Ammonium sorption onto polymeric adsorbing material from corn stalks oxidized and loaded with magnesium

Abstract

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Ammonium Sorption onto Polymeric Adsorbing Material from Corn Stalks Oxidized and Loaded with Magnesium

Le Liu, Meiting Ju,* Ruite Lai, Shiqiu Zhang, and Weizun Li

Corn stalks were modified by magnesium loading and an oxidation process and then characterized by a series of methods. The ammonium in wastewater showed high sorption efficiency onto the polymeric adsorbing material during the process, with a biochars dosage of 20 g/L for 8h. Mg²⁺ was the dominant cation exchanger on the modified corn stalk, and it played an important role in ammonium sorption. Large amounts of NH₄⁺ were exchanged onto the corn stalk, and they formed strong complexes with oxygen-containing groups on the corn stalk surface through polar bonds, resulting in the removal of NH₄⁺ from the solution. Na⁺ present in wastewater was the major influence on ammonium sorption onto the corn stalk.

Keywords: Ammonium sorption; Polymeric adsorbing material; Modified corn stalk

Contact information: College of Environmental Science and Engineering, Nankai University, Tianjin 300071, PR China; *Corresponding author: yingying0913@126.com

INTRODUCTION

Ammonium (NH₄⁺) is the inorganic ion form of N contained in domestic and industrial wastewater or decomposed from organic N compounds in those wastewaters. However, high concentrations of N in wastewater increase oxygen demand and eutrophication (Wu et al. 2004). Ammonium may have harmful effects on animal and human health, and it attacks the rubber components of water plumbing systems.

Some methods of NH₄⁺ removal are biological nitrification-denitrification, air-stripping, and ion exchange. The ion exchange method usually utilizes organic resins, but they are very expensive (Li et al. 2010). The use of polymeric adsorbing material as an ion exchanger has become one of the more efficient methods for removing NH₄⁺. NH₄⁺ ions are removed from aqueous solutions by biochar via cation exchange or by adsorption into the pores of the aluminosilicate structures. Biochar is a carbonaceous organic matter generated by heating under oxygen-limited conditions, comparable to charcoal (Sun et al. 2010). Ammonium retention by biochar may be readily explained by electrostatic adsorption onto the negatively charged oxygen-containing surface functional groups (Ebiura et al. 2005). Freshly produced biochars typically have very low ability to adsorb ammonium (Zhang et al. 2011). Over time, the biochar surfaces are oxidized, and cation retention increases to greater levels (Cantrell et al. 2005; Xie et al. 2006). Oxidation may also be achieved intentionally as part of an effort to improve nutrient retention by biochars. Wastewater may contain various cations (calcium, magnesium, sodium, and potassium), anions (chloride, phosphate, and sulfate), and organic acids (oxalic, citric, and malic), which may affect NH₄⁺sorption by the adsorbing material. In the present study, polymeric adsorbing material was produced from corn stalks and characterized by a series of methods to determine NH₄⁺ removal from wastewater. The influence of various parameters in the process on ammonium sorption was also studied. This study provides the necessary theoretical basis for the application of modified corn stalks for ammonium sorption from wastewater.

EXPERIMENTAL

Materials

Samples of corn stalks were collected from agricultural fields in Tianjin in northern China. After drying at 60 C in an oven for 16 h, the samples were ground into particles and passed through a 0.7-mm screen. The powdered biomass was tightly placed in a ceramic pot and then pyrolyzed in a muffle furnace under a N₂ atmosphere. The pyrolysis was programmed to drive the internal biomass chamber temperature to 500 °C at a rate of 5 °C/min and then to hold at the peak temperature for 2 h before cooling to room temperature. This produced the polymeric adsorbing material denoted as B_o. MgO powder and the powdered biomass were mixed in deionized water and wet ball-milled to decrease the particle size. The mass ratio of MgO powder and the powdered biomass was set at 1:10 (Liu et al. 2013). The mixture was heated for 5 h at 105 C to remove water and pyrolyzed in a muffle furnace under the same conditions. This produced the polymeric adsorbing material denoted as B₁. The polymeric adsorbing material B₁ was then oxidized using 30% H₂O₂ at 30 °C for 6 h, using a solid-to-liquid ratio of 1:10 (w/v), and was denoted as B₂.

Characterization

Elemental (C, H, and N) analyses were conducted using a CHN Elemental Analyzer (Vario El cube, Germany). The oxygen content was determined by a mass balance. The O/C atomic ratio was calculated to evaluate the aromaticity and polarity of the polymeric adsorbing materials. Surface element analysis was conducted using energy dispersive X-ray spectroscopy (EDS 1791-N-016-000, HYDAC, Germany). Fourier transform infrared (FTIR) analysis (Tensor 27, Bruker, Germany) was performed in the 400 cm⁻¹ and 4000 cm⁻¹ region with 50 scans being taken at 2 cm⁻¹ resolution to identify the surface functional groups. The specific surface area and porosity properties of the polymeric adsorbing materials were measured by N₂ adsorption isotherms with the Brunauer-Emmett-Teller (BET) method using a surface area analyzer (Tristar 3000, Micromeritics, USA).

Adsorption Experiments

Batch adsorption experiments were conducted in centrifuge tubes at room temperature. All adsorption experiments were performed in triplicate. For these experiments, 0.1 g to 2.0 g of the dried adsorbing material were added into 200-mL centrifuge tubes containing 100 mL of solution with 50 mg of ammonium-nitrogen (NH₄-N) per liter. As a comparison, the dried adsorbing material was added into 200-mL centrifuge tubes containing 100 mL of wastewater with 50 mg of NH₄-N, 9.3 mg of Ca²⁺, 2.3 mg of Mg²⁺, 13.3 mg of Na⁺ , and 1.3 mg of K⁺ per liter. The tubes were shaken at 400 rpm in a mechanical shaker for 1 to 11 h. The supernatant was carefully aspirated using a Pasteur pipette. The initial NH₄-N concentration (C_o) and residual NH₄-N concentration (C_e) in the supernatant were determined using an inductively-coupled plasma spectrometer (ICP) (VISTA-MPX, Varian, USA), which was also used for concentration determination of the other cations. Ammonium sorption efficiency (η) onto the adsorbing material, was obtained according to Eq. 1,

η = (C_o – C_e)/ C_o (1)

where C_o and C_e (mg/L) are the initial and residual aqueous sorbate (NH₄⁺) concentrations, respectively.

RESULTS AND DISCUSSION

Characteristics of the Polymeric Adsorbing Materials

The basic physicochemical properties of the polymeric adsorbing materials derived from corn stalks are presented in Table 1. The C and O contents of the different materials were in the range of 55.32% to 63.21% and 19.15% to 29.35%, respectively. The molar O/C ratio of B₂ was higher than in the other materials, suggesting that more oxygen-containing functional groups were preserved, and that the surface remained hydrophilic. Polarity and hydrophilic properties are beneficial to the sorption of polar water-soluble contaminants, such as NH₄⁺ (Liu et al. 2013). The surface area (SA) of the materials ranged from 10.38 m²/g to 11.22 m²/g, but the pore diameter (PD) of most of the samples was around 2 nm. Total pore volume (TPV) of the materials ranged from 0.036 cm³/g to 0.052 cm³/g. B_o had a larger surface area than the other materials, indicating that abundant MgO may have been located onto the surface of B₁ and B₂.

Table 1. Physicochemical Properties of Polymeric Adsorbing Materials Derived from Corn Stalks

Table 2. Major Functional Groups in Polymeric Adsorbing Materials Detected by FTIR

^a Peak values in the FTIR spectra of B_o were set as the reference value 1.00.

^bBiochars B_o, B₁, and B₂ used after NH₄⁺ sorption in wastewater were denoted as B_o-N, B₁-N, and B₂-N, respectively.

A large number of functional groups were shown on the surfaces of the materials (Table 2), which may be beneficial for sorption. All materials had similar peaks on the FTIR spectra, which may be attributed to the uniform pyrolysis temperature used. The band at 3397 cm⁻¹ represents the stretching vibration of hydroxyl groups. The peaks at 3056 cm⁻¹ can be assigned to the C-H stretching vibrations of olefin. The bands at 2908 cm⁻¹ were assigned to -CH₂ groups. The two bands at 1590 cm⁻¹ and 1387 cm⁻¹ represent the anti-symmetric and symmetric C=O stretching vibrations of carboxyl groups. The band at 1135 cm⁻¹ is assigned to C-O-C stretching vibrations in the aliphatic ethers, which represent the oxygenated functional groups of lignin. The peaks from 900 cm⁻¹ to 750 cm⁻¹ are assigned to the aromatic C-H out-of-plane bending vibrations (Fu and Mazza 2011; Liu et al. 2012).

Effects of Polymeric Adsorbing Material Dosage on Ammonium Sorption

As shown in Fig. 1A, the ammonium in the solutions showed high sorption efficiency onto the polymeric adsorbing materials during the sorption process. An increase in materials dosage (from 2 g/L to 16 g/L) increased the sorption efficiency rapidly. When the dosage reached 20 g/L, the sorption efficiency maintained a steady level. Different materials can also affect the ammonium sorption, which was the main cause of different degrees of ammonium sorption in this process (Zhang et al. 2005). Because Mg²⁺ is the dominant cation exchanger (Wang et al. 2011), material B₂ played a more important role in ammonium sorption than the other materials in this study. This indicated that different types of materials have different characteristics, and thus may exhibit different NH₄⁺cation selectivity (Tang et al. 2005; Pinkert et al. 2009). Because of the complete cation exchange, the sorption efficiency of ammonium on B₂ was high during the process (Wang et al. 2011).

Effects of Sorption Time on Ammonium Sorption

Figure 1B shows the effects of sorption time on ammonium sorption onto the polymeric adsorbing materials. With increased sorption time, the ammonium sorption efficiency onto all 3 kinds of materials increased. When the sorption time reached 8 h, the sorption efficiency maintained a steady level. Comparing the sorption efficiency curves, the ammonium sorption efficiency onto B_o and B₁ was lower than the sorption efficiency onto the oxidized material, B₂. The role of the oxygen-containing functional groups on the materials must also be taken into consideration. The oxidized material, B₂, had larger sorption capacity for NH₄⁺, which corresponded to its high molar O/C ratio. According to the FTIR spectra analysis, B₂ had more oxygen-containing functional groups compared to the other materials. These results indicated that the oxygen-containing functional groups were involved in the sorption of ammonium. Apparently, the polarity and hydrophilic properties of the material surfaces were beneficial to ammonium sorption by forming surface complexes with carboxyl and carbonyl functional groups.

Effects of Adsorption System on Ammonium Sorption

As shown in Fig. 1C, the ammonium sorption efficiency in wastewater was lower than the other systems with an adsorbing materials dosage of 20 g/L for 8 h. This result was due to the impact of various cations in the wastewater, which affected NH₄⁺ removal by the adsorbing materials (Lee et al. 2009; Fu and Mazza 2011). In wastewater, the concentration of major competing cations ranged from 13.18 mg/L to 133.09 mg/L (Table 3). The amounts of Na and Ca were abundant. The presence of Na⁺, K⁺, Ca²⁺, and Mg²⁺ cations in the solution decreased NH₄⁺ adsorption efficiency. Previous studies have reported that NH₄⁺ adsorption was mainly influenced by the presence of competing cations in the concomitant system (Zhang et al. 2005; Li et al. 2010). As shown in Fig. 1C, the ammonium sorption efficiency in the ammonium-Na solution containing an equal amount of Na⁺ was approximately the same as that of the wastewater system. However, the ammonium sorption efficiency in the ammonium-Ca solution containing an equal amount of Ca²⁺was higher than that of the wastewater system. This result indicated that Na⁺ in wastewater might have been the major influence on ammonium sorption onto the adsorbing materials.

Table 3. ICP-Detectable Major Cations in Various Solutions

^a Not detectable

Figure 1C compares the ammonium sorption efficiency onto the adsorbing materials in various adsorption systems. In these systems, the ammonium sorption efficiency onto B_o was the lowest, while the sorption efficiency onto B₂ was highest. Because of the strong adsorbability, material B₂ kept a higher level of ammonium sorption efficiency than the other adsorbing materials in wastewater containing interfering ions. Cation exchange was considered to contribute to ammonium sorption onto the adsorbing materials. The amount of NH₄⁺sorbed was highly correlated with the amount of Mg²⁺ on the biochars that was released into the equilibrium solution. Generally speaking, polymeric adsorbing materials derived from corn stalks are electronegative because they are saturated with cations (Wu et al. 2004; Oh et al. 2005). The exchange ability between cations depends on the ion valence and the radius of the hydration ion. Mg²⁺ was the dominant cation exchanger on the modified corn stalks, and thus played a more important role in ammonium sorption.

Mechanisms of Ammonium Sorption onto Polymeric Adsorbing Materials

Cation exchange is a possible mechanism involved in NH₄⁺sorption onto polymeric adsorbing materials (Zhang et al. 2011). To evaluate the contribution of cation exchange to NH₄⁺ sorption, the amounts of cations released from the adsorbing materials after NH₄⁺ sorption were determined (Table 4). The amount of NH₄⁺sorbed had a correlation with the amount of Mg²⁺released from the adsorbing materials. The B₂ material released more Mg²⁺, approximately10 times that of B_o, and consistently exhibited greater sorption capacity. These results implied that Mg²⁺contributed to NH₄⁺sorption through cation exchange. Furthermore, the content of Na⁺ on the surface of the materials increased. Thus, Na⁺ in the solution was adsorbed onto the adsorbing materials, which showed strong ability for cation exchange. Compared to B₂, B₁ had a different sorption capacity with a similar amount of Mg²⁺ released, indicating that cation exchange was not the sole mechanism responsible for NH₄⁺ sorption onto the biochars.

The role of functional groups was also considerable in NH₄⁺sorption onto the polymeric adsorbing materials. The modified corn stalks were rich in oxygen-containing groups, which could form strong surface complexes with the NH₄⁺ (Xu et al. 2013). The FTIR spectra of B₂before and after NH₄⁺sorption showed an obvious reduction in the peaks of carboxyl C=O and C-O-C stretching vibrations at 1590, 1387, and 1135 cm⁻¹, as shown in Table 2, indicating that the surface complexation of NH₄⁺ with oxygen-containing groups may have contributed to NH₄⁺sorption. A similar change occurred in the other adsorbing materials after the NH₄⁺ sorption process, indicating that the same functional groups were involved in NH₄⁺ sorption onto the adsorbing materials.

Fig. 1. Effects of polymeric adsorbing material dosage (A), sorption time (B), and adsorption system (C) on ammonium sorption efficiency (η)

Table 4. EDS-Detectable Major Cations on the Polymeric Adsorbing Materials

^a B_o, B₁, and B₂ used after NH₄⁺ sorption in wastewater were denoted as B_o-N, B₁-N, and B₂-N, respectively.

One possible mechanism for ammonium sorption onto modified corn stalks in wastewater is proposed in Fig. 2. Large amounts of Mg²⁺ on the polymeric adsorbing materials were released into the wastewater, and they produced more sorption sites for cation exchange onto the materials. The cation exchange ability from wastewater onto the adsorbing materials depended on the ion valence and radius of the hydration ion, and it increased in the order of Ca²⁺< Mg²⁺<NH₄⁺< Na⁺. First, a small amount of the cations Na⁺ and K⁺ in the equilibrium solution could have been exchanged onto the adsorbing materials. Afterwards, as a major cation, large amounts of NH₄⁺were exchanged onto the adsorbing materials in the same manner, forming strong surface complexes with oxygen-containing groups through polar bonds, resulting in the removal of NH₄⁺from the solution.

Fig. 2. Mechanism for ammonium sorption onto polymeric adsorbing material in wastewater

CONCLUSIONS

1. Ammonium in wastewater showed high sorption efficiency onto the modified polymeric adsorbing materials during the process, with a material dosage of 20 g/L for 8 h.

2. Mg²⁺ and Na⁺ was the dominant cations exchanger on the modified corn stalks, and it played an important role in ammonium sorption.

3. Large amounts of NH⁴⁺ were exchanged onto the adsorbing materials and formed strong complexes with oxygen-containing groups on the material surface through polar bonds, resulting in the removal of NH₄⁺ from the solution.

ACKNOWLEDGMENTS

This work was supported by the Key Projects in the Science & Technology Pillar Program of Tianjin, China (12ZCZDSF01700); Projects in Tianjin Agricultural Science and technology achievement transformation and extension Program, China (201302150); and International Cooperation Projects in the Science & Technology Pillar Program of Tianjin, China (13RCGFSF14300).

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Article submitted: June 2, 2016; Peer review completed: July 30, 2016; Revised version received and accepted: October 19, 2016; Published: December 8, 2016.

DOI: 10.15376/biores.12.1.923-931