Abstract

Metal ions in sediment are inherent Ca and Fe sources for biochar modification. In this work, the effect of Ca2+ and Fe2+ released from sediment on biochar for phosphorus adsorption was evaluated. Results showed that raw peanut shell biochar (PSB) was poor in phosphorus adsorption (0.48 mg/g); sediment-triggered biochar (S-PSB) exhibited a P adsorption capacity of 1.32 mg/g in capping reactor and maximum adsorption capacity of 10.72 mg/g in the Langmuir model. Sediment released Ca2+ of 2.2–4.1 mg/L and Fe2+/Fe3+ of 0.2–9.0 mg/L. The metals loaded onto the biochar surface in the forms of Ca-O and Fe-O, with Ca and Fe content of 1.47 and 0.29%, respectively. Sediment metals made point of zero charge (pHpzc) of biochar shifted from 5.39 to 6.46. The mechanisms of enhanced P adsorption by S-PSB were surface complexation of CaHPO4 followed by precipitation of Ca3(PO4)2 and Ca5(PO4)3(OH). Sediment metals induced the modification of biochar and improvement of P adsorption, which was feasible to overcome the shortcomings of biochar on phosphorus control in sediment capping.

HIGHLIGHTS

  • Biochar was triggered by sediment Ca2+ and Fe2+.

  • Metals loaded on biochar surface as Ca-O and Fe-O.

  • q of S-PSB was 2.5 times higher than PSB.

  • Metal-loaded biochar was a seed for Ca-P complexation and precipitation.

INTRODUCTION

Phosphate concentration as low as 0.02 mg/L is sufficient to stimulate algal growth and deplete dissolved oxygen in water body. Sewage discharge and agricultural runoff transfer P to sediment. In turn, sediment releases phosphorus to overlying water when water body suffers from anaerobic conditions, alkaline pH, or wave disturbance (Lewandowski et al. 2007; Yin et al. 2013). Sediment as a sink/source of phosphorus poses a great challenge for water body remediation. Endogenous P release from sediment is a critical process for eutrophication control in water body. In recent decades, various approaches have been applied to manage sediment, including sediment dredging, in situ capping, co-precipitation of P with Fe and Al salts, etc. In situ capping refers to phosphate adsorbents used as a barrier layer at the sediment–water interface. Commonly used capping agents are soil, coal ash, active carbon, zeolite (Zhu et al. 2019). The core of this technology is an eco-friendly adsorbent.

Biochar is a low-cost adsorbent with a wide availability of feedstock. It is prepared through pyrolysis of biomass (crops, forestry waste, and sewage sludge) under insufficient or zero oxygen conditions. The features of biochar that make it feasible for adsorption include high specific surface area, pore structure, high C content, and oxygen-containing functional groups. Biochar has an essential advantage of environmental friendliness, as it is often biodegradable and has very low toxic (Zhu et al. 2019). Subsequently the P-laden biochar could be recovered through desorption or direct use as a soil fertilizer (Shakoor et al. 2021).

Biochar has limited adsorption capacity for anions of PO43− and NO3, due to alkaline and negatively charged surfaces, namely hydroxyl and carboxyl groups (Zhang et al. 2020). Modification strategies of acid activation, electrochemical treatments and metal ion modification are developed to alleviate this issue. Metal ions particularly Mg2+, Ca2+, Fe3+ are deemed efficient modification methods (Jung et al. 2015). For instance, by modifying with Fe/Ca, P adsorption capacity of peanut shell biochar was enhanced from 0 to 18.94 mg/g (Pei et al. 2021). Ca-rich biochar has higher P uptake capability and lower effect on pH compared with Mg-rich biochar (Saadat et al. 2018). Ca and Mg are often fixed on the biochar surface in metal oxide or hydroxide. Besides surface precipitation, Mg/Ca oxide on the surface of biochar would be protonated to MgOH+/CaOH+ (Deng et al. 2021), improving the electrostatic attraction towards phosphate. Also, ligand exchange forming covalent bonds between phosphate and the metal cation (inner-sphere surface complexation) (Bacelo et al. 2020) is an important adsorption mechanism. In a context of resource recycling and sustainable development, waste materials rich in Ca and Fe, such as construction and demolition wastes (dos Reis et al. 2020), eggshell (Yang et al. 2021b), marble and sepiolite (Deng et al. 2021) are used for phosphate adsorption. However, in previous research metal ions were obtained by dosage of chemical agents or waste materials, metals in ambient environment are ignored. In sediment remediation, sediment itself has a mineral content (Si, Ca, Fe, Al, Mg, etc.) as much as 78.4% being relevant to P recovery capacity (Yang et al. 2021a). Accompanied with dissolution of Ca-P and Fe-P in sediment, Fe3+ and Ca2+ were also released. Until now, there is a knowledge gap about the released Ca2+ and Fe2+ from sediment and their modification effect on biochar.

In this work, it is hypothesized that spontaneous release of Ca2+ and Fe2+ from sediment could modify biochar to some extent. Peanut shell biochar (PSB) was selected as sediment capping material. The study was aimed at (1) optimize sediment capping to obtain the sediment-modified biochar (S-PSB); (2) compare raw PSB and S-PSB from the aspects of P adsorption capacity, isotherms, and kinetics; (3) reveal the mechanism of S-PSB promoting P adsorption via Ca2+ and Fe2+/Fe3+ released by sediment, characterization of S-PSB by FTIR, XRD, EDS and zero point charge.

MATERIALS AND METHODS

Sediment & biochar of PSB and S-PSB

Sediment was collected from the Xunsi River, a black-odorous urban river in Wuhan, China. The sediment was grabbed approximately 10 cm below the surface sediment by a gravity sediment collector. Physiochemical parameters of the overlying water were pH 7.80, TP 4.8 mg/L, NH4+-N 13.7 mg/L, DO 0.5 mg/L.

Peanut shell biochar (PSB) was prepared as follows: peanut shell was washed with tap water to remove dusts and followed by a distilled water rinse. Then, it was heated at a rate of 7 °C/min to target temperature (300, 500 and 700 °C, respectively) in a muffle furnace and hold for 2 h, afterwards cooled down inside the furnace to room temperature. PSB was washed in 1 mol/L HCl for 24 h (Antunes et al. 2018) to exclude the effect of intrinsic metals on P adsorption. Then PSB was rinsed repeatedly with deionized water until neutral. Dried in oven at 105 °C and sieved into 0.3 mm, 0.6 mm, and 1.5 mm for further use.

S-PSB: after raw PSB was dosed into capping reactor for several days, contact of sediment metals with PSB caused modification of biochar, and the resulting biochar was labeled as S-PSB. An orthogonal experiment was carried out to seek optimal capping condition for sufficient modification of biochar by sediment metals. Pyrolysis temperature (300 °C, 500 °C, 700 °C), dosage (10 g/L, 20 g/L, 30 g/L) and particle size (0.3 mm, 0.6 mm, 1.5 mm) were set as influencing factors of orthogonal experiment.

Sediment capping with biochar

Schematic of sediment capping by biochar was shown in Figure 1. The sediment capping was done in a cylindrical plastic reactor with total volume of 400 mL. First, 80 mL of fresh sediment was added to the bottom of the reactor. Then, biochar was evenly spread on the surface of the sediment. A layer of gauze was used to prevent biochar from floating. Fill the reactor by siphon method with 320 mL of distilled water, which was previously deoxygenated by Na2SO3. The reactor was sealed and put in the dark to avoid the growth of algae (Zhu et al. 2019). After 5 mL of overlying water was sampled every day, the same volume of distilled water was replenished to keep the overlying water volume constant.

Figure 1

Schematic of sediment capping by biochar.

Figure 1

Schematic of sediment capping by biochar.

P adsorption ability of S-PSB

  • (1)

    P adsorption capacity of S-PSB in capping reactor: it was calculated based on P mass balance among sediment, overlying water, and biochar. For tracking the residual P adsorption capacity, 0.05 g biochar was taken out from the capping reactor every day and then dosed into 5 mg/L of KH2PO4 solution until the P adsorption reached saturation. Samples were filtered through 0.45 μm membrane and the orthophosphate concentration was measured by the molybdate method.

  • (2)

    P adsorption kinetics and isotherms of S-PSB in KH2PO4 solution: to evaluate maximum P adsorption capacity and the relevant adsorption behaviors, kinetics and isotherms were conducted. Kinetics of P adsorption on biochar: 0.2 g biochar was dosed into 250 mL Erlenmeyer flask containing 100 mL of 5 mg/L KH2PO4, continually stirred and sampled at 2, 5, 10, 15, 30, 60, 120, 240 and 300 min. A pseudo-first-order model and pseudo-second-order model were used to fit the kinetics.

Pseudo-first-order:
formula
(1)
where qt is the amount of P adsorbed at time t (mg/g); qe is the amount of P adsorbed at equilibrium (mg/g); k1 is equilibrium rate constant of the pseudo first order model (/min); t is the time (min).
Pseudo-second-order:
formula
(2)
where qt is the amount of P adsorbed at time t (mg/g); qe is the amount of P adsorbed at equilibrium (mg/g); k2 is the equilibrium rate constant of the pseudo-second-order model (g/mg/min). This linearized equation yields the best fit at low adsorbate concentration (Ugwu et al. 2021).

Adsorption isotherms: 0.2 g of biochar was dosed into 250 mL in an Erlenmeyer flask containing 100 mL KH2PO4 with P concentrations of 2.0, 5.0, 10.0, 20.0, 30.0 and 50.0 mg/L, and then flasks were placed on a horizontal shaker at 50 rpm for 24 h. All experiments were conducted at 25 °C. The Langmuir and Freundlich models were selected to fit the adsorption isotherms.

Langmuir model:
formula
(3)
where Ce is the equilibrium concentration (mg/L); qm is the maximum monolayer adsorption capacity (mg/g); and KL is Langmuir constant (L/mg).
Freundlich model:
formula
(4)
where qe is equilibrium adsorption capacity, KF is the Freundlich constant related to adsorption capacity [(mg/g) × (L/mg)1/n], qm is the maximum adsorption capacity (mg/g), Ce is the equilibrium concentration, and 1/n is Freundlich isotherm constants related to adsorption intensity (Othmani et al. 2017).

Characterization of sediment metals on S-PSB

In the capping reactor, anaerobic sediment released P as well as Ca2+ and Fe2+. Once the metal ions contacted with biochar, the modification of biochar occurred. Given the metals in sediment were gradually released into overlying water, total Fe (Fe2+/Fe3+) and Ca2+ concentrations in overlying water were measured over time. Furthermore, the metals loading on biochar surface was confirmed by SEM-EDS (Zeiss Sigma300, Germany) and surface functional groups were analyzed by FTIR (Thermo Scientific Nicolet 6,700, US). Crystal products of metal ions and phosphate were observed by XRD (Rigaku Ultima IV, Japan). Point of zero charge (pHpzc) for biochar was determined according to previously published literature (Tomul et al. 2020).

RESULTS AND DISCUSSION

Sediment capping by biochar and obtainment of S-PSB

An orthogonal experiment was carried out to seek optimal capping conditions (Table 1) and guarantee sufficient modification of biochar by sediment metals. The optimal capping condition was obtained by biochar of 1.5 mm pyrolyzed at 500 °C with a dosage of 20 g/L. Range analysis (R value) could be applied to evaluate relative importance of the three influencing factors. The factors followed the order: pyrolysis temperature > particle size > dosage. The R value of the pyrolysis temperature was obviously larger than the other two factors, demonstrating the key role of pyrolysis temperature in the biochar preparation. Generally, rise in pyrolysis temperature removed the volatile material resulting in increased micropore volume and surface area of biochar. Nevertheless, the high temperature of 700 °C could destroy the carbon skeleton, reduce porosity (Shakoor et al. 2021), and increase hydrophobicity (Ahmad et al. 2012).

Table 1

Orthogonal experiment analysis

NumberPyrolysis temperature (°C)Dosage (g/L)Particle size (mm)P in overlying water (mg/L)
300 10 0.3 0.405 
300 20 0.6 0.433 
300 30 1.5 0.405 
500 10 0.6 0.192 
500 20 1.5 0.116 
500 30 0.3 0.213 
700 10 1.5 0.199 
700 20 0.3 0.213 
700 30 0.6 0.213 
k1 0.414 0.265 0.277 – 
k2 0.174 0.254 0.279 – 
k3 0.208 0.277 0.240 – 
0.240 0.023 0.039 – 
Order Pyrolysis temperature > Particle size > Dosage 
Optimal capping 500 °C, 1.5 mm, 20 g/L 
NumberPyrolysis temperature (°C)Dosage (g/L)Particle size (mm)P in overlying water (mg/L)
300 10 0.3 0.405 
300 20 0.6 0.433 
300 30 1.5 0.405 
500 10 0.6 0.192 
500 20 1.5 0.116 
500 30 0.3 0.213 
700 10 1.5 0.199 
700 20 0.3 0.213 
700 30 0.6 0.213 
k1 0.414 0.265 0.277 – 
k2 0.174 0.254 0.279 – 
k3 0.208 0.277 0.240 – 
0.240 0.023 0.039 – 
Order Pyrolysis temperature > Particle size > Dosage 
Optimal capping 500 °C, 1.5 mm, 20 g/L 

Figure 2(a) shows the performance of biochar under optimal capping conditions. Sediment alone continually released P into overlying water and P concentration reached 0.5 mg/L a week later. Sediment with biochar capping maintained a stable P concentration in overlying water of approximately 0.1 mg/L, regardless of more or less P released from sediment. After one week, the P in the overlying water reduced by 75%. With regard to P in sediment, it was the sum of iron/aluminum-bound phosphorus (Fe/Al-P), calcium-bound phosphorus (Ca-P), and organic phosphorus (OP). Through mass balance (Figure 2(b)), the reduction of Fe/Al-P, Ca-P, OP in sediment was 70.9, 8.2, and 48.4%, respectively. Given the P adsorbed by S-PSB was calculated as 10.55 mg and biochar dosage was 8 g, the adsorption capacity of S-PSB was 1.32 mg/g, which was three times higher than that of raw PSB (0.42 mg/g). The improvement of biochar adsorption capacity was due to sediment. Sediment released soluble reactive phosphorus, NH4+, Fe2+, Ca2+, HS and heavy metals (Lewandowski et al. 2007; Zhu et al. 2019). Among these ions, Fe2+ and Ca2+ were plausible to modify biochar. Therefore, the biochar contacting with sediment (labeled as S-PSB) was used for the characterization experiment (next section).

Figure 2

P in overlying water (a) and P mass balance at day 7 (b).

Figure 2

P in overlying water (a) and P mass balance at day 7 (b).

Enhanced P adsorption capacity of S-PSB

Figure 3 shows a comparison of P adsorption capacity between raw PSB and S-PSB. Generally, P adsorption capacity exhausted over time, because the adsorption site was occupied by phosphate until saturation. The P adsorption capacity of raw PSB was 0.42 mg/g determined in KH2PO4 solution. For S-PSB, P adsorption capacity on day 1 was 0.48 mg/g, similar to that of PSB. On day 2 and day 3, residual P adsorption capacity of S-PSB increased to 0.75, 1.24 mg/g, respectively, and then exhausted over time. That was modification of biochar by sediment occurred in initial 3 days and P adsorption capacity of S-PSB was enlarged to 1.24 mg/g, in accord with the value of 1.32 mg/g calculated from P mass balance (Figure 2(b)), which was 3 times higher than that of raw PSB.

Figure 3

P adsorption capacity of raw PSB and S-PSB.

Figure 3

P adsorption capacity of raw PSB and S-PSB.

Figure 4(a) shows the adsorption isotherm of PSB and S-PSB. The Langmuir model assumed monolayer adsorption on a homogenous adsorbent surface (Ugwu et al. 2021). The Freundlich model was used for multi-molecular layer on a heterogeneous adsorbent surface (Othmani et al. 2017). With high regression coefficients (R2) and minimum value of reduced chi-square (χ2) in Table 2, thereby the Langmuir model better fitted P adsorption than the Freundlich model, indicating that the adsorption process was dominated by monolayer chemical adsorption. In the Langmuir model, the maximum P adsorption capacity (qm) of PSB, S-PSB was 4.85, 10.72 mg/g, respectively. The kinetic curves and their parameters are shown in Figure 4(b) and Table 2. According to R2 and χ2 in Table 2, the pseudo-second-order kinetic fitted better than first-order kinetics, and the calculated qe (1.29 mg/g) were close to the experimental qe (1.21 mg/g). The pseudo-second-order kinetics linked closely with the formation of chemical bonds (Gupta et al. 2020). It took a longer time to achieve equilibrium by S-PSB (120 min) than PSB (60 min), meanwhile P adsorption speed of S-PSB (k2: 0.039 g/mg/min) was much slower than that of PSB (k2: 0.205 g/mg/min). P adsorption by PSB was mainly attributed to ligand exchange of PO43− with –COOH and –OH. The speed control step of P adsorption on S-PSB may be surface reaction between metal oxide and P, and the tardy formation of hydrogen bond between metal oxide and P slowed down adsorption rate (Pei et al. 2021).

Table 2

The parameters of adsorption isotherms and kinetics

BiocharLangmuir
Freundlich
Pseudo-first order
Pseudo-second order
qm (mg/g)KL (L/mg)R2χ2KF (mg/g)(L/mg)1/n1/nR2χ2qe (mg/g)k1 (/min)R2χ2qe (mg/g)k(g/mg/min)R2χ2
PSB 4.85 0.03 0.91 0.107 0.24 0.66 0.86 0.181 0.46 0.090 0.94 0.004 0.50 0.205 0.95 0.002 
S-PSB 10.72 0.07 0.96 0.244 0.98 0.58 0.92 0.653 1.16 0.044 0.96 0.039 1.29 0.039 0.97 0.035 
BiocharLangmuir
Freundlich
Pseudo-first order
Pseudo-second order
qm (mg/g)KL (L/mg)R2χ2KF (mg/g)(L/mg)1/n1/nR2χ2qe (mg/g)k1 (/min)R2χ2qe (mg/g)k(g/mg/min)R2χ2
PSB 4.85 0.03 0.91 0.107 0.24 0.66 0.86 0.181 0.46 0.090 0.94 0.004 0.50 0.205 0.95 0.002 
S-PSB 10.72 0.07 0.96 0.244 0.98 0.58 0.92 0.653 1.16 0.044 0.96 0.039 1.29 0.039 0.97 0.035 
Figure 4

Adsorption isotherms (a) and kinetics (b).

Figure 4

Adsorption isotherms (a) and kinetics (b).

Metals adsorbed and loaded on S-PSB

Figure 5(a) shows the metals adsorbed by biochar. The difference between sediment curve and S-PSB curve was the metals adsorbed by S-PSB. On day 1, Fe-P in sediment dissolved the most which led to maximum total Fe (Fe2+/Fe3+) adhering to biochar of 9.03 mg/L. However, Fe adsorbed by biochar was sharply declined over time to 0.2 mg/L at day 6. This was ascribed to the formation of FeS by Fe2+ and HS (Hansel et al. 2015), also FeS on biochar surface was confirmed by XRD in Figure 7(b). In contrast, Ca adsorbed by biochar was stable at 2.80 mg/L. The result suggested that Ca was strongly adsorbed by biochar while Fe was not bound. The different adsorption behavior of Ca2+ and Fe2+/Fe3+ on biochar was mainly due to selective adsorption of biochar among Ca2+, Fe2+, Fe3+ (Maneechakr & Karnjanakom 2019) and Mg2+ (Yi & Chen 2018). Another possible explanation was that Fe loading on biochar required the formation of amorphous FeOOH (Bakshi et al. 2021). Furthermore, Ca as the main metal adsorbed on S-PSB was confirmed by desorption of P species on S-PSB and EDS analysis. Fe/Al-P accounted for 11% and Ca-P accounted for 89% of P on S-PSB (Figure 5(b)). EDS (Table 3) revealed a biochar surface with Ca content of 1.47% whereas Fe content was merely 0.29%. Ca and Mg content of biochar were positively correlated with phosphate adsorption (Zhang et al. 2020). Ca loading on biochar might be robust, since Ca content was not impacted by biochar surface, pore volume and pore size (Antunes et al. 2018).

Table 3

Metal content of peanut shell biochar (EDS, %)

ElementsAlCaNaFeKMg
PSB 0.18 – – – – – 
S-PSB – 1.47 0.48 0.29 0.15 0.06 
ElementsAlCaNaFeKMg
PSB 0.18 – – – – – 
S-PSB – 1.47 0.48 0.29 0.15 0.06 
Figure 5

Metals adsorbed by biochar (a) and P species on S-PSB (b).

Figure 5

Metals adsorbed by biochar (a) and P species on S-PSB (b).

Notably, although P release was mainly originated from Fe/Al-P in sediment, P adsorbed on S-PSB consisted of 89% Ca-P and 11% Fe/Al-P. The prevalence of Ca-P on S-PSB could not be explained by chemical precipitation, because solubility product of Fe-P (KspFe3(PO4)2 = 1.3 × 10−30) was much smaller than Ca-P (KspCa3(PO4)2 = 10−26, KspCaHPO4 = 10−6.6). In addition, higher metal ions (Figure 5(a)) and pH (Figure 5(b)) of overlying water in sediment reactor did not favor P removal, indicating that Ca-P did not form in bulk solution but on the biochar surface. Therefore, biochar was inferred to be a carrier of Ca for complexation. In the pH range of 8–9, the predominant specie of P was HPO42− (dos Reis et al. 2020) and it reacted with Ca2+ to form CaHPO4. The complexation of CaHPO4 on the biochar surface was also observed by Marshall et al. (2017), who found that biochar acted as a seed material to adsorb Ca2+ on the edge of the carbon lattice and form CaHPO4, while that biochar without Ca2+ did not adsorb phosphate.

Comparison of P adsorption capacity between S-PSB and other metal-modified adsorbents is listed in Table 4. qm in some research was obtained through isotherm assays in high P concentrations (100–800 mg/L), but the adsorption could be less effective in low concentration (<10 mg/L). In this work, initial P concentration was 5–50 mg/L and qm was 10.72 mg/g. qm of Ca-modified biochar was impacted by raw biomass, initial P concentration, and Ca content (Antunes et al. 2018). Structure of Ca-modified biochar was better than that of Mg-modified biochar for P adsorption (Yin et al. 2021). P removal efficiency of sludge-derived biochar modified with Ca, Mg, Fe was 85, 68, 10%, respectively (Saadat et al. 2018). It was noted that, qm of Ca-modified biochar was a bit higher than Ca-modified adsorbents of zeolite, clay and red mud.

Table 4

P adsorption capacity of metal-modified adsorbents

AdsorbentPretreatment
Initial P (mg/L)qm (mg/g)Reference
Peanut shell biochar Ca Sediment metal 5–50 10.72 This work 
Peanut shell biochar Ca(OH)2 40 11.94 Pei et al. (2021)  
Grapevine cane biochar CaCl2 100 25.90 Marshall et al. (2017)  
Sludge biochar Eggshell 800 121.00 Yang et al. (2021b)  
Sheep manure Oyster shells 200 78.11 Feng et al. (2021)  
Zeolite Ca(OH)2 25–200 7.78 Mitrogiannis et al. (2017)  
K-clay CaO 80 4.39 Yang et al. (2013)  
Red mud Seawater (Ca and Mg) – 0.66 Cusack et al. (2018)  
Peanut shell biochar Mg MgCl2 40 16.96 Pei et al. (2021)  
Peanut shell biochar MgO 20 4.63 Wu et al. (2019)  
Corn cobs biochar MgCl2 0.7–68.5 15.80 Ding et al. (2021)  
Peanut shell biochar Fe FeCl3 40 3.01 Pei et al. (2021)  
Rice husk biochar Fe2(SO4)3 150 4.45 Ajmal et al. (2020)  
AdsorbentPretreatment
Initial P (mg/L)qm (mg/g)Reference
Peanut shell biochar Ca Sediment metal 5–50 10.72 This work 
Peanut shell biochar Ca(OH)2 40 11.94 Pei et al. (2021)  
Grapevine cane biochar CaCl2 100 25.90 Marshall et al. (2017)  
Sludge biochar Eggshell 800 121.00 Yang et al. (2021b)  
Sheep manure Oyster shells 200 78.11 Feng et al. (2021)  
Zeolite Ca(OH)2 25–200 7.78 Mitrogiannis et al. (2017)  
K-clay CaO 80 4.39 Yang et al. (2013)  
Red mud Seawater (Ca and Mg) – 0.66 Cusack et al. (2018)  
Peanut shell biochar Mg MgCl2 40 16.96 Pei et al. (2021)  
Peanut shell biochar MgO 20 4.63 Wu et al. (2019)  
Corn cobs biochar MgCl2 0.7–68.5 15.80 Ding et al. (2021)  
Peanut shell biochar Fe FeCl3 40 3.01 Pei et al. (2021)  
Rice husk biochar Fe2(SO4)3 150 4.45 Ajmal et al. (2020)  

Effect of metals on pHpzc of biochar

Figure 6 shows the pHpzc of PSB and S-PSB. pHpzc corresponded to pHf − pH0 = 0 (Tomul et al. 2020). Surface charge of the biochar was either positive (pH < pHpzc) or negative (pH > pHpzc). Compared to pHpzc of 5.39 for PSB, the pHpzc of S-PSB increased to 6.46, demonstrating the formation of hydroxyl ligands on the surface (Yin et al. 2013), which was consistent with strengthened adsorption peaks of -OH on S-PSB (Figure 7(a)). However, pH in the capping reactor ranging from 7.9 to 8.2 was higher than pHpzc of 6.46 for S-PSB, thereby S-PSB was still negatively charged and repelled with H2PO4 and HPO42−. In this work, on the one hand, sediment released Ca2+ of 2.2–4.1 mg/L, which was lower than Ca concentration (50.3 mg/L) or Ca content (4%–15.28%) reported in literature (Antunes et al. 2018). On the other hand, sediment released Fe2+/Fe3+ of 0.2–9.0 mg/L. Bakshi et al. (2021) found that FeSO4 pretreatment declined pHpzc of biochar from 8.48 to 4.31. As a result, the elevation of pHpzc was not enough to make S-PSB positively charged, and electrostatic attraction was not the main mechanism for improved P adsorption.

Figure 6

pHpzc of PSB and S-PSB.

Figure 6

pHpzc of PSB and S-PSB.

Figure 7

FTIR (a) and XRD (b) spectra of biochar.

Figure 7

FTIR (a) and XRD (b) spectra of biochar.

FTIR and XRD analysis of functional groups

Figure 7(a) shows FTIR spectra of PSB and S-PSB. On the PSB surface, there were -COO at 1,590 cm−1 and -OH at 3,460 cm−1, and these functional groups were formed through dehydration of lignin and cellulose during pyrolysis. New functional groups on the S-PSB surface appeared at 877 cm−1 (Ca-O) (Deng et al. 2021), 756 cm−1 (Mg-O), and 626 cm−1 (Fe-O) (Tuna et al. 2013), demonstrating that Fe, Mg, and Ca had been successfully loaded onto the biochar surface. Similar to our result, CaO and Ca(OH)2 were surface active sites for P adsorption in previous research that used calcium-rich sepiolite (Deng et al. 2021), egg shell (Yang et al. 2021b) as Ca sources. Surface complexes Ca-O-P (Deng et al. 2021) and cation bridging (Jung et al. 2015) were primary mechanisms for P adsorption on Ca-modified biochar. In addition, adsorption peak of -OH (3,460 cm−1) was strengthened, owing to the formation of Ca5(PO4)3(OH) and hydroxyl ions induced by Ca(OH)2 or CaO (Cao et al. 2020).

XRD spectra are shown in Figure 7(b). The newly formed diffraction peaks of S-PSB were assigned to CaHPO4, Ca3(PO4)2, Ca5(PO4)3(OH), and FePO4·2H2O. The solubility product of CaHPO4 (Ksp = 10−6.6) was much higher than that of Ca3(PO4)2 (Ksp = 10−26) and Ca5(PO4)3(OH) (Ksp = 10−48.6) (Barat et al. 2008), thus CaO on biochar surface complexed with HPO42− in the pH range of 8–9 (Figure 5(b)) to form CaHPO4 first, and followed by precipitation of Ca3(PO4)2. CaHPO4 and Ca3(PO4)2 were the most common precursors for Ca5(PO4)3(OH). The transformation to Ca5(PO4)3(OH) was feasible at pH 6.8–10.0 with a reaction time of 1–11 h (Montastruc et al. 2003). Taken together, biochar loading with CaO acted as a seed to induce complexation of CaHPO4 and precipitation of Ca3(PO4)2 and Ca5(PO4)3(OH). The reactions were expressed as Equations (5)–(7) (Tas & Bhaduri 2004; Barat et al. 2008; dos Reis et al. 2020):
formula
(5)
formula
(6)
formula
(7)

CONCLUSION

Raw PSB had a poor P adsorption ability. Sediment and biochar contacting for 3 days enabled the modification of biochar (S-PSB). Compared with PSB (qm of 4.85 mg/g), the P adsorption capacity of S-PSB (qm of 10.72 mg/g) was 2.2 times higher. FTIR confirmed Ca-O, Fe-O, and Mg-O were successfully loaded on the biochar surface and EDS determined Ca and Fe content on the biochar surface was 1.47%, 0.29%, respectively. Sediment metals made the pHzpc of biochar shifted from 5.39 to 6.46. The mechanism of enhanced P adsorption by S-PSB was surface complexation of Ca-O-P, surface precipitation of Ca5(PO4)3(OH) and CaHPO4. Sediment metals as environmental factors of capping should be considered, because synergistic effect between sediment and biochar could enhance the phosphorus control in real application.

COMPETING INTERESTS

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

FUNDING

This work was funded by The National College Student Innovation and Entrepreneurship Training Program Project (201810488032), National Environmental Protection Mining and Metallurgical Resource Utilization and Pollution Control Key Laboratory Open Fund (HB201915).

DATA AVAILABILITY STATEMENT

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

REFERENCES

Ahmad
M.
,
Lee
S. S.
,
Dou
X. M.
,
Mohan
D.
,
Sung
J. K.
,
Yang
J. E.
&
Ok
Y. S.
2012
Effect of pyrolysis temperature on soybean stover and peanut shell derived biochar properties and TCE adsorption in water
.
Bioresource Technology
118
,
536
544
.
Ajmal
Z.
,
Muhmood
A.
,
Dong
R.
&
Wu
S.
2020
Probing the efficiency of magnetically modified biomass-derived biochar for effective phosphate removal
.
Journal of Environmental Management
253
,
109730
.
Bacelo
H.
,
Pintor
A. M. A.
,
Santos
S. C. R.
,
Boaventura
R. A. R.
&
Botelho
C. M. S.
2020
Performance and prospects of different adsorbents for phosphorus uptake and recovery from water
.
Chemical Engineering Journal
381
,
122566
.
Bakshi
S.
,
Laird
D. A.
,
Smith
R. G.
&
Brown
R. C.
2021
Capture and release of orthophosphate by Fe-modified biochars: mechanisms and environmental applications
.
ACS Sustainable Chemistry & Engineering
9
(
2
),
658
668
.
Barat
R.
,
Montoya
T.
,
Borras
L.
,
Ferrer
J.
&
Seco
A.
2008
Interactions between calcium precipitation and the polyphosphate-accumulating bacteria metabolism
.
Water Research
42
(
13
),
3415
3424
.
Cao
H.
,
Wu
X.
,
Syed-Hassan
S. S. A.
,
Zhang
S.
,
Mood
S. H.
,
Milan
Y. J.
&
Garcia-Perez
M.
2020
Characteristics and mechanisms of phosphorous adsorption by rape straw-derived biochar functionalized with calcium from eggshell
.
Bioresource Technology
318
,
124063
.
Cusack
P. B.
,
Healy
M. G.
,
Ryan
P. C.
,
Burke
I. T.
,
Donoghue
L. M. T.
,
Ujaczki
E.
&
Courtney
R.
2018
Enhancement of bauxite residue as a low-cost adsorbent for phosphorus in aqueous solution, using seawater and gypsum treatments
.
Journal of Cleaner Production
179
,
217
224
.
Deng
W.
,
Zhang
D.
,
Zheng
X.
,
Ye
X.
,
Niu
X.
,
Lin
Z.
,
Fu
M.
&
Zhou
S.
2021
Adsorption recovery of phosphate from waste streams by Ca/Mg-biochar synthesis from marble waste, calcium-rich sepiolite and bagasse
.
Journal of Cleaner Production
288
,
125638
.
dos Reis
G. S.
,
Cazacliu
B. G.
,
Correa
C. R.
,
Ovsyannikova
E.
,
Kruse
A.
,
Sampaio
C. H.
,
Lima
E. C.
&
Dotto
G.
2020
Adsorption and recovery of phosphate from aqueous solution by the construction and demolition wastes sludge and its potential use as phosphate-based fertilizer
.
Journal of Environmental Chemical Engineering
8
,
103605
.
Feng
Y.
,
Luo
Y.
,
He
Q.
,
Zhao
D.
,
Zhang
K.
,
Shen
S.
&
Wang
F.
2021
Performance and mechanism of a biochar-based Ca-La composite for the adsorption of phosphate from water
.
Journal of Environmental Chemical Engineering
9
(
3
),
105267
.
Gupta
S.
,
Sireesha
S.
,
Sreedhar
I.
,
Patel
C. M.
&
Anitha
K. L.
2020
Latest trends in heavy metal removal from wastewater by biochar based sorbents
.
Journal of Water Process Engineering
38
,
101561
.
Hansel
C. M.
,
Lentini
C. J.
,
Tang
Y. Z.
,
Johnston
D. T.
,
Wankel
S. D.
&
Jardine
P. M.
2015
Dominance of sulfur-fueled iron oxide reduction in low-sulfate freshwater sediments
.
The ISME Journal
9
,
2400
2412
.
Jung
K. W.
,
Hwang
M. J.
,
Ahn
K. H.
&
Ok
Y. S.
2015
Kinetic study on phosphate removal from aqueous solution by biochar derived from peanut shell as renewable adsorptive media
.
International Journal of Environmental Science and Technology
12
(
10
),
3363
3372
.
Marshall
J. A.
,
Morton
B. J.
,
Muhlack
R.
,
Chittleborough
D.
&
Kwong
C. W.
2017
Recovery of phosphate from calcium-containing aqueous solution resulting from biochar-induced calcium phosphate precipitation
.
Journal of Cleaner Production
165
,
27
35
.
Mitrogiannis
D.
,
Psychoyou
M.
,
Baziotis
I.
,
Inglezakis
V. J.
,
Koukouzas
N.
,
Tsoukalas
N.
,
Palles
D.
,
Kamitsos
E.
,
Oikonomou
G.
&
Markou
G.
2017
Removal of phosphate from aqueous solutions by adsorption onto Ca(OH)2 treated natural clinoptilolite
.
Chemical Engineering Journal
320
,
510
522
.
Montastruc
L.
,
Azzaro-Pantel
C.
,
Biscans
B.
,
Cabassud
M.
&
Domenech
S.
2003
A thermochemical approach for calcium phosphate precipitation modeling in a pellet reactor
.
Chemical Engineering Journal
94
(
1
),
41
50
.
Pei
L.
,
Yang
F.
,
Xu
X.
,
Nan
H.
,
Gui
X.
,
Zhao
L.
&
Cao
X.
2021
Further reuse of phosphorus-laden biochar for lead sorption from aqueous solution: isotherm, kinetics, and mechanism
.
Science of the Total Environment
792
,
148550
.
Shakoor
M. B.
,
Ye
Z. L.
&
Chen
S. H.
2021
Engineering biochars for recovering phosphate and ammonium from wastewater: a review
.
Science of the Total Environment
779
,
146240
.
Tas
A. C.
&
Bhaduri
S. B.
2004
Chemical processing of CaHPO4•2H2O: its conversation to hydroxyapatite
.
Journal of the American Ceramic Society
87
,
2195
2200
.
Tomul
F.
,
Arslan
Y.
,
Kabak
B.
,
Trak
D.
,
Kendüzler
E.
,
Lima
E. C.
&
Tran
H. N.
2020
Peanut shells-derived biochars prepared from different carbonization processes: comparison of characterization and mechanism of naproxen adsorption in water
.
Science of the Total Environment
726
,
137828
.
Tuna
A. Ö. A.
,
Özdemir
E.
,
Şimşek
E. B.
&
Beker
U.
2013
Removal of As(V) from aqueous solution by activated carbon-based hybrid adsorbents: impact of experimental conditions
.
Chemical Engineering Journal
223
,
116
128
.
Ugwu
E. I.
,
Othmani
A.
&
Nnaji
C. C.
2021
A review on zeolites as cost-effective adsorbent for removal of heavy metals from aqueous environment
.
International Journal of Environmental Science and Technology
.
https://doi.org/10.1007/s13762-021-03560-3
.
Wu
L.
,
Wei
C.
,
Zhang
S.
,
Wang
Y.
,
Kuzyakov
Y.
&
Ding
X.
2019
MgO-modified biochar increases phosphate retention and rice yields in saline-alkaline soil
.
Journal of Cleaner Production
235
,
901
909
.
Yang
S. J.
,
Zhao
Y. X.
,
Chen
R. Z.
,
Feng
C. P.
,
Zhang
Z. Y.
,
Lei
Z. F.
&
Yang
Y. N.
2013
A novel tablet porous material developed as adsorbent for phosphate removal and recycling
.
Journal of Colloid and Interface Science
396
,
197
204
.
Yang
F.
,
Chen
Y.
,
Nan
H.
,
Pei
L.
,
Huang
Y.
,
Cao
X.
,
Xu
X.
&
Zhao
L.
2021a
Metal chloride-loaded biochar for phosphorus recovery: Noteworthy roles of inherent minerals in precursor
.
Chemosphere
266
,
128991
.
Yang
J.
,
Zhang
M.
,
Wang
H.
,
Xue
J.
,
Lv
Q.
&
Pang
G.
2021b
Efficient recovery of phosphate from aqueous solution using biochar derived from co-pyrolysis of sewage sludge with eggshell
.
Journal of Environmental Chemical Engineering
9
(
5
),
105354
.
Yi
C.
&
Chen
Y. C.
2018
Enhanced phosphate adsorption on Ca-Mg-loaded biochar derived from tobacco stems
.
Water Science & Technology
78
(
11
),
2427
2436
.
Zhang
M.
,
Song
G.
,
Gelardi
D. L.
,
Huang
L.
,
Khan
E.
,
Masek
O.
,
Parikh
S. J.
&
Ok
Y. S.
2020
Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water
.
Water Research
186
,
116303
.
Zhu
Y. Y.
,
Tang
W. Z.
,
Jin
X.
&
Shan
B. Q.
2019
Using biochar capping to reduce nitrogen release from sediments in eutrophic lakes
.
Science of the Total Environment
646
,
93
104
.
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