as well as inner-sphere complexation as the main binding mechanisms (1). In addition, sulphate had higher adsorption capacity than phosphate and nitrate, which indicated that the number of specific active sites for sulphate ion adsorption are higher than nitrate and phosphate; this may be due to the symmetry (tetrahedral) of the sulphate, as the preferential sorption of sulfate can take place due to its higher charge density, which agrees with what was reported by Vinicius et al., (2). Meanwhile, nitrate adsorption could be attributed to electrostatic interactions rather than surface complexation because it is difficult for nitrate to bond to hydroxyl groups. This behavior has been previously informed by Wu et al. (3), for nitrate and phosphate specifically. The understanding of the reaction mechanisms of the co-adsorption of nitrate, sulphate and phosphate is a very important basis for further optimizing adsorbents as well as the scalability of the process to batch to fixed-bed system; thus, more studies and characterization are needed.

4. Conclusions

This study concluded that: (i) the equilibrium is reached at 180 min. (ii) the maximum adsorption capacity of nitrate and sulphate was obtained with B 1:2 and for phosphate with B 1:3. (iii) The adsorption of nitrate and phosphate with two bioadsorbents, showed a good fitting to the pseudo-first-order and pseudo-second-order model, while that of sulphate on B 1:2 is described by Elovich’s model, establishing that the rate-limiting step is chemisorption due to physicochemical interactions between the two phases. (iv) Freundlich’s model describes the equilibrium of adsorption of the three anions on B 1:1, of nitrate and phosphate using C 1:2 and C 1:3, while Dubinin-Radushkevich adjusts the removal of sulphate on C 1:2 and C 1:3; therefore it is suggested that adsorption occurs in multilayers. (v) The multicomponent study evidences the preference of biochar for phosphate, without indicating competition for the active centers of the material among the anions studied.