Competitive adsorption behaviour and mechanisms of cadmium, nickel and ammonium from aqueous solution by fresh and ageing rice straw biochars

Journal Pre-proofs Competitive adsorption behaviour and mechanisms of cadmium, nickel and ammonium from aqueous solution by fresh and ageing rice straw biochars Yiyi Deng, Shuang Huang, Caiqin Dong, Zhuowen Meng, Xiugui Wang PII: DOI: Reference:

S0960-8524(20)30122-X https://doi.org/10.1016/j.biortech.2020.122853 BITE 122853

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Bioresource Technology

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26 November 2019 15 January 2020 17 January 2020

Please cite this article as: Deng, Y., Huang, S., Dong, C., Meng, Z., Wang, X., Competitive adsorption behaviour and mechanisms of cadmium, nickel and ammonium from aqueous solution by fresh and ageing rice straw biochars, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech.2020.122853

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Competitive adsorption behaviour and mechanisms of cadmium, nickel and ammonium from aqueous solution by fresh and ageing rice straw biochars Yiyi Deng 1, Shuang Huang1*, Caiqin Dong1, Zhuowen Meng1, Xiugui Wang1 (1 State Key Laboratory of Water Resources and Hydropower Engineering Sciences, Wuhan University, Wuhan 430072, China (First author: [email protected]; Corresponding author: [email protected].)

Abstract In this study, competitive adsorption behaviour and mechanisms of Cd2+, Ni2+ and NH4+ by fresh and artificially ageing biochars produced from rice straw at 400 and 700 ºC (RB400, RB700, HRB400 and HRB700) were investigated. Cd2+ competed with Ni2+ and NH4+ for the overlapped adsorption sites on the biochars. For Cd2+ and Ni2+ adsorption, cation exchange (Qci) and mineral co-precipitation (Qcp) were the primary mechanisms for the low-temperature and high-temperature biochars, respectively. However, the other potential mechanisms (Qco) made the greatest contributions to NH4+ adsorption (> 60%). Cd2+ and Ni2+ competition increased the proportions of mineral co-precipitation (Qcp) and other potential mechanisms (Qco) but decreased that of cation exchange (Qci) mechanism. Biochar ageing increased the contribution of surface complexation (Qcf) mechanism, especially for the low-temperature biochars. This study indicated that biochar aging and types and states of adsorbates should be considered when biochars were applied to remove contaminants. Keywords: cadmium, nickel, ammonium, biochar, mechanism

1

1 Introduction Recently, due to the rapid development of industry and agriculture, human activities have caused the continuous degradation of the surface water quality, including heavy metal contamination and eutrophication, and thus posing a great threat to sustainable agriculture and human health (Cui et al., 2016b; Shi et al., 2009). In water and wastewater, high levels of ammonium will lead to eutrophication (Yang et al., 2018). Cd and Ni are toxic trace elements with high mobility, non-biodegradable property and facile accumulation via food chain, which are harmful to human physiology and other biological systems over the acceptable levels (Cui et al., 2016b; Deng et al., 2019). Ammonium (NH4+) is a common form of reactive nitrogen (N) in farmland. Cd2+ and Ni2+ are widely used in industrial materials, and these heavy metals will be imported to farm land by wastewater irrigation or improper fertilization, and thus NH4+ often coexisted with Cd2+ and Ni2+ in farmland, resulting in combined soil pollution (Cui et al., 2016a; Park et al.,2019; Yin et al., 2018). Traditional methods for the removal of contaminants from the water environment include ion exchange, chemical

precipitation,

filtration,

electrochemical

treatment,

chemical

electrocoagulation, evaporation and adsorption (Cui et al., 2016a; Fu and Wang, 2011). Adsorption is considered as an economical and efficient treatment technology among these. In this method, the selection of appropriate sorbents is the primary factor affecting the treating efficiency (Cui et al., 2016b). In recent years, biochar has been recognized as an environmentally friendly adsorbent with high efficiency and low expenses to immobilize pollutants in water or 2

soils (Ahmad et al., 2014; Hu et al., 2019). Biochar is an desired adsorption material because of its unique physicochemical properties, including high cation exchange, rich surface functional groups and huge specific surface area (Ahmad et al., 2014). Large amounts of waste biomasses can be converted to biochar to remove pollutants from wastewater or soil as a sorbent (Hu et al., 2019; Xu et al., 2014; Wu et al., 2019). The feedstocks, pyrolysis temperatures and the treatment before or after pyrolysis for the biochars influenced its properties and potential value in application (Kwak et al., 2019; Park et al., 2109). Kwak et al. (2019) claimed that canola straw biochar had highest and sawdust biochar had lowest Pb adsorption capacity, which increased with the increasing pyrolysis temperature and steam activation. Yin et al. (2018) reported that the biochar properties also obviously influence NH4+-N adsorption performance and that the adsorption amounts for NH4+-N were 3.45, 0.91 and 1.62 mg g-1 by sesame straw biochars prepared at 300, 500, and 700 ºC, respectively. Since rice straw is a common and easily accessible agriculture waste in China and traditional disposal of rice straw was open field burning causing air pollution and waste of resources, biochars produced from rice straw is considered to be economic, environmentally friendly and sustainable (Deng et al., 2019). Thanks to the improvement of instrumental technology, many studies investigated the mechanisms between contaminants and biochars (Ni et al., 2019; Yang et al., 2019). Researches have proved that the adsorption mechanisms for inorganic contaminants on biochars can be summarized as ion exchange, complexation, precipitation or co-precipitation with minerals, cation-π reaction, electrostatic attraction and physical 3

adsorption (Cui et al., 2016b; Wang et al., 2018; Wu et al., 2019). The adsorption mechanisms for inorganic contaminants on biochars in single metal conditions have been studied by many researchers. For instance, the cadmium adsorption behavior and mechanisms on biochars derived from corn straw, bamboo and swine manure at 300– 700 °C were investigated by Wang et al. (2018). Wu et al. (2019) claimed that the adsorption mechanisms of Pb and Cd on camellia seed husk biochars pyrolyzed at different temperatures mainly involved ion exchange, oxygen functional groups (OFGs) complexation, Pb/Cd–π interactions, and precipitation with minerals. Cui et al. (2016a) reported that cation exchange, the formation of magnesium ammonium phosphate (MAP) compounds, and oxygen-containing functional groups contributed to ammonium sorption on biochars. Hu et al. (2019) also indicated that electrostatic attraction is an important mechanism for NH4+-N adsorption on biochars because biochar is electronegative and saturated with cations (K+, Ca2+, Na+ and Mg2+). However, since inorganic contaminants are commonly concomitant in water or soils and their mutual competition affected the adsorption on biochars (Deng et al., 2019; Park et al., 2016), the investigation of competitive pollutants adsorption by biochars has attracted worldwide attention. For example, Ni et al. (2019) investigated the competitive adsorption of heavy metals in aqueous solution onto biochar derived from anaerobically digested sludge. Chen et al. (2011) focused on the competitive adsorption behaviour between Cu and Zn by hardwood and corn straw biochars. However, most studies are focused on competition among heavy metals adsorbed on biochars, information about the influence of competition between heavy metals and ammonium on the adsorption 4

mechanisms is limited. A precondition for biochars recommended as considerable sorbents for the contaminants removal is that its great adsorption capacity for adsorbates can keep for a long time (Ren et al., 2018). Although stable properties have been reported for biochars, slow changes are supposed to occur when exposed to the environment. Previous studies reported that after biochars exposed to the environment, its pH, zero point charge and carbon content decreased but oxygen content, CEC (cation exchange capacity) and surface acidic functional groups increased (Tan et al., 2019; Zhao et al., 2015). It was reported that the biochar aging process was affected by the ambient temperature that high temperature may accelerate the aging of biochars (Guo et al., 2014; Mia et al., 2017). Researches have shown that ageing process usually occurred on the surface of biochars, which will influence the heavy metal adsorption behaviour and mechanisms of biochars (Guo et al., 2014; Zhao et al., 2015). At present, studies about biochar aging mainly focused on the changes of physiochemical properties and adsorption capacity and most biochar aging processes were conducted by artificial methods due to its very slow process in natural conditions (Tan et al., 2019). For example, Tan et al. (2019) studied the impacts of three artificial ageing methods on physical and chemical properties and Pb adsorption amounts for various biochars. Mia et al. (2017) reported that biochar aging had influences on the functionality of biochars and affected the P and N adsorption behavior. The adsorption mechanisms for heavy metals largely depend on biochars’ properties and the adsorbed solution, such as solution pH, concentration of ions in solution and biochar surface characteristics, which are affected by ageing 5

process (Guo et al., 2014; Wu et al., 2019; Zhao et al., 2015). However, limited information about quantitative adsorption mechanisms of heavy metals and ammonium for ageing biochars is available. Therefore, we assumed that the proportion of mechanisms for cadmium, nickel and ammonium adsorbed on biochars varied in co-existed adsorption conditions and biochar aging affected the contribution of adsorption mechanisms to heavy metals and ammonium. In this study, biochars were pyrolyzed from rice straw at 400 °C and 700 °C (RB400 and RB700) and artificially aged at constant temperature (50 ± 1°C) and humidity (40%) for one year, and physicochemical properties of fresh and aged biochars were characterized. Adsorption kinetics and isotherms were carried out for single and binary cadmium, nickel and ammonium on fresh biochars (RB400 and RB700) and ageing biochars (HRB400 and HRB700). The objectives of this study were: i) to investigate the impact of biochar aging on adsorption characteristics of Cd2+, Ni2+ and NH4+ in single and binary adsorption conditions; and ii) to analyze the effect of biochar aging on the quantitative distributions of cadmium and nickel adsorption mechanisms in single and binary conditions and ammonium adsorption mechanisms.

2 Materials and methods 2.1 Biochar preparation The preparation process of rice straw biochars at 400 °C and 700 °C (RB400 and RB700) has been reported in our previous work (Deng et al., 2019). For the artificially ageing experiment of biochars, RB400 and RB700 samples were placed in a container and incubated at constant temperature (50 ± 1°C) and humidity (40%) in constant 6

temperature humidity chamber (Boxun Company, Shanghai, China). After 360 day aging time, samples were dried at 80 ± 1°C. After this, the treated biochar samples were called ageing biochars and labelled as HRB400 and HRB700, respectively. Biochars were washed with 0.1 mol L-1 HCl and the ratio of biochar to HCl was 50 g biochar L-1 to get the demineralized biochars. 2.2 Characterization of biochars The yield, ash content (combusted in an open crucible for 6 h at 750 °C), pH (1:20 water extraction) of biochars were measured. The point of zero charge (pHpzc) for RBs was measured according to the pH drift method. The surface area, pore volume and pore diameter were measured by an ASAP-2020 M analyser (Micromeritics Instrument Corp., NoRBross, GA, USA) via N2 adsorption multilayer theory. BET equation was used to calculate the surface area of biochars. The elemental composition (CHNO) was measured by a Vario EL cube CHNS/O analyser (Elementar analysis system GmbH, Hanau, Germany). Scanning electron microscopy (SEM) (Carl Zeiss, SIGMA, England) was used to observe the microstructure and morphology on biochar surface. X-ray diffraction (XRD) (PANalytical B.V., Netherlands) and Fourier transform infrared spectroscopy (FTIR) (Thermo Nicolet Corporation, Waltham, MA, USA) were applied to identify the crystalline minerals and functional groups on the biochar surfaces. 2.3 Adsorption experiments Stock solutions (1000 mg L-1) for Cd2+, Ni2+ and NH4+ were prepared by dissolving a certain mass of Cd (Cd(NO3)2·4H2O), Ni (Ni(NO3)2·5H2O) and NH4Cl in the background solution (0.005 mol L-1 CaCl2), and working solutions for Cd2+, Ni2+ and NH4+ used in all 7

experiments were diluted from the respective stock solutions with the background solution (0.005 mol L-1 CaCl2). All batch experiments in this study were carried out under the same conditions. The ratio of biochars (RB400, RB700, HRB400 and HRB700) to salt solution (Cd2+, Ni2+ and NH4+) in batch experiments was 1:1000 (g : ml), as 0.02g of biochar was added into 20ml of salt solution. The initial pH of salt solutions was adjusted to 5.5 ± 0.1 by 0.05 M NaOH or HNO3. All adsorption experiments were conducted under the same temperature condition (25 ± 1 °C). For the adsorption kinetics, studied biochars (0.02g) were mixed with salt solutions (100 mg L-1 for single Cd2+, single Ni2+ and binary Cd2+and Ni2+ , 50 mg L-1 for NH4+) and shaken in a digital water bath oscillator for 0, 5, 10, 30, 60, 90, 120, 240, 480, 720, 1440, or 2880 min, respectively. Then, the samples were centrifuged at 4000 rpm for 20 min and filtered through a 0.45-μm syringe filter. Flame atomic absorption spectrometry (Agilent 240 Duo AA, USA) and segmental flow analyzer (AA3 Auto Analyze, Brown Ruby, German) ware used to analyse the metal ions (Cd2+ and Ni2+) and NH4+-N concentrations in the filtrates, respectively. For isotherm experiments, the initial concentrations of 0, 2, 5, 10, 20, 40, 60, 80 100 and 200 mg L-1 were set for Cd2+, Ni2+ and NH4+ in single and binary conditions. Samples equilibrated for 24h were collected and the next treatment was the same with the adsorption kinetics experiment. To determine the contribution of minerals on biochars, the original and demineralized biochars were equilibrated with the salt solutions (100 mg L-1 for single Cd2+, single Ni2+ and binary Cd2+and Ni2+ , 50 mg L-1 for NH4+) and then the adsorbed Cd2+, Ni2+ and NH4+ amounts were measured. The original biochars were mixed with 8

0.005 mol L-1 CaCl2 (blank) and studied salt solutions, respectively, the contributions of the cation exchange mechanism to Cd2+, Ni2+ and NH4+ adsorption were calculated by subtracting the released amounts of cations (K+, Ca2+, Na+ and Mg2+) in the blank from that in the biochars mixed salt solutions. In the experiment, pH values of mixed solution were measured at initial stage and after adsorption equilibrium, respectively. The adsorption capacity for Cd2+, Ni2+ and NH4+ on the original and demineralized biochars after equilibrium were measured. ICP-MS (Perkin Elmer Ltd., USA) was used to analyse the K+, Ca2+, Na+ and Mg2+ concentrations in the filtrates for treated samples. The samples in the experiment were filtered after equilibrium with Cd2+, Ni2+ and NH4+, the Cd2+/Ni2+/ NH4+-adsorbed biochars were collected to SEM-EDS, XRD and FTIR measurements. 2.4 Quantitative contributions of adsorption mechanisms The contributions of cation exchange (Qci), mineral co-precipitation (Qcp), complexation with surface oxygen-containing functional groups (Qcf) and other potential mechanisms (Qco) to Cd2+, Ni2+ and NH4+ adsorption on the fresh and ageing biochars were identified. The details of the calculation method were described in our previous work (Deng et al., 2019). 2.5 Statistical analysis The amounts of Cd2+/Ni2+/NH4+ adsorbed (Qt) were calculated according to Eq. (1): Qt 

(C0  Ct )V m

(1)

where Qt (mg g-1) is the adsorbed amounts at time t, C0 (mg L-1) and Ct (mg L-1) are the metal concentration of solution before the adsorption and at time t, respectively, V (L) 9

is the volume of mixed solution, and m (g) is the dry mass of biochar. The isotherm data of Cd2+/Ni2+/NH4+ adsorption on the biochars was fitted by the Langmuire (LM, Eq. (2)) and Freundlich model (FM, Eq. (3)) (Tran et al., 2017):

Qm K LCe 1  K LCe

Qe 

(2)

Qe  KFCen

(3)

where Ce (mg L-1) is the concentration of Cd2+/Ni2+/NH4+ in the solution after equilibration, Qe (mg g-1) is the adsorbed amount of Cd2+/Ni2+/NH4+ on the biochar after equilibration, Qm (mg g-1) is the maximum Cd2+/Ni2+/NH4+ adsorption capacity for the biochar, KL (L mg-1) and KF (mg g-1) (mg L-1)-n are the adsorption coefficients of the LM and FM, respectively, and n (dimensionless) is a constant fitted by the FM relevant to the adsorption heterogeneity of biochar surface. The isotherm models for adsorption in binary system, the modified Langmuire model (Eq. (4)) and the modified Freundlich model (Eq. (5) and Eq. (6)), used in this study were described as follows, respectively (Aksu et al., 2002; Li et al., 2011).

Qe,i 

Qm,i K L,i (Ce,i / i ) 1  K L,i (Ce,i / i )  K L, j (Ce, j /  j )

Qe,1  Qe,2 

KF,1Ce,1n1x1 Ce,1x1  y1Ce,2z1 KF,2Ce,2n2 x2

Ce,2x2  y2Ce,1z2

(4)

(5)

(6)

where Qm,i (mg g-1) and KL,i (L mg-1) are the maximum capacity and Langmuire adsorption constants from the corresponding individual Langmuire isotherm, ηi and ηj are correction parameters estimated from binary adsorption data for the first and second 10

adsorbates, respectively, KF,1, KF,2 (mg g-1) (mg L-1)-n and n1, n2 (dimensionless) are constants fitted by the corresponding individual Freundlich equations and the other parameters (x1, y1, z1 and x2, y2, z2) are the modified Freundlich adsorption constants for the first and the second adsorbates. The distribution coefficient (Kd, L g-1), an important parameter for comparing the adsorption capacities of biochars for heavy metals, was calculated according to Eq. (7) (Park et al., 2016):

Kd 

Qe Ce

(7)

The adsorption kinetics of Cd2+/Ni2+/NH4+ by biochars were fitted to the pseudo-first-order model (PFO, Eq. (8)) and the pseudo-second-order model (PSO model, Eq. (9)) (Tran et al., 2017), which are described as follows:

Qt  Qe (1 ek1t )

(8)

k2 Qe 2 t Qt  1  k2 Qe t

(9)

where k1 (h-1) and k2 (g mg-1 h-1) are the rate constants of the PFO model and PSO model, respectively. In this study, the batch experiments for cadmium, nickel and ammonium adsorption were carried out in triplicate. The fittings of cadmium, nickel and ammonium adsorption kinetics and isotherms data were performed in Origin 9.0.

11

3

Results and discussion

3.1 Properties of the fresh and ageing biochars For the studied fresh and ageing biochars, the physicochemical properties (Table 1) and spectral properties (SEM, XRD and FTIR) were analysed. All studied biochars were alkaline, and the pH values of ageing biochars were lower than fresh biochars. The ash contents and pHpzc (point of zero charge) of biochars increased with increasing pyrolysis temperature and has little differences between ageing and fresh biochars. With increasing pyrolysis temperature, the BET surface area and pore volume of biochars dramatically increased, while the average pore diameter of biochars decreased. Especially, the BET surface area of RB700 biochar is significantly very high, 161.18 m2 mg-1, however, the average diameter is significantly low, 24.9 Å. It can be explained by reason that the evolution of water and CO2 at lower temperatures and H2, CH4, and CO at higher temperatures resulted in micropores or even nanostructures on biochars at high carbonization temperature, which could be confirmed by the results of SEM images and thermogravimetric analysis for biochars. Comparing to the fresh biochars, the contents of C and N were lower for ageing biochars but the contents of O, H and S were higher, thus resulting in the increase of H/C, O/C and (O+N+S)/C ratios. With increasing pyrolysis temperature, the C content of the biochars increased, whereas O, H and N contents decreased. High-temperature biochars with lower H/C and O/C values were highly carbonized with more aromatic structures and less surface polar functional groups. Researches have reported the significant positive correlation between the O/C and H/C atomic ratio and metal adsorption amounts for biochars, which were related to 12

the functional groups complexation with metal ions and π interaction with metal ions, respectively (Wang et al., 2018; Xu et al., 2013). 3.2 Kinetics of Cd2+, Ni2+ and NH4+ adsorption by biochars The kinetics of Cd2+ and Ni2+ adsorption by the different biochars in the unitary and mixed systems and NH4+ adsorption are presented in Fig. 1, and the fitting parameters of the pseudo-first order (PFO) model, the pseudo-second order (PSO) model are listed. The pseudo-first-order kinetic model presumed that adsorption occurred prior to physical diffusion, while the pseudo-second order model assumed that chemical adsorption including ion exchange, complexation and covalent forces may be the rate-limiting step for the process (Ho and McKay, 1998). As shown in Fig. 1, with the increasing adsorption time, the Cd2+, Ni2+ and NH4+ adsorption capacity of biochars increased gradually. The Cd2+, Ni2+ and NH4+ adsorbed on biochars all reached equilibrium within 48 h. And the adsorption process can be divided into three stages: 1) quick adsorption (0-240 min), 2) slow adsorption (240-720 min), and 3) plateau (720-2880 min). The adsorption amounts of Cd2+, Ni2+ and NH4+ by biochars for the 1) quick adsorption stage accounted for about 81.13~93.01% of the total adsorption amounts. The Cd2+, Ni2+ and NH4+ adsorption kinetics data for studied biochars were better fitted by the pseudo-second order (PSO) model, with R2adj values ranging from 0.87 to 0.98. Besides, the calculated Qe(cal.) values fitted by the PSO model were approximately the experimental Qe(exp.) values. Comparing with the fresh biochars RB400 and RB700 (Fig. 1(a)(c)(e)), the equilibrium amounts for Cd2+, Ni2+ and NH4+ adsorbed by the ageing biochars HRB400 and HRB700 (Fig. 1(b)(d)(f)) were 13

much lower, while the equilibrium time between them was similar. The same trend was exhibited by the kinetics of cadmium and nickel adsorption on the biochars in the unitary and mixed systems (Fig. 1). This result indicates that the PSO equation could predict the Cd2+, Ni2+ and NH4+ kinetic adsorption processes, suggesting the potential contributions of chemical interactions, e.g., cations exchange, surface complexation and mineral precipitation to the Cd2+, Ni2+ and NH4+ adsorption by the studied biochars (Ho and McKay, 1998). 3.3 Isotherms of Cd2+, Ni2+ and NH4+ adsorption by biochars The adsorption isotherms for Cd2+, Ni2+ and NH4+ on the fresh and aged biochars were fitted by the Freundlich and Langmuire models in single system and described by the modified Freundlich and Langmuire models in binary system (Fig. 2). The Langmuire model is considered to be a monolayer adsorption model, but the Freundlich model is more suitable for the heterogeneous multilayer adsorption (Tan et al., 2019; Tran et al., 2017). Individual adsorption constants may not define exactly the multi-component adsorption behaviour of metal ion mixtures. For that reason, better accuracy may be achieved by using modified isotherms related to the individual isotherm parameters and to correction factors. The results indicated that the isotherms data for cadmium, nickel and ammonium adsorption by the studied biochars might be well fitted by the Freundlich and Langmuire models, with the values of adjusted coefficient of determination (R2adj) ranging between 0.87 and 1.00. For the adsorption isotherms of Cd2+ and Ni2+, it was slightly better fitted by the Freundlich model (FM) than the Langmuire model (LM) for the fresh biochars 14

but it was fitted better by Langmuire model for the ageing biochars. Similarly, the NH4+ adsorption isotherms for the ageing biochars were better fitted by Langmuire model; however, for the fresh biochars, NH4+ adsorption isotherms of RB400 fitted better with Freundlich equation (R2adj = 0.99) and that of RB700 was better fitted by Langmuire equation (R2adj = 0.99). It can be explained by the heterogeneous physical structures and various chemical properties on biochar surfaces (e.g., surface charges and carbon crystallinity); the ageing process might have positive effects on the distribution homogeneity of Cd2+, Ni2+ and NH4+ adsorption sites on biochars by affecting the surface physicochemical properties of biochars (Chang et al., 2019; Tan et al., 2019). The extents of Cd2+, Ni2+ and NH4+ adsorption by HRB400 and HRB700 were lower than RB400 and RB700, suggesting that the ageing process of biochars diminished the adsorption capacity of biochars, which was consistent with Chang et al. (2019) and Guo et al. (2014). The isotherms for cadmium and nickel adsorbed by the fresh and ageing biochars were influenced by competition between them. According to Fig. 2, compared with single Cd2+ and Ni2+ system, the linear parts were shorter and equilibrium amounts for the adsorption isotherms were lower in the mixed Cd2+ and Ni2+ condition. For example, the Qm values for cadmium and nickel on RB400 reduced from 37.14 and 27.31 mg g-1, respectively, in the unitary condition to 24.22 and 25.20 mg g-1, respectively, in the mixed condition. But the decrement of cadmium adsorption amounts resulting from competition was greater than that of nickel. In addition, according to the correlation coefficients of competitive models, it was slightly better fitted by the modified Freundlich model for the fresh biochars but it was better described by the modified 15

Langmuire model for the ageing biochars. It was reported that metal ions with higher interaction coefficient η had smaller depressive effect on the adsorption of the other ions (Li et al., 2011). The interaction coefficient η in this study followed the order of Ni < Cd for RB400, suggesting that the nickel exerted more inhibitory effect on the adsorption of cadmium on RB400. However, the opposite trend was observed for the other studied biochars (RB700, HRB400 and HRB700). 3.4 Competitive behaviour of Cd2+ and NH4+ adsorbed on biochars The competitive adsorption of Cd2+ and NH4+ on the fresh and ageing biochars (RB400, RB700, HRB400 and HRB700) was measured in this study. As presented in Table 2, the adsorption amounts and selectivity sequences for Cd2+ and NH4+ on the biochars were influenced by the concentrations of coexisted Cd2+ ions. As the initial concentration of NH4+ maintained 50 mg L-1 and the initial concentrations of Cd2+ were set as 0, 5, 10, 20, 50, and 100 mg L-1, the NH4+ adsorption amounts by RB400, RB700, HRB400 and HRB700 decreased from 2.55, 4.17, 1.89 and 3.31 mg g-1 to 1.23, 1.55, 1.09 and 1.03 mg g-1, respectively. With increasing concentrations of Cd2+, the Cd2+ adsorption amounts and the total adsorption amounts of Cd2+ and NH4+ by all biochars increased but that of NH4+ gradually decreased. The competition between Cd2+ and NH4+ diminished the adsorption capacity of Cd2+ and NH4+ by biochars. The results further indicated that the adsorption sites for Cd2+ and NH4+ on biochars partly intersected and that a small part of Cd2+ tended to substitute NH4+ during adsorption process with the increasing concentrations of Cd2+ in the solution, but most of Cd2+ were adsorbed onto new sites on biochars (Deng et al., 2019; Park et al., 2016). Besides, the Qe values of Cd2+ 16

and NH4+ were larger for RB700 and HRB700 than RB400 and HRB400, respectively. Comparing the ageing biochars with fresh biochars, the equilibrium Cd2+ and NH4+ adsorption amounts were lower and the decrease amplitude for Cd2+ and NH4+ adsorbed on HRB700 were larger than HRB400 at Cd2+ concentration ≥ 50 mg L-1, implying that the ageing process had greater influence on the adsorption capacity of high-temperature biochars. As exhibited in Table 2, with increasing Cd2+ concentrations, the Kd values for NH4+ and Cd2+ decreased, and the Kd values for high-temperature biochars (RB700 and HRB700) were much larger than low-temperature biochars (RB400 and HRB400).α is the selectivity coefficient reflecting the a selective sequences in mixed adsorbates adsorption conditions (Deng et al., 2019; Li et al., 2015). Accordingly, the greater α values ( > 1) in this study indicated that Cd2+ was more selective to the adsorption on biochars than NH4+ and that biochars are preferential to adsorb Cd2+ over NH4+. The equation of Kd was: Kd = Qe / Ce. Thus, when the initial concentration of Cd2+ increased in solution, the equilibrated Cd2+ concentration (Ce) increased but the Kd values of Cd2+ decreased. The smaller Kd value of NH4+ was ascribed to decreasing Qe values of NH4+ resulting from the competition of concomitant Cd2+. Moreover, the decrement of Kd values of Cd2+ was greater compared to NH4+. It implied that the influence of initial concentration of adsorbates was more significant than its mutual competition on its corresponding Kd values. Competitive adsorption behaviour of contaminants on biochars is affected by many factors, including radii of the hydrated ions, hydrolysis constant, metal electronegativity, and the surface charge (Cui et al., 2016b; Shi et al., 17

2009). In comparison, cadmium is divalent while ammonium is monovalent, and the radii of cadmium ion (0.87Å) is much smaller than that of ammonium (1.43Å). Thus the adsorption capacity of cadmium is larger than that of ammonium, which was consistent with the results of many researches (Cui et al., 2016a; Cui et al., 2016b; Hou et al., 2016). 3.5 Adsorption mechanisms of Cd2+, Ni2+ and NH4+ by biochars 3.5.1 Cation exchange The cations including K+, Ca2+, Na+ and Mg2+ are retained on biochars by direct electrostatic attraction and forming complexes with carboxyl and hydroxyl groups (e.g., -COOM, -R-O-M) (Cui et al., 2016a). These ions can be exchanged by the adsorption process of Cd2+, Ni2+ and NH4+, and thus cation exchange is a possible mechanism for the adsorption of Cd2+, Ni2+ and NH4+ by biochars in this study. In order to elaborate this, the contents of cation (K+, Na+, Ca2+, Mg2+) in the solution of biochar (RB400, RB700, HRB400 and HRB700) before and after adsorption of Cd2+, Ni2+ and NH4+ were measured. It showed that the net amounts of divalent cations released were much larger than monovalent cations. As we known, K+ and Na+ (monovalent cations) are electrostatically adsorbed to the negatively charged sites on outer-sphere surface of biochars, while Ca2+ and Mg2+ (divalent cations) are mainly adsorbed on biochars by inner-sphere complexation with oxygen-containing functional groups and precipitation (such as Ca/MgCO3) (Cui et al., 2016a; Shen et al., 2017). The total net released amounts of K+, Na+, Ca2+, Mg2+ during Ni2+, Cd2+ and NH4+ adsorption in the unitary system were larger for RB400 and HRB400 than for RB700 and HRB700, respectively. It 18

might result from the formation of crystalline minerals and decarboxylation at high temperature, which can lead to lower CEC and less negatively charged carboxylate groups for the high-temperature biochars (Wang et al., 2018; Xu et al., 2013). Besides, there were greater net released amounts of K+, Na+, Ca2+, Mg2+ after the adsorption of nickel by biochars than cadmium, indicating that cation exchange mechanism contributed more to the nickel adsorption on biochars. Comparing with the fresh biochars, the net released amounts of K+, Na+, Ca2+, Mg2+ after Cd2+, Ni2+ and NH4+ adsorption by the ageing biochars obviously decreased, which may result from the lower CEC and specific surface areas in ageing biochars (Chang et al., 2019; Tan et al., 2019). 3.5.2 Mineral co-precipitation Previous researches have claimed that large amount of minerals contained in biochars can precipitate or co-precipitate with Cd2+, Ni2+ and NH4+ (Cui et al., 2016b; Ding et al., 2016). To identify the role of minerals played to Cd2+, Ni2+ and NH4+ adsorption by biochars, the adsorption capacity for original and demineralized biochars were compared. Dramatic decreases for the Cd2+, Ni2+ and NH4+ adsorption capacities by the demineralised biochars (ARB400, ARB700, AHRB400 and AHRB700) suggested the great contribution of minerals to biochars adsorption. Furthermore, the decrement for the adsorption capacity of Cd2+, Ni2+ and NH4+ was larger for the high-temperature biochars. And the contributions of minerals were affected by the competition between cadmium and nickel. Cadmium and nickel can precipitate or co-precipitate with the CO32−, PO43− and OH− released from biochars and the adsorption of NH4+ by biochars may result from the formation of magnesium ammonium phosphate (MAP) compounds 19

(Liu and Fan, 2018; Wang et al., 2018). Considerable scattered white granular crystals were observed on the biochar surfaces after Cd2+, Ni2+ and NH4+ adsorption in the SEM image, and its elemental composition investigated by EDS spectrum confirmed the presence of Cd, Ni and N, respectively. Furthermore, According to the biochar analysis by EDS, the atom% Cd, Ni and N values for the biochar surface sites were lower for the RB400 and HRB400 (low-temperature biochars) than RB700 and HRB700 (high-temperature biochar). Similarly, the atom% Cd, Ni and N values in the corresponding EDS spectra were lower for the ageing biochars (HRB400 and HRB700) than for the fresh biochar (RB400 and RB700). CdCO3 phases were clearly identified in the XRD analysis for the Cd-loaded RB400 and RB700, suggesting the observed white granular precipitates were CdCO3. Interestingly, for ageing biochars HRB400 and HRB700, there were CdCO3 phases identified in single-Cd adsorption condition but no Cd crystalline minerals were observed in binary adsorption system (coexisted cadmium and nickel). It reflected that the contributions of minerals to the cadmium and nickel adsorption on the ageing biochars were influenced by its competition. However, for the Ni-loaded and NH4+-loaded biochars, though white granular crystals were found by SEM-EDS, no Ni and N mineral phases were observed in corresponding XRD images. It may be explained by the reason that Ni2+ and NH4+ formed amorphous precipitates on biochar surface or that its concentrations were under the minimum detect limit of XRD. 3.5.3 Surface complexation with oxygen-containing functional groups Biochar has plentiful oxygen-containing functional groups (OFGs, e.g., -OH, -COOH, -R-OH) on its surface, which can form complex compound with adsorbates, 20

and thus it was considered to be a crucial adsorption mechanism for biochars (Cui et al., 2016a; Xu et al., 2013). The surface complexation of OFGs on biochar with cations usually accompanied with the release of H+, resulting in the decrease of pH values in solution. However, the release of alkaline minerals in biochar may lead to an increase in the pH value of the solution. Therefore, in order to minimize the disturbance by minerals, biochars were required to be demineralized by an acid dipping process (Session 2.1). For the acidified biochars, a pH decrease in the solution was found after Ni2+, Cd2+, Cd2++Ni2+ and NH4+ adsorption relative to the corresponding blank systems, and there were larger decrease of pH values for RB400 and HRB400 than RB700 and HRB700, implying that surface complexation with OFGs made more contributions to the adsorption capacity of the fresh and low-temperature biochars than the ageing and high-temperature biochars. On the other hand, XPS and FTIR were used by many researchers to qualify the surface complexation with oxygen-containing functional groups on biochars (Cui et al., 2016a; Shen et al., 2017; Wang et al., 2018). In this study, the fresh and ageing biochars (RB400, RB700, HRB400 and HRB700) loaded with Cd2+, Ni2+, Cd2++Ni2+ and NH4+ were qualified by FTIR analysis. After the adsorption of Cd2+ and Ni2+ in unitary and binary conditions, the –OH and C-O adsorption peaks at 1315 cm-1 weakened for RB400 and HRB400 (the low-temperature biochars) and disappeared for RB700 and HRB700 (the high-temperature biochars). However, there were no obvious variation for the adsorption peaks at 1315 cm-1 after NH4+ adsorption. New bands at 1385 cm-1 were observed for both fresh and ageing biochars and that were more prominent for biochars loaded with NH4+, likely attributing to the surface complexation 21

with carboxyl (-COOH) and/or hydroxyl (-OH) functional groups. 3.5.4 Other potential mechanisms Besides the above adsorption mechanisms for Cd2+, Ni2+ and NH4+, other potential mechanisms could also exist, for example, cation-π interactions, electrostatic attraction and physical adsorption (Deng et al., 2019; Tran et al., 2017). The cation-π interactions is attributed to the electrostatic interaction between aromatic rings and cations on alkaline carbon adsorbents (biochar, activated carbon and so on) (Tran et al., 2017). Biochar is highly aromatic and rich in cyclic aromatic structure, so it can be used as π-donor to combine with cations (Xu et al., 2014). The coordination with π electrons will be inhibited by the increasing pH values of solution due to the deprotonation of charged amino functional group and enol functional group at higher pH, and thus the capacity of the electron acceptor of the ox tetracycline molecule is weakened (Xu et al., 2013). The FTIR analysis for the biochars after adsorbed exhibited a weakened intensity of β-pyridine at 780 cm-1. The C=O and C=C stretching bands at 1580-1650 cm-1 presented a shift and transformed width and showed different variations between fresh and ageing biochars, suggesting the contribution of cation-π interactions to Cd2+, Ni2+ and NH4+ adsorption. Besides, the stronger variation intensity of peaks was observed for the NH4+-loaded biochars than for the Cd2+ or Ni2+-loaded biochars. Electrostatic attraction depends on the pH values of solution and pHpzc (point of zero charge) values of biochars (Mahdi et al., 2018). The pHpzc values for RB400, RB700, HRB400 and HRB700 were 2.68, 5.30, 2.54 and 5.11, respectively (Table 1). Accordingly, when the solution pH is > pHpzc, biochar surfaces are negatively charged resulting from the 22

dissociation or ionization of surface acidic groups (e.g., phenolic and carboxyl groups); thus, electrostatic attraction is expected to generate between positively charged cations and negatively charged functional groups. Physical adsorption is dependent on the micropore structures and specific surface areas of biochars (Tran et al., 2017). Because of the improved micropore structure and larger specific surface area of the high-temperature, fresh biochars (Table 1), physical adsorption might play a more important role in the adsorption of RB700 and HRB700. 3.6 Quantitative contribution of each adsorption mechanism In this section, the quantitative contributions of cation exchange (Qci), mineral co-precipitation (Qcp), surface complexation (Qcf) and other potential mechanisms (Qco) to Cd2+, Ni2+ and NH4+ adsorption by the fresh and ageing biochars (RB400, RB700, HRB400 and HRB700) in the unitary and binary conditions were estimated (Fig. 3). Accordingly, compared with RB400 and HRB400, the values of Qci and Qci/Qc for RB700 and HRB700 decreased. The Qci and Qci/Qc values of cadmium and nickel are larger than that of NH4+ all biochars, indicating that cation exchange mechanism contributed more to cadmium and nickel adsorption than to ammonium adsorption. For instance, the Qci/Qc values of cadmium on RB400 and RB700 were 56.1% and 26.1%, respectively, which were much larger than that of NH4+ 8.5% and 3.6%, respectively (Fig. 3). Moreover, the Qci/Qc values in the binary (Cd+Ni) conditions were smaller than that in the unitary conditions; similarly, compared to the fresh biochars, the Qci and Qci/Qc values for the ageing biochars were obviously lower. In contrast, the contributions of mineral co-precipitation (Qcp and Qcp/Qc) were larger for the high-temperature biochars, and the 23

Qcp and Qcp/Qc values in the binary (Cd+Ni) conditions were larger than that in the unitary conditions. The Qcp and Qcp/Qc values of Cd2+, Ni2+ and NH4+ were in the order Cd2+ > Ni2+ > NH4+. The Qcp values for the fresh biochars were obviously larger than the aging biochars, but the Qcp/Qc values for the fresh biochars were obviously lower than the aging biochars. Competition between Cd2+ and Ni2+ and biochar ageing process might reduce the relative contribution of cation exchange mechanism to Cd2+, Ni2+ and NH4+ adsorption. The values of Qcf and Qcf/Qc of RB400 and HRB400 (the low-temperature biochars) were larger than that of RB700 and HRB700 (the high-temperature biochar), and these values for the ageing biochars were much larger than the fresh biochars. By contrast, surface complexation (Qcf) only made a small part of Cd2+, Ni2+ and NH4+ adsorption on biochars, especially for RB700 and HRB700. Therefore, the impacts of competition between Cd2+ and Ni2+ on the Qcf values is hard to determine. The Qco/Qc values for cadmium and nickel decreased with the pyrolysis temperature of biochars, but it showed the opposite trend for NH4+. Relative to the fresh biochars, larger Qco and Qco/Qc values for Cd2+, Ni2+ and NH4+ were reported for the ageing biochars. Besides, the Qco and Qco/Qc values in the binary (Cd+Ni) conditions were larger than that in the unitary conditions. These results suggested that for cadmium and nickel adsorption, cation exchange (Qci) was the dominant mechanism for RB400 and HRB400 (the low-temperature biochars) (ranging between 40.5% and 56.5%) and mineral co-precipitation (Qcp) was foremost for the high-temperature biochars (RB400 and HRB400) (accounting for about 59.1%-79.2%). However, the other potential mechanisms (Qco) made the greatest 24

contributions to ammounium adsorption on all studied biochars (accounting for > 60%). The mechanism of surface complexation (Qcf) seem to only militate in low-temperature biochars (RB400 and HRB400) and be almost negligible in the high-temperature biochars (RB400 and HRB400). Competition between cadmium and nickel reduced the proportions of cation exchange mechanism while increased the relative contributions of mineral co-precipitation (Qcp) and the other potential mechanisms (Qco) for the cadmium and nickel adsorption on biochars. The biochar ageing process only increased the contributions of surface complexation (Qcf) to cadmium, nickel and ammonium adsorption, especially on the low-temperature biochars, but decreased the contributions of the other three adsorption mechanisms (cation exchange (Qci), mineral co-precipitation (Qcp) and other potential mechanisms (Qco)).

4

Conclusions Cadmium competed with nickel and ammonium for binding sites and its adsorption

sites on the biochars largely overlapped. Cation exchange and mineral co-precipitation were the dominant mechanisms of cadmium and nickel adsorption (ranging between 40.5% and 79.2%), however, the other potential mechanisms dominated ammonium adsorption (accounting for > 60%). Competition between cadmium and nickel decreased the contributions of cation exchange mechanism but increased that of minerals precipitation and other potential mechanisms. Biochar aging increased the contribution of surface complexation mechanism. This study proves theoretical basis to evaluate the effectiveness of biochar application to practical contamination treatment.

25

Acknowledgements This work was jointly supported by the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2015BAD05B02) and the National Natural Science Foundation of China (No. 51109166)

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562, 517-525. 6.

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14. Kwak, J.H., Islam, M.S., Wang, S., Messele, S.A., Naeth, M.A., El-Din, M.G., Chang, S.X., 2019. Biochar properties and lead(II) adsorption capacity depend on feedstock type, pyrolysis temperature, and steam activation. Chemosphere 231, 393-404. 15. Li, K., Wang, Y., Huang, M., Yan, H., Yang, H., Xiao, S., Li, A., 2015. Preparation of chitosan-graft-polyacrylamide magnetic composite microspheres for enhanced selective removal of mercury ions from water. J. Colloid Interface Sci. 455, 261-270. 16. Li, L., Liu, F., Jing, X., Ling, P., Li, A., 2011. Displacement mechanism of binary competitive adsorption for aqueous divalent metal ions onto a novel IDA-chelating resin: isotherm and kinetic modeling. Water Res. 45(3), 1177-1188. 17. Liu, L., Fan, S., 2018. Removal of cadmium in aqueous solution using wheat straw biochar: effect of minerals and mechanism. Environ. Sci. Pollut. Res. Int. 25(9), 8688-8700. 18. Mahdi, Z., Yu, Q.J., El Hanandeh, A., 2018. Investigation of the kinetics and mechanisms of nickel and copper ions adsorption from aqueous solutions by date seed derived biochar. J. Environ. Chem. Eng. 6(1), 1171-1181. 19. Mia, S., Dijkstra, F.A., Singh, B., 2017. Aging Induced Changes in Biochar’s Functionality and Adsorption Behavior for Phosphate and Ammonium. Environ. Sci. Technol. 51(15), 8359-8367. 20. Ni, B.J., Huang, Q.S., Wang, C., Ni, T.Y., Sun, J., Wei, W., 2019. Competitive adsorption of heavy metals in aqueous solution onto biochar derived from anaerobically digested sludge. Chemosphere 219, 351-357. 21. Park, J.H., Ok, Y.S., Kim, S.H., Cho, J.S., Heo, J.S., Delaune, R.D., Seo, D.C., 2016. Competitive adsorption of heavy metals onto sesame straw biochar in aqueous solutions. Chemosphere 142, 77-83. 28

22. Park, J.H., Wang, J.J., Kim, S.H., Kang, S.W., Jeong, C.Y., Jeon, J.R., Park, K.H., Cho, J.S., Delaune, R.D., Seo, D.C., 2019. Cadmium adsorption characteristics of biochars derived using various pine tree residues and pyrolysis temperatures. J. Colloid Interface Sci. 553, 298-307. 23. Ren, X., Sun, H., Wang, F., Zhang, P., Zhu, H., 2018. Effect of aging in field soil on biochar's properties and its sorption capacity. Environ. Pollut. 242(Pt B), 1880-1886. 24. Shen, Z., Zhang, Y., Jin, F., McMillan, O., Al-Tabbaa, A., 2017. Qualitative and quantitative characterisation of adsorption mechanisms of lead on four biochars. Sci. Total Environ. 609, 1401-1410. 25. Shi, T., Jia, S., Chen, Y., Wen, Y., Du, C., Guo, H., Wang, Z., 2009. Adsorption of Pb(II), Cr(III), Cu(II), Cd(II) and Ni(II) onto a vanadium mine tailing from aqueous solution. J. Hazard. Mater. 169, 838-846. 26. Tan, L., Ma, Z., Yang, K., Cui, Q., Wang, K., Wang, T., Wu, G.L., Zheng, J., 2019. Effect of three artificial aging techniques on physicochemical properties and Pb adsorption capacities of different biochars. Sci. Total Environ. 699, 134-143. 27. Tran, H.N., You, S.J., Hosseini-Bandegharaei, A., Chao, H.P., 2017. Mistakes and inconsistencies regarding adsorption of contaminants from aqueous solutions: A critical review. Water Res. 120, 88-116. 28. Wang, R.Z., Huang, D.L., Liu, Y.G., Zhang, C., Lai, C., Zeng, G.M., Cheng, M., Gong, X.M., Wan, J., Luo, H. 2018. Investigating the adsorption behavior and the relative distribution of Cd(2+) sorption mechanisms on biochars by different feedstock. Bioresour. Technol. 261, 265-271. 29. Wu, J., Wang, T., Zhang, Y., Pan, W.P., 2019. The distribution of Pb(II)/Cd(II) adsorption 29

mechanisms on biochars from aqueous solution: Considering the increased oxygen functional groups by HCl treatment. Bioresour. Technol. 291, 121859. 30. Xu, D., Cao, J., Li, Y., Howard, A., Yu, K., 2019. Effect of pyrolysis temperature on characteristics of biochars derived from different feedstocks: A case study on ammonium adsorption capacity. Waste Manag. 87, 652-660. 31. Xu, D., Zhao, Y., Sun, K., Gao, B., Wang, Z., Jin, J., Zhang, Z., Wang, S., Yan, Y., Liu, X., Wu, F., 2014. Cadmium adsorption on plant- and manure-derived biochar and biochar-amended sandy soils: impact of bulk and surface properties. Chemosphere 111, 320-326. 32. Xu, X., Cao, X., Zhao, L., 2013. Comparison of rice husk- and dairy manure-derived biochars for simultaneously removing heavy metals from aqueous solutions: role of mineral components in biochars. Chemosphere 92(8), 955-961. 33. Yang, H.I., Lou, K., Rajapaksha, A.U., Ok, Y.S., Anyia, A.O., Chang, S.X., 2018. Adsorption of ammonium in aqueous solutions by pine sawdust and wheat straw biochars. Environ. Sci. Pollut. Res. Int. 25(26), 25638-25647. 34. Yin, Q., Zhang, B., Wang, R., Zhao, Z., 2018. Phosphate and ammonium adsorption of sesame straw biochars produced at different pyrolysis temperatures. Environ. Sci. Pollut. Res. Int. 25(5), 4320-4329. 35. Zhao, R., Coles, N., Kong, Z., Wu, J., 2015. Effects of aged and fresh biochars on soil acidity under different incubation conditions. Soil Till. Res. 146, 133-138.

30

Table 1 Physicochemical properties of fresh and ageing biochars prepared from rice straw at 400 and 700 ºC

Biochars

Ash content (%)

pH value

RB400 RB700 HRB400 HRB700

40.7±3.5 52.5±1.8 40.8±2.7 53.0±2.2

10.5±0.1 11.3±0.0 9.4±0.2 10.9±0.1

pHpzc

BET area (m2mg-1)

Pore volume (cm3 kg-1)

Average diameter (Å)

2.7 5.3 2.5 5.1

4.4±0.1 161.2±0.6 4.0±0.3 38.1±0.4

15.3±1.0 85.9±2.3 14.8±0.7 37.6±1.7

120.8±2.6 24.9±1.2 148.1±1.8 39.5±1.4

Elemental content (%)

Atomic ratio

C

H

N

O

S

H/C

O/C

(O+N+S)/C

51.7±0.5 52.6±0.7 45.7±0.5 47.2±0.4

2.9±0.0 0.6±0.1 3.0±0.1 1.6±0.1

1.0±0.1 0.8±0.0 0.9±0.0 0.5±0.1

13.3±0.2 7.6±0.2 16.8±0.3 10.0±0.2

0.2±0.0 0.3±0.0 0.6±0.0 0.6±0.0

0.66 0.14 0.79 0.41

0.19 0.11 0.28 0.16

0.11 0.06 0.30 0.17

31

Table 2 Effects of concentrations of competitive ions (Cd) on adsorption capacity and selectivity sequences of biochars Biochar

RB400

RB700

HRB400

HRB700

a

of Cd2+ (mg L-1)

Cd2+

NH4+

Kd (L g-1) c Cd2+ + NH4+

NH4+

αd

2.90±0.35

2.55±0.22

5.45±0.29

1.19

0.050

23.7

10

3.91±0.32

2.48±0.26

6.39±0.28

0.64

0.049

13.1

20

7.28±0.48

2.04±0.20

9.32±0.56

0.55

0.040

13.7

50

12.56±0.44

1.42±0.15

13.98±0.33

0.33

0.027

12.2

100

16.14±0.74

1.23±0.17

17.37±0.64

0.20

0.023

8.51

5

4.94±0.34

4.17±0.18

9.11±0.54

12.32

0.080

154.7

10

9.33±0.38

4.04±0.18

13.37±0.46

13.30

0.077

172.6

20

17.02±0.64

3.67±0.26

20.69±0.68

4.70

0.069

67.7

50

35.76±1.16

1.82±0.20

37.58±0.98

2.33

0.034

68.6

100

48.65±1.54

1.55±0.09

50.20±1.45

0.98

0.028

34.5

5

2.51±0.40

1.89±0.10

4.40±0.52

2.57

0.038

67.9

10

3.35±0.37

1.77±0.11

5.12±0.42

1.32

0.035

37.5

20

6.73±0.82

1.65±0.08

8.38±0.70

0.81

0.033

24.7

50

11.57±0.92

1.37±0.06

12.94±0.85

0.38

0.027

14.0

100

14.97±0.88

1.09±0.08

16.06±0.82

0.21

0.022

9.7

5

4.14±0.52

3.31±0.16

7.45±0.36

11.89

0.066

179.8

10

8.02±0.48

3.05±0.10

11.07±0.66

9.20

0.061

150.8

20

16.72±1.22

2.80±0.12

19.52±0.90

7.24

0.056

129.6

50

30.17±1.86

1.43±0.04

31.60±1.65

1.54

0.029

53.7

100

32.23±1.36

1.03±0.09

33.26±1.30

0.49

0.021

23.8

Initial concentration of NH4 in solution was fixed at 50 mg e

Cd2+

5

+

bQ c

Qe (mg g-1) b

Concentration

L-1.

is the adsorption capacity of biochars at equilibrium in the experiment, mg g-1.

Kd is the distribution coefficient, Kd = Qe / Ce, L g-1, Ce is the concentration of ions at equilibrium

in the experiment. dα

is the selectivity coefficient,

32

Fig. 1 Adsorption kinetics of Cd2+, Ni2+ and NH4+ on the fresh and ageing biochars in single and binary systems ((a) and (b) for Cd2+, (c) and (d) for Ni2+, (e) and (f) for NH4+, respectively)

33

Fig. 2 Adsorption isotherms of Cd2+, Ni2+ and NH4+ on the fresh and ageing biochars in single and binary systems ((a) and (b) for Cd2+, (c) and (d) for Ni2+, (e) and (f) for NH4+, respectively)

34

Fig. 3 The estimated contributions of the four mechanisms Qci, Qcp, Qcf and Qco to Cd2+, Ni2+ and NH4+ adsorption on biochars ((a)(e) for RB400, (b)(f) for RB700, (c)(g) for HRB400 and (d)(h) for HRB700, respectively)

35

Credit Author Statement Shuang Huang: Conceptualization, Writing- Reviewing and Editing, Resources Yiyi Deng: Data curation, Formal analysis, Investigation, Writing- Original draft preparation Caiqin Dong: Investigation, Visualization Zhuowen Meng: Investigation Xiugui Wang: Supervision, Validation

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1

Highlights



The adsorption sites for Cd2+, Ni2+ and NH4+ on the biochars were competitive and largely overlapped.



Cation exchange and mineral co-precipitation were the dominant mechanisms of Cd2+ and Ni2+ adsorption.



The other potential mechanisms dominated NH4+ adsorption.



Competition between Cd2+ and Ni2+ decreased the contribution of cation exchange mechanism.



Biochar aging decreased the adsorption capacity for Cd2+, Ni2+ and NH4+, but increased the contribution of surface complexation mechanism.

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