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Article

Phosphorus and Nitrogen Adsorption Capacities of Biochars Derived from Feedstocks at Different Pyrolysis Temperatures

1
Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing 210044, China
2
School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
3
Key Laboratory of Agro-Environment in Downstream of Yangze Plain, Ministry of Agriculture, Nanjing 210014, China
4
Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, Nanjing 210044, China
5
Department of Geography and Environmental Science, University of Reading, Reading RG6 6AB, UK
*
Authors to whom correspondence should be addressed.
Water 2019, 11(8), 1559; https://doi.org/10.3390/w11081559
Submission received: 14 June 2019 / Revised: 10 July 2019 / Accepted: 20 July 2019 / Published: 28 July 2019
(This article belongs to the Special Issue Advances in Constructed Wetland)

Abstract

:
This study investigates the P and NO3 adsorption capacities of different biochars made from plant waste including rice straw (RSB), Phragmites communis (PCB), sawdust (SDB), and egg shell (ESB) exposed to a range of pyrolysis temperatures (300, 500 and 700 °C). Results indicate that the effect of pyrolysis temperature on the physiochemical properties of biochar varied with feedstock material. Biochars derived from plant waste had limited adsorption or even released P and NO3, but adsorption of P capacity could be improved by adjusting pyrolysis temperature. The maximum adsorption of P on RSB700, PCB300, and SDB300, produced at pyrolysis temperature of 700, 300 and 300 °C, was 5.41, 7.75 and 3.86 mg g−1, respectively. ESB can absorb both P and NO3, and its adsorption capacity increased with an increase in pyrolysis temperature. The maximum NO3 and P adsorption for ESB700 was 1.43 and 6.08 mg g−1, respectively. The less negative charge and higher surface area of ESB enabled higher NO3 and P adsorption capacity. The P adsorption process on RSB, PCB, SDB and ESB, and the NO3 adsorption process on ESB were endothermic reactions. However, the NO3 adsorption process on RSB, PCB and SDB was exothermic. The study demonstrates that the use of egg shell biochar may be an effective way to remove, through adsorption, P and NO3 from wastewater.

1. Introduction

Discharges of phosphorus (P) and nitrate (NO3) into the natural environment from agricultural, industrial, and domestic wastewater have increased in many countries. The resulting nutrient enrichment and eutrophication of water-bodies has become a serious environmental concern around the world [1,2]. Under eutrophic conditions, rapid growth of organisms, especially algae, may be stimulated resulting in depletion of dissolved oxygen and deterioration of the aquatic environment [3]. In addition, human health could be affected by high levels of NO3 in water. For example, infant methemoglobinemia and several types of cancers could occur due to human uptake of excess NO3 [4]. Therefore, removing excess P and NO3 from wastewaters has important ecological and social implications.
Technologies used to remove P and NO3 from wastewaters are mainly divided into three categories: chemical, biological and physical methods [5]. Chemical treatments may produce additional pollutants, such as sludge, due to the precipitation of P [6]. Biological treatments may increase the cost of wastewater treatment because of the requirement for aeration or other pre-treatments [7]. Physical treatments include many methods, such as electrodialysis, reverse osmosis and adsorption. Compared to electrodialysis and reverse osmosis, adsorption is a widely used and promising method of purifying wastewater in situ because it is relatively low-cost and efficient and is less likely to generate secondary pollution [8,9]. Therefore, investigation of economical and effective adsorbents to remove P and NO3 from wastewaters has much potential value.
Large amounts of bio-wastes, such as straw and manure, are produced from agriculture and industrial activities [10]. For example, the annual production of agricultural waste is approximately 0.7 billion tonnes in China, of which 70% is rice straw, wheat straw, and corn stalk [11]. Egg shell is a waste from daily life, and its annual production is about 3.7 million tonnes in China [12]. Large quantities of bio-waste have resulted in many environmental problems, such as water pollution due to its leachate, and air pollution due to its burning. Therefore, it is very important to find sustainable solutions to solve bio-waste issues, and biochar production offers a viable approach. As well as possibly aiding the removal of P and NO3 from wastewaters, biochars may be applied to soil to serve as an amendment to increase plant growth, and reduce bioavailability, phytotoxicity, and uptake of heavy metals in contaminated soil [13]. Biochars that are used as adsorption agents in wastewater, could, therefore, be subsequently re-used as a soil amendment.
There have, however, been inconsistent results on NO3 and P adsorption by biochars. Yao et al., for example, found that NO3 leaching was reduced by 14.0–34.0% when a biochar produced from Brazilian pepper wood and peanut hull at 600 °C was added to soil [14]. However, Hale et al. [15] found that cacao shell and corn cob biochar did not adsorb NO3 and, in fact, sometimes increased the release of NO3 into the environment. Jung and Hong [16], for example, found an adsorption of 8.3 mg g−1 of phosphate in a biochar from seaweed pyrolysis at 600 °C. Michalekova-Richveisova et al. [17] demonstrated that the maximum phosphate adsorption capacity was 0.036 mg g−1, 0.132 mg g−1 and 0.296 mg g−1, respectively for a corn cobs biochar, garden wood waste biochar and wood chips biochar. However, Novaisa et al. [18] demonstrated that biochars would not be able to adsorb significant amounts of phosphate due to the large amount of phenolic and carboxylic groups and a high proportion of fulvic and humic acids. Similarly, some biochars, such as wild fire biochar, Jarrah biochar, greenwaste biochar, sugarcane bagasse biochar and blady grass biochar, do not have the capacity to adsorb phosphate and can actually release P into solution [19]. Therefore, research to identify the physiochemical properties of biochars most suitable for removal of P and N from wastewater remains very important.
Adsorption of nitrogen (N) and P may be influenced by many factors, including its CEC, acidic functional groups, surface oxygen-containing functional groups [20], surface charges and available anion exchange sites [21]. In addition, Chintala et al. [22] found that P adsorption by biochars was significantly affected by initial P concentration in the solution. Therefore, P and NO3 adsorption is related to the biochar’s physiochemical properties, and the initial P and NO3 concentration. However, there is little information on how biochars derived from plant waste and egg shell affect P and NO3 adsorption at varying pyrolysis temperatures. The objectives of this study therefore include: (1) to examine capacity of P and NO3 adsorption of different biochars produced under different pyrolysis temperatures; (2) to investigate the adsorption process of P and NO3 by the biochars. The results will offer a theoretical and practical foundation for removal of P and NO3 from eutrophic or wastewaters using biochars.

2. Materials and Methods

2.1. Materials

Four types of materials, i.e., rice straw, Phragmites communis, sawdust and egg shell were obtained from Nanjing University of Information Science and Technology, Nanjing, China. The materials were rinsed with water and air-dried, then ground and sieved to < 2.0 mm particles using a stainless grinding machine. The powdered biomass was tightly placed in a ceramic pot, and then pyrolyzed in a muffle furnace. The pyrolysis was programmed to drive temperature to 300 °C, 500 °C and 700 °C at a rate of 5 °C min−1, respectively, and held at the peak temperature for 2 h before being cooled to room temperature. The biochars produced from rice straw, Phragmites communis, sawdust and egg shell under 300 °C, 500 °C and 700 °C were referred to as RSB300, RSB500, RSB700, PCB300, PCB500, PCB700, SDB300, SDB500, SDB700, ESB300, ESB500 and ESB700, respectively. All biochar samples were ground to pass through a 0.5 mm sieve prior to use.

2.2. Adsorption Kinetics Experiments

The stock solutions of P and NO3-N were prepared using KH2PO4 and KNO3, respectively. Sorption kinetics were evaluated at room temperature (30 °C) and the initial pH for each sorption solution was adjusted to 7 prior to the experiments. The biochar (0.2 g) was added into 20 mL solutions containing 20 mg P or NO3-N L−1. Subsamples were taken after 30 min, 60 min, 180 min, 300 min, 420 min, 780 min, 1080 min and 1440 min, and shaken at 180 rpm in a mechanical shaker. The subsamples were then filtered with a syringe filter. The concentrations of NO3-N and P in the filtrate were determined by ultraviolet spectrophotometry and ammonium molybdate spectrophotometry, respectively.
The amount of P and NO3 adsorbed by the biochars was calculated by the following equation (Equation (1)).
q t = ( C 0 C t ) × V W
where qt (mg g−1) is the amount of P and NO3 adsorbed by the biochars; C0 and Ct (mg L−1) are the initial and t time concentrations of the pollutants, respectively; V (L) is the volume of adsorption solution; and W (g) is the mass of biochar.
The experimental results were fitted using three typical kinetic models (Pseduo-first-order Equation (2), Pseduo-second-order Equation (3) and Intraparticle diffusion Equation (4).
ln ( q e q t ) = ln q e k 1 t
t q t = 1 k 2 q e 2 + t q e = 1 h + t q e
q t = k 3 t 0.5 + C
where qe and qt (mg g−1) are the amounts of P and NO3 adsorbed by the biochars at the equilibrium time and at time t, respectively; k1 (h−1), k2 (g mg−1 h−1), and k3 (mg g−1 h−0.5) are the rate constants of the corresponding model; and C (mg g−1) is a constant.

2.3. Adsorption Isotherm Experiment

Sorption isotherms of P or NO3-N were determined using batch experiments in the centrifugal tube under the same conditions as above, and the concentration of P varied from 0 to 320 mg L−1 (0, 5, 10, 20, 40, 80, 160 and 320 mg L−1) or concentration of NO3-N varied from 0 to 320 mg L−1 (0, 5, 10, 20, 40, 80, 160 and 320 mg L−1). After being shaken for 24 h, the final suspensions were centrifuged, filtered, and the supernatant solution was separated for analysis of P or NO3. The concentration of P or NO3 was calculated Equation (5).
q e = ( C 0 C e ) × V W
where qe is the equilibrium P or NO3 concentration in mg g−1; V is the volume of P or NO3 aqueous solution in L; W is the adsorbent mass in g; C0 is the initial P or NO3 concentration in mg L−1; and Ce is the aqueous P or NO3 concentration at equilibrium in mg L−1.
Sorption isotherms were fitted to the Langmuir (Equation (6) and Freundlich (Equation (7)) equations to quantify the adsorption capacity of different biochars.
q e = q max K L C e 1 + K L C e
q e = K F C e 1 n
where qe (mg g−1) is the adsorption capacity; Ce (mg L−1) is the equilibrium concentration after the adsorption or desorption; 1/n is the intensity of adsorption or affinity; KF (mg g−1) is the Freundlich adsorption constant; qmax (mg g−1) is the maximum sorption capacity; KL (L mg−1) is a Langmuir constant.

2.4. Adsorption Thermodynamics

Sorption data of RSB700, PCB300, SDB300, and ESB700 using initial P or NO3 concentration (20, 40, 80, 160 and 320 mg L−1) at a temperature range of 20, 30 and 40 °C were collected after 24 h equilibration time. Three parameters (Gibb’s free energy change (ΔG0), enthalpy change (ΔH0) and entropy change (ΔS0)) were calculated using the following equations.
The thermodynamic equilibrium constant Kc was defined as (Equation (8)):
K c = C 0 C e C e
where C0 and Ce (mg L−1) are the initial and equilibrium concentration of P or NO3 solution.
ΔG0 was calculated by the following equation (Equation (9)):
Δ G 0 = R T L n K c
where, T is temperature in K, R the ideal gas constant = 8.314 J mol−1 K−1.
ΔH0 and ΔS0 was calculated by the following equation (Equation (10)).
Ln   Kc = Δ G 0 R T = Δ H 0 R T + Δ S 0 R
Thus, from the linear plot of ln Kc versus 1/T, the enthalpy (ΔH0) and entropy (ΔS0) values were calculated from the slope (ΔH0/RT) and intercept (ΔS0/R), respectively.

2.5. Analysis Method

The pH of biochars was measured by adding the biochars to deionized water at a mass/water ratio of 1:20 (PHS-3C). Each sample was analyzed in duplicate. The specific surface area and porosity properties of the biochars were measured by N2 adsorption isotherms at 77 K with the Brunauer–Emmett–Teller (BET) method and by CO2 isotherms at 273 K using a Quadrasorb Si-MP surface area analyzer. Zeta-potential measurements were performed at pH 7 with a potential analyzer (Zetasizer Nano ZS90, Malvern, UK).

2.6. Statistical Analysis

The average was calculated from three replicates of each experimental treatment using Origin Pro 8.0, and the results were indicated as mean ± standard deviation. The kinetics and sorption isotherms were fitted using Origin Pro 8.0, and R2 values were used to compare the performance of different models. Statistical analysis was performed using SPSS 12.0. A one-way analysis of variance (ANOVA) was conducted for biochar characteristics. The tukey test was performed to detect the statistical significance of differences (p < 0.05) among means of treatments.

3. Results and Discussion

3.1. Characteristics of Biochar

According to Table 1, total volume, specific surface area and pH of RSB, PCB, SDB and ESB increased with pyrolysis temperatures, and the parameters were significantly higher at 700 °C than at 300 °C (p < 0.05).
Zeta potential of PCB and ESB increased with pyrolysis temperatures (Table 1). The zeta potential significantly decreased for RSB and SDB produced from pyrolysis at 500 °C than at 300 °C (p < 0.05). However, zeta potential significantly increased for RSB and SDB produced from pyrolysis at 700 °C than at 300 °C or 500 °C (p < 0.05). Results showed that the response of zeta potential of biochars on pyrolysis temperature varied with biochar type.
The SEM images of the studied biochars are shown in Figure 1, the more and uniform hollow channels for each biochar occurred with increasing pyrolysis temperature. Compared to RSB, PCB and SDB biochar, ESB biochar had less small pore size with relatively lower porosity (Figure 1).

3.2. Sorption Kinetics of Biochar

Adsorption or desorption of P on the biochars is shown in Figure 2. Release of P occurred in RSB300 and RSB500, PCB500 and PCB700, and SDB500 and SDB700 (Figure 2a–c), respectively. Similarly, PCB300 and SDB300 presented lower P adsorption capacity (Figure 2b,c): Novais et al. [23] showed that some biochars have very low or zero P adsorption, due to its behavior as “great anion”, which prevents the adsorption of anions such as phosphates. Schneider and Haderlein [24] demonstrated that the dissolved organic matter released from the biochar pyrolysis at 200 °C when placed into the P solution competed for sorption sites and inhibited P sorption. However, our results show that P was adsorbed by ESB300, ESB500 and ESB700 (Figure 2d), P adsorption capacity is therefore dependent on biochar type and the rate of adsorption is affected by pyrolysis temperature.
Release of NO3 occurred in RSB, PCB500, PCB700 and SDB700 (Figure 2e–g), respectively. However, adsorption of NO3 by ESB increased with time, and increased with pyrolysis temperature (Figure 2h). Therefore, ESB could adsorb both P and NO3. As shown in Figure 2d,h, when the initial P and NO3 concentration was 20 mg/L, rapid adsorption on ESB was observed in the first 8h, which suggested that P and NO3 in solution was impelled to adhere to the surface of biochar.
To investigate adsorption mechanisms of P and NO3, the sorption data were fitted with kinetic models, including the pseudo-first-order, pseudo-second-order and intra-particle diffusion model. According to Table 2, the pseudo-second-order kinetic model for adsorption P on RSB700, PCB300 and SDB300 showed the best fit to the experimental data with the highest R2 in a range of 0.994–0.999. These results are similar to those of Elsa et al. [25], who demonstrated that the pseudo-second-order kinetic model fits the experimental data better than the pseudo-first-order kinetic model for P adsorption. However, the pseudo-first-order kinetic model for ESB showed the best fit to the experimental data with the highest R2 in a range of 0.984–0.995 (Table 2). In this study, the intra-particle diffusion model didn’t fit the data well with low R2 in a range of 0.601–0.893 (Table 2), indicating that intrapore diffusion does not dominate the adsorption process of P on the biochars.
According to Table 3, the pseudo-second-order kinetic model for NO3 adsorption on PCB300, SDB300 and SDB500 showed the best fit to the experimental data with the highest R2 in a range of 0.974–0.992. This suggests that NO3 adsorption process is mainly chemical adsorption. However, the pseudo-first-order kinetic model for NO3 adsorption on ESB showed the best fit to the experimental data with the highest R2 in a range of 0.906–0.989. The results suggest that the intra-particle diffusion model didn’t fit the data well with low R2 in a range of 0.784–0.914 (Table 3). The pseudo-first-order model is widely used to describe reversible physical adsorption between the adsorbent and the adsorbate [26]. The pseudo-second-order model indicated that the adsorption of P onto calcium-flour biochar was a chemisorptions-dominated process [27]. Therefore, in this study, the pseudo-second-order model could be used to predict the kinetic process for P and NO3 sorption on PCB and SDB, which was a chemisorptions-dominated process. However, the pseudo-first-order model could be suitable to predict the kinetic process for P and NO3 sorption on EBS, which is a physisorption-dominated process.

3.3. Sorption Isotherms of Biochar

The amount of P adsorption by RSB700 increased with P concentration in the initial solution. Release of P from RSB300 and RSB500 occurred when P concentration was < 80 mg L−1 in the initial solution (Figure 3a). The amount of P adsorption by PCB300 and PCB500 increased with P concentration in the initial solution (Figure 3b). However, release of P from PCB700 occurred for different P concentrations in the initial solution (Figure 3b). Zhang et al. [19] showed that timber biochar and peanut shell biochar released P when the P concentration was < 100 mg P L−1, but they retained 1–2% P when the P concentration in the solution was 200 mg P L−1. It can be seen from Figure 3c that SDB300, SDB500 and SDB700 released P when the P concentration was < 5 mg L−1, but they adsorbed P when P concentration was >80 mg L−1. Therefore, the P adsorption capacity of the biochars was influenced by the P concentration in the initial solution. These results are similar to those of Chintala et al. [22], who demonstrated that P adsorption on biochars was significantly affected by initial P concentration and biochar types. According to Figure 3d, ESB300, ESB500 and ESB700 had a capacity to adsorb P, and ESB700 exhibited the highest P sorption capacity.
It can be seen from Figure 3e,f, RSB and PCB cannot adsorb NO3 in the lower initial NO3 concentration. The results are similar to those of Hale et al. [15], who found that cacao-shell derived biochar could not adsorb NO3. The amount of adsorption NO3 on SDB and ESB increased with initial NO3 concentration (Figure 3g,h). The order of NO3 adsorption capacity on ESB was that of ESB700 > ESB500 > ESB300 (Figure 3h). However, the order of NO3 adsorption on SDB was shown as following: SDB500 > SDB300 > SDB700 (Figure 3g). These results showed that the NO3 adsorption capacity on biochars was also influenced by types of biochar and pyrolysis temperatures.
RSB700, PCB300, PCB500, SDB300, SDB500, and ESB have the ability to adsorb P from P solutions, so their P adsorption isotherms were investigated to elucidate the adsorption mechanisms. The model parameters for P on biochars for both Langmuir and Freundlich models are presented in Table 4, which show that the Langmuir model fits the experimental data better (0.981 > R2 > 0.852) than the Freundlich model (0.897 > R2 > 0.728). Similarly, PCB300, SDB300, SDB500, ESB300, ESB500 and ESB700 have the ability to adsorb NO3. Therefore, their model parameters for NO3 on biochars for both Langmuir and Freundlich models are presented in Table 5, which show that the Langmuir model fits the experimental data better (0.996 > R2 > 0.919) than the Freundlich model (0.949 > R2 > 0.850). Therefore, the Langmuir equation fitted the data better than the Freundlich equation for adsorption P and NO3 (Table 4 and Table 5). This is consistent with Tan et al. [28], who showed that the Langmuir equation had the best fit for the experiment. Elsa et al. [25] demonstrated that the Freundlich constant (KF) increased with the calcium content, which confirms the increase in adsorption intensity of the biochar due to the chemical reaction between phosphate ions and Ca2+. In this study, KF for P and NO3 adsorption on ESB increased with pyrolysis temperature.
The maximum adsorption of P and NO3 (qmax) on ESB was lower in the biochar produced through pyrolysis at 300 °C than at 700 °C (Table 4 and Table 5), indicating that the maximum adsorption of P and NO3 increased with pyrolysis temperature. Our results are similar to those of Zhang et al. [20], who found that biochar, produced from horse manure and bedding compost at pyrolysis of 200 °C, released the P and NO3. However, the maximum adsorption of P (qmax) on PCB and SDB was lower in the biochar produced at pyrolysis temperature of 500 °C than at 300 °C. Therefore, the maximum adsorption of P on biochar was influenced by pyrolysis temperature. The maximum adsorption of P (qmax) on RSB700, PCB300, SDB300 and ESB700 was 5.407, 7.747, 3.859 and 6.084 mg g−1, respectively. The maximum adsorption of P (qmax) on PCB300 was approximately two times that of SDB300. The maximum adsorption of NO3 (qmax) on PCB300, SDB300 and ESB700 was 0.443, 1.574 and 1.426 mg g−1, respectively. Kameyama et al. [21] found that only 1.2 and 0.7 mg g−1 NO3 could be adsorbed by the bamboo powder charcoal and sugarcane bagasse derived biochar, respectively. In this study, the NO3 adsorption capacity was between 0.443 to 1.426 mg g−1. Therefore, the capacity to absorb NO3 was lower but varied with the biochars from different feedstock materials.

3.4. Adsorption Thermodynamics of Biochars

The thermodynamics of P and NO3 adsorption on the biochars at 293, 303, and 313 K were analyzed (Figure 4 and Figure 5). As shown in Table 6, the ΔH0 value for RSB700, PCB300, SDB300 and ESB700 was 83.54, 44.63, 27.53 and 39.68 kJ mol−1, respectively, indicating that P adsorption process was endothermic. The ΔS0 value for RSB700, PCB300, SDB300 and ESB700 was 0.33, 0.19, 0.13 and 0.19 kJ mol−1 K−1, respectively, indicating increased disorder and randomness of liquid-solid phase interaction at the biochar surface [29]. Zhang et al. [30] demonstrated that P tended to be adsorbed on the surface of biochar when the ΔS0 values were positive. The ΔG0 values were in a range of −10.48 to −18.41 kJ mol−1 indicating that the process of PO43− adsorption onto the biochar was mainly spontaneous [31,32]. Furthermore, the ΔG0 decreased with increasing adsorption reaction temperature, indicating a better P adsorption efficiency at a higher solution temperature [33].
The ΔH0 value for RSB500, PCB300 and SDB500 was −70.8, −25.37 and −27.44 kJ mol−1 (Table 6), respectively, indicating that NO3 adsorption process was an exothermic reaction. However, the ΔHo value for ESB was 39.2, indicating this was endothermic. The ΔS0 value for RSB, PCB and SDB was −0.19, −0.05, and −0.06 kJ mol−1 k−1, respectively, indicating decreased disorder. However, the ΔS0 value for ESB was 0.17 kJ mol−1 k−1, suggesting increased disorder. The ΔG0 values were in the range of −8.67 to −13.47 kJ mol−1 indicating that the process of NO3 adsorption onto the biochars was also spontaneous in nature. The ΔG0 of RSB, PCB and SDB increased with adsorption reaction temperatures, indicating a better NO3 adsorption efficiency at a lower solution temperature. However, the ΔG0 of ESB decreased with increasing adsorption reaction temperature, indicating a better NO3 adsorption efficiency at a higher solution temperature [33]. Results show that NO3 adsorption process on the biochars produced from feedstocks rich in cellulose, hemicellulose and lignin was an exothermic reaction, but an endothermic reaction occurred for the biochar produced from egg shell.

3.5. Effect of Characteristic of Biochar on Capacity of Nitrate and Phosphate Adsorption

Cellulose, hemicellulose, and lignin are essential constituents of plant cell walls. In this study, RSB, PCB and SDB derived from the plants waste showed limited adsorption of, or even released, NO3 and P. These findings agree with Zhang et al. [34] who found that the pristine biochar surface is negatively charged, and thus cannot easily adsorb the negatively charged NO3 and P. Plant-derived biochar used in this study had a low zeta potential (−16.6 to −41.9 mV), meaning that electrostatic repulsion between the negatively charged surface sites and electronegative phosphate species resulted in the lower phosphate adsorption [35]. Yang et al. [36] demonstrated that positive zeta potential is desirable to the adsorption of anion by electrostatic attraction and in this study egg shell biochar (ESB) had a positive zeta potential in the range −2.9 to −4.9 mV.
The NO3 and P adsorption capacity of egg shell biochar was influenced by surface area and porosity characteristic. The SEM images of ESB showed that there was more porosity of ESB produced at 700 °C than at 300 °C (Figure 1). The P and NO3 adsorption capacities were higher in ESB700 than in ESB300, which was related to more porous structure and higher surface area in ESB700 than in ESB300 (Table 1). These findings agreed with those of Yin et al. [37], who demonstrated that the porous structure and higher surface area could contribute to better PO43− and NO3 adsorption. Therefore, the higher surface area of egg shell biochar seems to improve adsorption of NO3 and P.
The NO3 and P adsorption capacity of biochars could be improved by adjusting pyrolysis temperature. Many methods have been used to improve NO3 and P adsorption capacity. It was reported that the dashed activated carbon could adsorb 9.84 mg NO3 g−1 [38]. The NO3 adsorption on the La-modified biochar was 8.81 mg g−1, which was considerably higher than that on the pristine biochar (2.81 mg g−1) [39]. The Mg-modified sugar beet tailing biochar showed a P adsorption capacity of 6.67 mg g−1 [35]. The sesame straw biochar activated by ZnCl2 showed the highest phosphorus adsorption capacity of 9.39 mg g−1 [40]. The poplar chips biochar modified by Al showed its high P adsorption capacity of 43.98 mg g−1 [37]. Results of this study showed that the NO3 and P adsorption capacity could be enhanced by adjusting pyrolysis temperature. For example, the maximum sorption of P on RSB, produced at pyrolysis temperature of 700 °C, was 5.40 mg g−1. Compared with RSB300 and RSB500, RSB700 with higher P adsorption capacity could be attributed to higher surface area and total volume (Table 1). Similarly, the maximum sorption of NO3 on SDB, derived from pyrolysis at 300 °C, was 1.574 mg g−1. Iida et al. [38] demonstrated that the NO3 adsorption process would be restricted under basic conditions because the OH competes with NO3 for the adsorption sites on the biochar surface. Ketcha et al. [41] also found that the optimum pH value of NO3 adsorption on activated carbon was in the range of 2.8–6.5. Therefore, the ability of SDB300 to adsorb NO3 could be related to their lower pH derived from pyrolysis at 300 °C (Table 1). Results suggested that the P and NO3 adsorption capacity could be improved through screening biochars derived from different feedstock materials and pyrolysis temperatures.

4. Conclusions

The physicochemical characteristics of biochars were influenced by feedstock type and pyrolysis temperature, which affected the N and P adsorption capacity. Biochars derived from plant wastes, including RSB, PCB and SDB, had limited adsorption or even released NO3 and P. However, their PO43− adsorption capacity improved by adjusting pyrolysis temperature. Egg shell biochar (ESB) has an ability to adsorption both NO3 and P, which increased with increasing pyrolysis temperatures. The maximum NO3 and P adsorption on ESB700, produced at pyrolysis temperature of 700 °C was 1.426 and 6.084 mg g−1, respectively. Surface area and zeta potential of ESB could be the two predominant factors affecting NO3 and P adsorption. The influences of pyrolysis temperature on adsorption of both NO3 and P varied between feedstock materials.

Author Contributions

L.Z., Q.P., and J.W. carried out the data analysis; D.X. and Y.L. wrote the manuscript; L.X. and A.H. revised it.

Funding

This research was funded by the Key Laboratory of Agro-Environment in downstream of Yangze Plain, Ministry of Agriculture, P. R. China (AE2018001), Six Talent Peaks Project in Jiangsu Province (JNHB-057), and Qing Lan Project (20161507).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Scanning electron microscope (SEM) photos of RSB300 (a), RSB500 (b), RSB700 (c), PCB300 (d), PCB500 (e), PCB700 (f), SDB300 (g), SDB500 (h), SDB700 (i), ESB300 (j), ESB 500 (k) and ESB 700 (l), respectively.
Figure 1. Scanning electron microscope (SEM) photos of RSB300 (a), RSB500 (b), RSB700 (c), PCB300 (d), PCB500 (e), PCB700 (f), SDB300 (g), SDB500 (h), SDB700 (i), ESB300 (j), ESB 500 (k) and ESB 700 (l), respectively.
Water 11 01559 g001aWater 11 01559 g001b
Figure 2. Adsorption kinetics of P and NO3 on RSB (a), PCB (b), SDB (c), ESB (d), RSB (e), PCB (f), SDB (g), and ESB (h), respectively. Data are means ± SD of n = 3.
Figure 2. Adsorption kinetics of P and NO3 on RSB (a), PCB (b), SDB (c), ESB (d), RSB (e), PCB (f), SDB (g), and ESB (h), respectively. Data are means ± SD of n = 3.
Water 11 01559 g002
Figure 3. Adsorption isotherms of P and NO3 on RSB (a), PCB (b), SDB (c), ESB (d), RSB (e), PCB (f), SDB (g), and ESB (h), respectively. Data are means ± SD of n = 3.
Figure 3. Adsorption isotherms of P and NO3 on RSB (a), PCB (b), SDB (c), ESB (d), RSB (e), PCB (f), SDB (g), and ESB (h), respectively. Data are means ± SD of n = 3.
Water 11 01559 g003aWater 11 01559 g003b
Figure 4. Amount of P on RSB700 (a), PCB300 (b), SDB300 (c) and ESB700 (d) under different temperature. Data are means ± SD of n = 3.
Figure 4. Amount of P on RSB700 (a), PCB300 (b), SDB300 (c) and ESB700 (d) under different temperature. Data are means ± SD of n = 3.
Water 11 01559 g004aWater 11 01559 g004b
Figure 5. Amount of NO3 on RSB500 (a), PCB300 (b), SDB500 (c) and ESB300 (d) under different temperature, respectively. Data are means ± SD of n = 3.
Figure 5. Amount of NO3 on RSB500 (a), PCB300 (b), SDB500 (c) and ESB300 (d) under different temperature, respectively. Data are means ± SD of n = 3.
Water 11 01559 g005
Table 1. Textural properties of different biochars.
Table 1. Textural properties of different biochars.
SampleTotal Volume (cm3 g−1)Specific Surface Area (m2 g−1)pHZeta Potential (mV)
RSB3000.022 ± 0.003 a5.9 ± 0.6 a6.61 ± 0.05 a−30.50 ± 0.70 a
RSB5000.072 ± 0.008 b34.0 ± 3.6 b9.28 ± 0.07 b−41.90 ± 1.05 b
RSB7000.189 ± 0.002 c122.6 ± 14.3 c10.06 ± 0.1 c−19.35 ± 1.92 c
PCB3000.008 ± 0.0009 a3.5 ± 0.4 a6.43 ± 0.03 a−39.30 ± 0.76 a
PCB5000.106 ± 0.002 b131.5 ± 14.3 b6.82 ± 0.02 b−30.33 ± 2.01 b
PCB7000.415 ± 0.05 c441.7 ± 45.6 c9.42 ± 0.04 c−23.41 ± 0.70 c
SDB3000.006 ± 0.0007 a2.9 ± 0.4 a4.55 ± 0.14 a−25.63 ± 0.54 a
SDB5000.233 ± 0.03 b378.7 ± 39.6 b6.03 ± 0.02 b−39.90 ± 1.20 b
SDB7000.278 ± 0.04 b594.9 ± 60.3 c7.88 ± 0.08 c−16.60 ± 0.29 c
ESB3000.004 ± 0.0005 a2.0 ± 0.2 a7.89 ± 0.04 a−9.99 ± 0.92 a
ESB5000.006 ± 0.0007 ab3.7 ± 0.4 ab8.02 ± 0.11 a−4.94 ± 1.63 b
ESB7000.009 ± 0.0008 b5.3 ± 0.5 b9.46 ± 0.07 b−2.99 ± 1.37 b
Data are means ± SD of n = 3. Different letters in the same column indicate significant differences in different pyrolysis temperature for each biochar (p < 0.05).
Table 2. Parameters of P adsorption kinetics of different biochars.
Table 2. Parameters of P adsorption kinetics of different biochars.
SamplePseudo-First-OrderPseudo-Second-OrderIntra-particle Diffusion
K1
(h−1)
qe
(mg g−1)
R2K2
(g mg−1 h−1)
qe
(mg g−1)
R2K3
(mg g−1 h−0.5)
C
(mg g−1)
R2
RSB7000.0050.9980.8650.0091.7900.9990.0320.7030.778
PCB3000.0060.4300.8680.6220.0840.9980.016−0.3920.706
SDB3000.0080.1000.8004.4800.0240.9940.002−0.0460.601
ESB3000.0061.8790.9660.0120.8270.9510.038−0.4110.800
ESB5000.0042.0270.9950.0201.5140.9190.054−0.2310.893
ESB7000.0061.5340.9840.0111.0260.9470.033−0.0530.753
Table 3. Parameters of NO3 adsorption kinetics of different biochars.
Table 3. Parameters of NO3 adsorption kinetics of different biochars.
SamplePseudo-First-OrderPseudo-Second-Order Intra-Particle Diffusion
K1
(h−1)
qe
(mg g−1)
R2K2
(g mg−1 h−1)
qe
(mg g−1)
R2K3
(mg g−1 h−0.5)
C
(mg g−1)
R2
PCB3000.0010.0280.7640.1530.0530.9840.0010.0150.771
SDB3000.0010.0230.7000.2870.2160.9920.0010.1770.328
SDB5000.0060.3000.9680.0100.2670.9740.006−0.0020.928
ESB3000.0040.0470.9700.4490.0440.9660.002−0.0090.804
ESB5000.0080.1270.9060.0970.050.8780.003−0.0210.814
ESB7000.0030.1930.9890.0240.1120.9350.005−0.0360.914
Table 4. Parameters of P adsorption isotherms of different biochars.
Table 4. Parameters of P adsorption isotherms of different biochars.
SampleLangmuirFreundlich
qmax
(mg g−1)
KL
(L mg−1)
R2KF
(mg g−1)
nR2
RSB7005.4070.0390.9810.6112.5580.876
PCB3007.7470.0070.9420.1251.5010.897
PCB5003.2700.0080.9330.0261.2460.760
SDB3003.8590.0030.9030.0191.2300.872
SDB5003.3950.0020.9110.0020.8710.728
ESB3004.5380.0180.8520.2442.0090.760
ESB5004.9220.0300.9200.4822.4580.800
ESB7006.0840.0310.9730.5982.5420.855
Table 5. Parameters of NO3 adsorption isotherms of different biochar.
Table 5. Parameters of NO3 adsorption isotherms of different biochar.
SampleLangmuirFreundlich
qmax
(mg g−1)
KL
(L mg−1)
R2KF
(mg g−1)
nR2
PCB3000.4330.0120.9550.0212.0060.876
SDB3000.8040.0210.9960.0702.4160.949
SDB5001.5740.0110.9890.0671.9410.941
ESB3000.6710.0100.9190.0201.7350.850
ESB5000.9370.0100.9630.0321.8100.901
ESB7001.4260.0100.9870.0511.8550.941
Table 6. Thermodynamic parameters for P and NO3 adsorption.
Table 6. Thermodynamic parameters for P and NO3 adsorption.
P NO3
T(K)SampleH0
(kJ mol−1)
S0
(kJ mol−1K−1)
G0
(kJ mol−1)
SampleH0
(kJ mol−1)
S0
(kJ mol−1 K−1)
G0
(kJ mol−1)
293RSB70083.540.33−11.95RSB500−70.80−0.19−13.47
303 −17.70 −12.69
313 −18.41 −9.73
293PCB30044.630.19−11.27PCB300−25.37−0.05−10.79
303 −13.92 −10.70
313 −15.07 −9.73
293SDB30027.530.13−10.48SDB500−27.44−0.06−10.79
303 −11.99 −10.70
313 −13.07 −8.67
293ESB70039.680.19−14.53ESB30039.200.17−9.81
303 −17.47 −12.45
313 −18.20 −13.11

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Zhou, L.; Xu, D.; Li, Y.; Pan, Q.; Wang, J.; Xue, L.; Howard, A. Phosphorus and Nitrogen Adsorption Capacities of Biochars Derived from Feedstocks at Different Pyrolysis Temperatures. Water 2019, 11, 1559. https://doi.org/10.3390/w11081559

AMA Style

Zhou L, Xu D, Li Y, Pan Q, Wang J, Xue L, Howard A. Phosphorus and Nitrogen Adsorption Capacities of Biochars Derived from Feedstocks at Different Pyrolysis Temperatures. Water. 2019; 11(8):1559. https://doi.org/10.3390/w11081559

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Zhou, Lei, Defu Xu, Yingxue Li, Qianchen Pan, Jiajun Wang, Lihong Xue, and Alan Howard. 2019. "Phosphorus and Nitrogen Adsorption Capacities of Biochars Derived from Feedstocks at Different Pyrolysis Temperatures" Water 11, no. 8: 1559. https://doi.org/10.3390/w11081559

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