Adsorption characteristics of ammonium onto biochar from an aqueous solution

Presence of excessive ammonium could adversely affect the natural ecosystems (Tang et al. 2019). Ammonium is released into the environment from municipal and industrial sources (Glavan & Pintar 2010). Ammonium contaminated water is detrimental to the health of infants, leading to serious illness and sometimes death; symptoms include shortness of breath and ‘blue-baby syndrome’ (EPA 2016). One major effect that excess ammonium has on the environment is eutrophication. Eutrophication causes excessive algal growth, known as algal bloom, which has various detrimental effects, such as increase of water treatment cost, decrease of oxygen content and recreational use of water (Dodds et al. 2009). Due to the adverse effects of ammonium on human life and environment, legislation on the standards of wastewater discharge has been strengthened in many countries (Wang et al. 2009). As a result, effective processes for removing ammonium from wastewater are important.

Processes for ammonium removal include adsorption (Saleh et al. 2012; Gai et al. 2014; Wang et al. 2015), ion-exchange (Leakovic et al. 2000), oxidation (Isaka et al. 2007), chemical precipitation (Zhang et al. 2009), and microwave radiation (Lin et al. 2009). Among these, adsorption is considered an effective, inexpensive, low sludge production, and simple technique for removing ammonium from water. Activated carbon is one of the most prevalent adsorbents used in adsorption process for water and wastewater treatment, but it is not the most cost efficient (Worch 2012). Therefore, various low-cost biosorbents have been used for the treatment (Hyder et al. 2015; Begum et al. 2016; Ahsan et al. 2018; Islam et al. 2019). Biochar is a low-cost, easily available and efficient carbonaceous biosorbent, which can be derived from waste materials including woody debris, corn stalks, cottonseed hulls (Uchimiya et al. 2011), rice husks (Zhang et al. 2015), sewage biosolids (Knowles et al. 2011), and animal manure (Song & Guo 2012). Biochar has been used extensively for the removal of heavy metals and organic contaminants from water (Hyder et al. 2014; Inyang & Dickenson 2015). Recent studies have also shown the potential to remove ammonium (NH₄⁺-N) using biochars (Gao et al. 2015; Kizito et al. 2015; Wang et al. 2015; Zhang & Wang 2016; Li et al. 2018). Adsorption capacities of biochars depend on its physical and chemical properties on the basis of type and pyrolysis temperature of feedstock materials (Mukome et al. 2013; Li et al. 2018). Therefore, the overarching objective of this study was to evaluate the effectiveness of a commercial biochar (a bi-product from a power plant) obtained by pyrolysis of various wood chips (spruce, pine, and fir) together at 700 °C to remove ammonium from aqueous solution. The specific objectives of this study were as follows: (1) evaluate the effects of adsorbent dose, initial ammonium concentration, contact time and pH on the adsorption process; (2) determine the adsorption isotherm; and (3) investigate the surface functional groups, which can contribute to the removal of ammonium.

The biochar used in this study was purchased from Phoenix Energy, California, USA. It is a carbonaceous granular material prepared by pyrolysis of wood chips at 700 °C. The analytical specifications of the biochar were provided by the company. The carbon content, ash, bulk density, total surface area and particle size range were 90.8%, 3.9%, 0.11 g/cm³, 186 m²/g and 19–150 mm, respectively. The biochar was crushed and sieved to obtain the particle with a size range 53–250 μm. The surface area and pore volume of the reduced sized biochar were determined using a NOVA 1200 surface area analyzer (Quantachrome Instruments, Boynton Beach, FL) with the Bruanuer–Emmett–Teller nitrogen adsorption method at 77 °K.

In preparation of ammonium solution, 1,000 mg/L ammonium-nitrogen (NH₄⁺-N) stock solution was prepared by dissolving approximately 381.9 mg of ammonium chloride (NH₄Cl) salt in deionized water. The NH₄⁺-N stock solution was diluted to working concentration of 10 mg/L for batch tests. The effect of biochar dosage on ammonium adsorption was studied in batch test using 0.10 L solution with varying biochar dosages ranging from 10 to 100 g/L. The pH of the mixture was not adjusted during batch tests. The mixture was shaken at a speed of 150 rpm using a platform shaker (New Brunswick Scientific, model: innova 2300) at room temperature for 5 days. Samples were collected at the end of test and filtered through 0.45 μm syringe filter (Sterlitech, WA, USA). Residual NH₄⁺-N concentration of the filtered solution was determined using a spectrophotometer (HACH DR2800).

In order to evaluate the effect of initial ammonium concentration and contact time, batch adsorption experiments were conducted separately with varying NH₄⁺-N concentrations from 10 to 80 mg/L at 10 g/L of biochar dosage. The pH of the mixture was not adjusted during the batch test and pH value was recorded at 5. The mixture was then stirred at 150 rpm at room temperature for 5 days. Residual NH₄⁺-N concentration was determined similarly as described in the previous section.

The pH of the solution is an important factor for the adsorption because pH affects the surface charge of adsorbent, chemical species and also the degree of ionization of surface functional groups of adsorbent during reaction (Li et al. 2012). Therefore, the effect of pH on the ammonium adsorption onto biochar was investigated by varying initial pH values from 2 to 10 using initial NH₄⁺-N concentration of 10 mg/L. The pH values of the solution were adjusted by using 0.1 M HCl and 0.1 M NaOH. Then, 40 g/L of biochar was added to the solution. Batch tests and analysis of residual NH₄⁺-N concentration were conducted according to the similar procedure described in the above section. At the end of tests using pH 4, 6 and 8, biochars were collected and air-dried overnight to analyze surface functional groups using FTIR spectroscopy at a wavenumber varying from 500 to 4,000 cm⁻¹.

Batch tests were performed in duplicate, and average values were used to represent the results. All working solutions were prepared freshly prior to batch tests. The glasswares were presoaked in 5% HNO₃ solution overnight, and then washed with tap water, rinsed three times with deionized water and dried in air before use.

The Langmuir isotherm indicates that the free energy of adsorption is independent on the surface coverage of adsorption sites, and the surface saturation of the adsorbent occurs by a monolayer coverage of adsorbate at high C_e values and a linear relationship at low C_e values (Khezami & Capart 2005).

Independent two-sample t-test was performed between the replicate values, which were obtained during the batch tests at a 95% confidence interval to determine whether the replicate values were significantly different. To verify the optimum pH value for maximum percentage of ammonium removal, a one-way ANOVA test was conducted at a 95% confidence interval. A Tukey Honestly Significant Difference (HSD) test was performed for multiple comparisons when the ANOVA test showed significant differences (p value < 0.05).

After reducing the particle size between 53 and 250 μm, the surface area (BET), pore volume, and average pore diameter of the biochar were found as 285 m²/g, 0.1507 cm³/g and 2.117 nm, respectively. The surface area of the biochar increased from a value of 186 m²/g to 285 m²/g due to the size reduction.

The biochar dosage has a significant influence on the adsorption of ammonium. The maximum percentage of ammonium removal was 80% using 100 g/L of biochar at an initial ammonium concentration of 10 mg NH₄⁺-N/L (Figure 1(a)). The percentage of ammonium removal increased with the increase of biochar dosage, which could be due to the increased number of available adsorption sites as the biochar amount increased in the solution. Other researchers also observed the increased ammonium adsorption with the increase of biochar amount up to a certain value. However, further increase of biochar dosage decreased the ammonium adsorption onto biochars (Kizito et al. 2015; Xue et al. 2019).

To observe the effects of initial ammonium concentration on the adsorption of ammonium onto biochar, initial NH₄⁺-N concentrations were varied from 10 to 80 mg/L (Figure 1(b)). The percentage of ammonium removal decreased steadily with the increase of initial NH₄⁺-N concentration. About 31% ammonium was removed with an initial NH₄⁺-N concentration of 10 mg/L with a biochar dosage of 10 g/L at a pH of 5. The ammonium removal was 12% at an initial NH₄⁺-N concentration of 80 mg/L by keeping other parameters constant. It may be due to the presence of lower amount of ammonium ions at low initial concentration compared to the availability of excess unoccupied active sites on the biochar (Qu et al. 2008; Nidheesh et al. 2012). Thus, the percentage of ammonium removal was higher at low initial NH₄⁺-N concentration. Similar results were also observed by other researchers (Vassileva et al. 2008; Kizito et al. 2015).

To identify the effect of pH, further batch tests were conducted using biochar dosage of 40 g/L with the variation of pH from 2 to 10 at an initial NH₄⁺-N concentration of 10 mg/L. The percentage of ammonium removal increased from 38% to 51% with the increase of pH value from 2 to 4 as shown in Figure 1(c) and remained stable up to pH value of 6. Subsequently, the percentage of ammonium removal decreased slowly with the increase of pH, and finally reached a value of 43% at pH 10. The less removal of ammonium at pH lower than 4 can be attributed to the high protonation of functional groups on the biochar surfaces resulting in a partial positive charge. It can compete with NH₄⁺ ions in aqueous solution for adsorption onto biochar surfaces as ammonium exists mainly in the form of NH₄⁺ in acidic environment (Maranon et al. 2006). At high pH, the decreasing trend of ammonium removal was observed which may be due to the conversion of NH₄⁺ to NH_3(aq) form. The electrostatic attraction mechanism was not effective at this pH resulting in the reduction of ammonium removal (Thurston et al. 1979; Vu et al. 2017).

The one-way ANOVA test results show that the percentage of ammonium removal was significantly different for various pH values. Therefore, Tukey HSD test was performed for multiple comparisons among pH values to identify the optimum pH value (Table 1). From Table 1, the percentage ammonium removal was not significantly different between pH values of 4 and 6, 4 and 8, and 6 and 8. Even though the percentage removal of ammonium is maximum at pH 4 (Figure 1(c)), Tukey HSD test indicates that the percentages of ammonium removal at pH 4, 6 and 8 were not significantly different. Thus, the pH value between 6 and 8 can be considered as the optimum pH as they are close to the pH value of natural waters.

Ammonium concentration in the solution decreased with time until 3 days. Then, ammonium concentration reached a stable value for all initial ammonium concentration when contact time was further increased as shown in Figure 2. This indicates that the adsorption of ammonium onto biochar reached the point of saturation after 3 days (Vu et al. 2017). The ammonium removal rate was faster until 2 days. The slight uptake of ammonium was observed up to an equilibrium level in day 3 for all initial ammonium concentrations. The ammonium concentrations decreased to 7 mg/L, 14.5 mg/L, 32.6 mg/L, 51.8 mg/L and 70.4 mg/L for an initial concentrations of 10 mg/L, 20 mg/L, 40 mg/L, 60 mg/L and 80 mg/L respectively in 3 days. The maximum ammonium adsorption capacities were 0.3, 0.55, 0.74, 0.82, and 0.96 mg/g for initial ammonium concentrations of 10 mg/L, 20 mg/L, 40 mg/L, 60 mg/L and 80 mg/L, respectively. The ammonium adsorption capacities of various biochars are presented in Table 2.

Generally, the adsorption capacity of the adsorbent is expected to improve at a higher initial ammonium concentration due to the greater driving force from concentration gradient (Ho 2004). A higher initial concentration leads to higher collisions between the biochar and ammonium species, resulting in a driving force that is able to overcome all mass transfer resistances between the solid and aqueous phases (Anupam et al. 2011).

Langmuir and Freundlich isotherm models were used to investigate the relationship between adsorbate in liquid and solid (Hameed et al. 2008). The adsorption isotherms of ammonium onto biochar were tested by varying initial NH₄⁺-N concentrations from 10 to 80 mg/L at 10 g/L of biochar dosage with pH of 5 for 5 days. These two models are the most common isotherms used to describe solid-liquid adsorption systems, and presented in Figures 3(a) and 3(b) for ammonium removal. The isotherm coefficients for these two models are presented in Table 3 for ammonium removal. Since, the maximum adsorption capacity (q_m) obtained from the experimental result and Langmuir model was not similar, the deviations of q_m value obtained from the experimental result to Langmuir model were 13% for ammonium adsorption. The value of R_L, the essential characteristics of Langmuir dimensionless separation factor or equilibrium parameter, was obtained between 0 and 1 which indicates the favorable adsorption process of ammonium onto the biochar (Hameed et al. 2009).

It is clear from Figures 3(a) and 3(b) that the adsorption data of ammonium onto biochar fit the Langmuir model better with R² values of 0.9796 as compared to the Freundlich model R² values of 0.9657. The suitability of Langmuir model can be interpreted to mean that the adsorption of ammonium took place largely by chemisorption within the monolayer on the biochar. On the contrary, R² value from Freundlich model is higher than 0.95 which suggests some level of physical adsorption (Kizito et al. 2015). Other studies also reported that the ammonium adsorption onto biochar followed both the Langmuir and Freundlich models (Liu et al. 2010, 2013; Zhu et al. 2012; Kizito et al. 2015). The results of these studies indicate the heterogenous nature of the biochar surfaces (Kizito et al. 2015).

The FTIR spectra of raw biochar and biochar tested at pH 4, 6 and 8 are presented in Figure 4. It shows that the IR spectrum of biochar tested at pH 4, 6 and 8 changed after being used in the tests. The FTIR spectroscopy confirmed the presence of different functional groups in the raw biochar and biochar used in the batch tests at pH 4, 6 and 8 to remove ammonium from the water. The FTIR spectra of raw biochar and biochar tested at pH 4, 6 and 8 showed the C–C triple bond stretching at wavenumbers of 2,106.34 cm⁻¹. At pH 4, 6 and 8, the FTIR spectra of biochar showed almost similar pattern. The symmetric and asymmetric stretching vibrations of N–H bond were observed at 3,859.69 and 3,603.15 cm⁻¹, respectively, which might correspond to the amine and amide functional groups. The peak at 2,270.29 cm⁻¹ could be due to the C–N triple bond stretching, which indicates the alkyl nitrile formation. In addition, the N–H bending vibration at 1,535.39 cm⁻¹ could also be an indication of the amine and amide functional groups, and C = O stretching vibration at 1,687.77 cm⁻¹ could be attributed to the amide functional group (Silverstein et al. 2005). Therefore, the higher ammonium removal at pHs of 4, 6 and 8 could be influenced by the formation of amine, amide and nitrile functional groups on the surface of biochar.

In this study, biochar derived from woodchips was used to adsorb ammonium from the aqueous solution. About 80% ammonium removal was achieved using 100 g/L of biochar at an initial NH₄⁺-N concentration of 10 mg/L at pH 6.6. Ammonium removal decreased by 15% with the increase of initial ammonium concentration by 50 mg/L. Even though the highest ammonium removal was obtained at pH 4, there was not significant difference in ammonium removal at pH 4, 6 and 8. Therefore, the optimum pH value was considered between 6 and 8 for ammonium removal as it is close to the pH value of natural waters. The maximum adsorption capacity of ammonium was 0.96 mg/g for 80 mg/L of initial ammonium concentration after 3 days. Both Langmuir and Freundlich models showed a good fit for ammonium adsorption onto the biochar. The ammonium removal could occur due to the formation of amine, amide and nitrile functional groups on the biochar surface. The results obtained in this study suggest that biochar derived from the combination of various wood chips at a pyrolysis temperature of 700 °C is a promising adsorbent to remove ammonium from water and wastewater resources. Therefore, a filtration unit incorporating the biochar as an adsorbent can be operated based on the effects of operating conditions, and adsorption isotherm obtained in this study in a full-scale water and wastewater treatment plant for the removal of ammonium from the contaminated waters.

This study was made possible by the support of the United States Department of Energy – National Nuclear Security Administration (DOE-NNSA). The contents of this paper are the sole responsibility of the authors and do not reflect the views of the US Department of Energy or the United States Government.

The authors declare that there are no conflicts of interest regarding the publication of this article.

All relevant data are included in the paper or its Supplementary Information.