Abstract

In this study, an efficient route to synthesizing polyethyleneimine-modified ultrasonic-assisted acid hydrochar (PEI-USAH) is developed and reported. Ultrasonic irradiation technique was used as surface modification method to shorten the crosslinking reaction for hydrochar and polyethyleneimine (PEI). The PEI-USAH showed an excellent adsorption capacity for Cr(VI) from aqueous solution. The physicochemical properties of this PEI-modified adsorbent were comparatively characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analysis and CNHS analysis. The effects of contact time, initial pH, and biosorbent dose on adsorption capacities were investigated. The batch adsorption experiments showed that PEI-USAH posessed the maximum adsorption capacities of 94.38 mg/g and 330.84 mg/g for initial Cr(VI) concentration of 100 mg/L and 500 mg/L, respectively. Furthermore, this adsorption process could be fitted to Langmuir adsorption and described by the pseudo second order kinetic model. Based on the above findings, PEI-USAH could be used as a potential adsorbent for removal of Cr(VI) from wastewater.

INTRODUCTION

The release of excessive heavy metals from various industries has become a global concern. The toxic heavy metals such as chromium, copper, zinc, cadmium, lead and nickel can be accumulated and passed into the food chain instead of being biodegraded (Garg et al. 2007). Among numerous harmful heavy metals, chromium, mainly existing in two oxidation states (Cr(VI) and Cr(III)), gets major attention due to its stability in the natural environment and widespread use for production of chemicals and pigments and electroplating and coating operations (Cao et al. 2018). In particular, Cr(VI) is more toxic and mobile than Cr(III) because of its stronger oxidizability and higher water solubility (Sillerova et al. 2014). According to the World Health Organization, the maximum permitted limit for hexavalent chromium and total chromium are 0.05 mg/L and 2 mg/L respectively (Baral & Engelken 2002). Consequently, it is necessary to remove hexavalent chromium from industrial wastewater before disposal and cycling into the natural environment.

In general, there are several conventional methods which have been developed over the past decades for the removal of heavy metals from industrial wastewater, such as adsorption, precipitation, ion exchange, electrochemical processes and membrane technology (Kongsricharoern & Polprasert 1996; Owlad et al. 2008; Korus & Loska 2009; Górka et al. 2011). Biosorption is an adsorption-based process that can remove substances from solution by biological material. Therefore, it is easy to operate (Al-Othman et al. 2012). Many researches also found that biological material can effectively remove heavy metals from the aqueous environment (Gao et al. 2009; González Bermúdez et al. 2012; Choinska-Pulit et al. 2018). Fungi, bacteria, algae and agricultural waste are the most common biosorbents that are abundant in nature (Sheng et al. 2007). Among them, algae are applied mainly in the fuel production field and medicine field because algae are rich in fat, polysaccharide and protein. But a wide gap in hazardous substances adsorption between activated carbon and algae still exists (González Bermúdez et al. 2012; Daneshvar et al. 2017). Therefore, algae are a relatively unexploited waste in environmental protection. It is meaningful for sustainable development of the environment and economy to manage algae resources scientifically.

Hydrothermal carbonization has been found as a very economic method for converting biomass waste into functional hydrochar materials (Mäkelä et al. 2015). And the solid product hydrochar shows advantages in its oxygen-containing functional groups, surface area and porosity, which confirms the potential as a novel adsorbent (Liu et al. 2010). However, a variety of factors, such as negative charge and the lack of functional groups, limit the adsorption efficiency of hydrochar (Fang et al. 2018). Therefore, some chemicals like hydrogen peroxide, sulfamic acid and alkali have been chosen to strengthen the desired performance of hydrochar for heavy metal removal (Xue et al. 2012; Sun et al. 2015; Petrovic et al. 2016; Deng et al. 2019). And ultrasonic irradiation is well known to accelerate the chemical reaction, due to the phenomenon of acoustic cavitation (Cravotto & Cintas 2007).

Polyethyleneimine (PEI) has been used as a crosslinker due to abundant primary and secondary amine groups in its backbone and branches, and it exhibits strong adsorption performance for heavy metal ions through electrostatic interaction or complexation (Deng & Ting 2005; Mao et al. 2011). However, lots of studies chose methanol as the PEI solvent, which is environmentally unfriendly (Liu & Huang 2011; Ma et al. 2014; Liang et al. 2018).

In this work, hydrochar synthesized from brown algae was treated by HCl under the ultrasonic condition to reduce the crosslinking reaction. And deionized water was chosen as PEI solvent for crosslinking modification to reduce wastewater emissions. The performance of the PEI-modified ultrasonic-assisted acid hydrochar (PEI-USAH) was assessed for Cr(VI) removal from aqueous solution. The effects of pH, biosorbent dose and contact time were investigated in this study. The surface characteristic and functional groups of biosorbent were determined by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) analysis and CNHS analysis.

EXPERIMENTAL

Materials

Potassium dichromate (K2Cr2O7) was purchased from Linfeng Chemical Reagent (Shanghai, China). PEI (99% (w/w)) and glutaraldehyde (GA, 25% (w/w)) were obtained from Adamas (Shanghai, China). All acid reagents (HCl and HNO3) used in the experiments were of G.R. grade from Aladdin (Shanghai, China). Deionized water was used for the preparation of various solutions.

The brown algal biomass, Sargassum horneri, was collected from the sea coast of Wenzhou, China. It was washed with deionized water and dried in an oven at 378 K for 24 hours. After that, the dried S. horneri was crushed in a grinder and sieved using standard sieves of 245–350 μm to maintain size fraction. The powdered S. horneri was stored for the next experiments.

Preparation of the biosorbent

The hydrothermal carbonization method was carried out in an 80-mL hydrothermal reactor under autogenic pressure. Specifically, the powdered S. horneri (10 g) was loaded with 30 mL of deionized water. The reactor was heated to 523 K for 2 hours. The reaction mixture was filtered and the retained solid was washed and dried in an oven at 373 K for 24 hours before use.

Before modification, 4.0 g of hydrochar was added into 1 mol/L HCl and the mixture was sonicated for 0.5 h with input power 100 W and frequency 40 kHz (AS-7240B, China). After that it was stirred for 4 hours at 298 K. Then the treated hydrochar was washed and dried at 333 K for 24 h.

To synthesize the PEI-USAH, the treated hydrochar was added to 100 mL of 1% (w/v) PEI solution, and stirred at 120 rpm and 298 K for 12 h. Then 100 mL 1% (w/v) glutaraldehyde solution was dropwise added into the mixture for crosslinking. It was agitated at 120 rpm and 303 K for 2 h. Finally, the PEI-USAH was washed with deionized water and dried in an oven at 333 K for 24 h before use.

Method

The Cr(VI) stock solution was 1,000 mg/L, and it was prepared by dissolving K2Cr2O7 in deionized water before each experiment. Different concentrations of Cr(VI) solution were obtained by diluting the initial aqueous solution accurately. The batch adsorption experiments were performed in 100 mL Erlenmeyer flasks. One of the modified adsorbents was added into 50 mL of known concentration of Cr(VI) solution. The initial pH of each Cr(VI) solution was adjusted using 0.1 mol/L NaOH and 0.1 mol/L HCl. The mixture was stirred in a thermostatic shaker at 120 rpm and 298 K for 6 h. After filtration, the filtrates were collected to measure the remaining Cr(VI) and Cr(III) concentration in the liquid phase. Cr(VI) concentrations were determined using a UV/vis spectrophotometer (PerkinElmer Lambda 35, USA) at 540 nm by 1,5-diphenylcarbazide method (Sanchez-Hachair & Hofmann 2018). Total chromium concentrations were measured by atomic absorption spectroscopy (AAS, PerkinElmer 900F, USA) at wavelength 358 nm.

The heavy metal removal efficiency (%) and biosorption capacity (qe, mg/g) were calculated by Equations (1) and (2) 
formula
(1)
 
formula
(2)
where, qe is the amount of absorbed Cr(VI) per gram of biosorbent at equilibrium, Ci and Ce represent the initial and equilibrium Cr(VI) concentration in the solution, respectively. V is the volume of solution, and m is the mass of biosorbent. In order to ensure the accuracy of experimental results, each experiment was repeated at last three times. The error bars were calculated by standard deviation.

Kinetic study

In the kinetic experiments, the adsorption process was performed at different concentrations of Cr(VI) solution (100 and 500 mg/L) and the initial pH of Cr(VI) solution was adjusted to pH 2. After agitation on a thermostatic shaker at 298 K at 120 rpm, the liquid samples were taken at periodic time intervals and measured by AAS. Finally, experimental data were fitted to different kinetic models respectively to investigate the biosorption mechanism of Cr(VI) removal (He & Chen 2014).

The pseudo first order model is given as follows: 
formula
(3)
The pseudo second order model is given as follows: 
formula
(4)
where qe represents equilibrium biosorption capacity (mg/g), qt is biosorption capacity (mg/g) at any time t, k1 and k2 are the equilibrium rate constant of pseudo first order kinetics and pseudo second order kinetics.
Along with coefficient of determination (R2), normalized standard deviation (NSD) was used for evaluating the fitness of kinetics models. NSD is defined as follows: 
formula
(5)
where qexp represents exprimental biosorption capacity (mg/g) at time t, qcal is model-predicted biosorption capacity (mg/g) at time t, and N is the number of measurements made.

Adsorption isotherm

In the adsorption isotherm experiments, the adsorbent was added to the Cr(VI) solutions with different concentrations (100, 200, 300, 400, 500 and 600 mg/L) at initial pH 2. The mixture was stirred at 120 rpm at different temperatures (288, 298, 308 and 318 K). The equilibrium data from isotherm experiments were fitted to the Langmuir and Freundlich adsorption isotherm models to obtain the maximum adsorption capacity (Giles et al. 1974; Liu & Liu 2008).

The Langmuir model represents an ideal localized monolayer adsorption. It is expressed as follows: 
formula
(6)
Also, the linear form of the Langmuir model can be given as 
formula
(7)
where qe represents the amount adsorbed per unit of biosorbent (mg/g), Ce is the equilibrium concentration of adsorbate in the solution (mg/L), qm is the maximum biosorption capacity (mg/g), and KL is the Langmuir constant which is dependent on the rate of adsorption (L/mg).
To predict the favorability of biosorption, the dimensionless constant separation factor RL can be used as an important characterized parameter (Golie & Upadhyayula 2017). 
formula
(8)
where C0 is the initial concentration of adsorbate.
The Freundlich isotherm model is non-ideal multilayer biosorption on a heterogeneous surface. The Freundlich isotherm can be given as follows: 
formula
(9)
The equation can be linearized as 
formula
(10)
where KF is the Freundlich constant (mg/g), Ce is the equilibrium concentration in the solution (mg/L) and 1/n is the Freundlich intensity parameter and gives an indication of the favorability of adsorption. In general, values of 1/n of 0.1–0.5 represent good adsorption characteristics, values of 0.5–1 represent moderately difficult, and values more than 1 indicate poor adsorption characteristics.
The residual root mean square error (RMSE) and the chi-square test (x2) were calculated to evaluate the fitness of these isotherm models for understanding the adsorption process. RMSE is given as follows: 
formula
(11)
where qe represents exprimental biosorption capacity (mg/g) at equilibrium time, qp is the isotherm model-predicted biosorption capacity (mg/g) corresponding to Ce and n is the number of observations. x2 is given as follows: 
formula
(12)

Thermodynamic study

In order to examine whether the biosorption is spontaneous or not, the thermodynamic parameters of free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) play a significant role in the biosorption process (Golie & Upadhyayula 2017; Ahmad et al. 2018).

According to the experimental data of biosorption isotherms at different temperatures (288, 298, 303, 308, and 313 K), the characteristics of thermodynamic parameters were calculated using the following equations: 
formula
(13)
 
formula
(14)
where K is the equilibrium constant, calculated as ratio of equilibrium metal concentration on the biosorbent surface and in the solution (Liu 2009). R represents the universal gas constant (8.314 × 10−3 kJ/mol·K) and T is biosorption temperature in Kelvin.

Characterization of biosorbent

FTIR spectroscopy was carried out on a Bruker TENSOR II (4,000–450 cm−1, resolution 2 cm−1) to identify the presence of functional groups on the surface of biosorbents. The morphology was observed by scanning electron microscopy (SEM, S-4700 II, Hitachi, Japan) at various magnifications with a voltage of 15.0 KV. The specific surface area was determined by nitrogen adsorption at 77 K (BET, 3H-2000PS1, BeiShiDe, China). The surface states of the adsorbents were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250Xi, USA). Elemental analysis was performed by an elemental analyzer (Elementar vario MACRO, Germany).

RESULT AND DISCUSSION

Effect of pretreatment on biosorption efficiency

Figure 1 shows the Cr(VI) biosorption capacities of the PEI-modified hydrochar prepared by different pretreatments. First, as observed from Figure 1, PEI-modified hydrochar with deionized water as the solvent for crosslinking had better average adsorption capacity than with methanol as the solvent for crosslinking. The Cr(VI) uptake capacities of PEI/water-modified hydrochar by 1 mol/L HCl and 1 mol/L NaOH pretreatments before modification were 257.20 mg/g and 234.22 mg/g, and they were 18.46% and 7.88% higher than that without pretreatment before modification, respectively. This result showed that acid treatment had a more significant effect on Cr(VI) biosorption capacity in this study. And it could be observed from Figure 1 that PEI-USAH had best Cr(VI) uptake performance and the adsorption capacity was 330.84 mg/g, which was 28.64% higher than that of PEI-modified hydrochar with no ultrasonic-assisted pretreatment. The Cr(VI) adsorption capacity of PEI-USAH was compared with that of different adsorbents from some researches (Table 1). PEI-USAH had an excellent Cr(VI) adsorption performance. It confirmed that PEI-USAH was a potential adsorbent for the Cr(VI) adsorption process.

Table 1

Cr(VI) adsorption capacity of different adsorbents from research studies

AdsorbentAdsorption capacity (mg/g)pHKinetics modelEquilibrium modeReference
PEI-USAH 330.84 Pseudo second order Langmuir This study 
PEI-APF 222 Pseudo second order Langmuir Tangtubtim & Saikrasun (2019)  
Fe3O4-PEI-SERS 280.11 Pseudo second order Langmuir Zhang et al. (2018)  
CMMS 87.32 Pseudo second order Langmuir Albadarin et al. (2017)  
PAN-NH2 156 Pseudo second order Langmuir Avila et al. (2014)  
PEI-ESM 160 Pseudo second order Langmuir Liu & Huang (2011)  
AC from red algae 66 Pseudo second order Langmuir Lei & Chen (2008)  
MSW 31.24 Pseudo second order Langmuir Ahmed et al. (2015)  
AdsorbentAdsorption capacity (mg/g)pHKinetics modelEquilibrium modeReference
PEI-USAH 330.84 Pseudo second order Langmuir This study 
PEI-APF 222 Pseudo second order Langmuir Tangtubtim & Saikrasun (2019)  
Fe3O4-PEI-SERS 280.11 Pseudo second order Langmuir Zhang et al. (2018)  
CMMS 87.32 Pseudo second order Langmuir Albadarin et al. (2017)  
PAN-NH2 156 Pseudo second order Langmuir Avila et al. (2014)  
PEI-ESM 160 Pseudo second order Langmuir Liu & Huang (2011)  
AC from red algae 66 Pseudo second order Langmuir Lei & Chen (2008)  
MSW 31.24 Pseudo second order Langmuir Ahmed et al. (2015)  

PEI-APF, polyethyleneimine-carbamate linked alkali pineapple leaf fiber; Fe3O4-PEI-SERS, polyethyleneimine functionalized Fe3O4/steam-exploded rice straw composite; CMMS, chemically modified masau stone; PAN-NH2, polyacrylonitrile nanofibers functionalized with amine groups; PEI-ESM, polyethyleneimine modified eggshell membrane; AC, activated carbon; MSW, modified seaweed.

Figure 1

Uptake of Cr(VI) and nitrogen content onto different modified adsorbents and comparison of nitrogen content for different adsorbents (temperature 298 K, biosorbent dose 1 g/L, initial concentration 500 mg/L).

Figure 1

Uptake of Cr(VI) and nitrogen content onto different modified adsorbents and comparison of nitrogen content for different adsorbents (temperature 298 K, biosorbent dose 1 g/L, initial concentration 500 mg/L).

Nitrogen contents of different adsorbents were measured by Elementar. As shown in Figure 1 and Table 2, the increasing nitrogen content of adsorbents favored the Cr(VI) biosorption capacity, which confirmed that nitrogen-containing groups played an important role in Cr(VI) biosorption. In general, ultrasonic-assisted acidic pretreatment was a useful method to purify and activate hydrochar in this study, which could favor the PEI crosslinking reaction.

Table 2

CNHS elemental analysis of different modified adsorbents

MethodN%C%H%S%
Sargassum horneri 1.84 35.89 5.49 0.21 
Hydrochar 2.04 55.16 5.88 0.13 
PEI/water + hydrochar 6.74 50.78 6.67 0.05 
PEI/methanol + hydrochar 6.52 52.2 6.16 0.73 
PEI/water + alkali hydrochar 7.75 50.92 6.46 0.24 
PEI/methanol + alkali hydrochar 5.25 50.06 6.09 0.18 
PEI/water + acid hydrochar 9.82 51.59 7.36 0.02 
PEI/methanol + acid hydrochar 7.91 52.15 6.67 0.05 
PEI/water + ultrasounic-assisted acid hydrochar 12.30 57.10 7.49 0.52 
MethodN%C%H%S%
Sargassum horneri 1.84 35.89 5.49 0.21 
Hydrochar 2.04 55.16 5.88 0.13 
PEI/water + hydrochar 6.74 50.78 6.67 0.05 
PEI/methanol + hydrochar 6.52 52.2 6.16 0.73 
PEI/water + alkali hydrochar 7.75 50.92 6.46 0.24 
PEI/methanol + alkali hydrochar 5.25 50.06 6.09 0.18 
PEI/water + acid hydrochar 9.82 51.59 7.36 0.02 
PEI/methanol + acid hydrochar 7.91 52.15 6.67 0.05 
PEI/water + ultrasounic-assisted acid hydrochar 12.30 57.10 7.49 0.52 

Effect of pH

pH is an important parameter influencing the biosorption process. The effect of pH on Cr(VI) biosorption is demonstrated in Figure 2. It was observed that Cr(VI) biosorption process using PEI-USAH was pH-dependent. The maximum Cr(VI) adsorption capacity for initial concentration 100 mg/L and 500 mg/L are 93.96 mg/g and 307.58 mg/g at pH 2.0, respectively. The amount of Cr(VI) absorbed by PEI-USAH increased with increasing initial pH of aqueous solution at lower pH (1–2). The biosorption capacity was decreased at higher pH (2–6). And the Cr(VI) final concentration and total Cr amount after adsorption are shown in Figure 3. It was found that there was still a certain amount of total chromium present in the solution, but Cr(III) ions hardly existed at pH >3. This meant Cr(VI) ions were reduced to Cr(III) due to its higher redox potential at low pH, and increased pH promoted the formation of chelation between Cr(III) and PEI.

Figure 2

Effect of pH on the uptake of Cr(VI) onto PEI-USAH (temperature 298 K, initial concentration 100 mg/L and 500 mg/L, adsorbent dose 1 g/L).

Figure 2

Effect of pH on the uptake of Cr(VI) onto PEI-USAH (temperature 298 K, initial concentration 100 mg/L and 500 mg/L, adsorbent dose 1 g/L).

Figure 3

Residual concentration in aqueous solution after Cr(VI) adsorption in different intial concentrations of 500 mg/L (top) and 100 mg/L (bottom).

Figure 3

Residual concentration in aqueous solution after Cr(VI) adsorption in different intial concentrations of 500 mg/L (top) and 100 mg/L (bottom).

Many researchers have reported that the optimal pH for Cr(VI) adsorption was around 2 (Murphy et al. 2008; Yang & Chen 2008; González Bermúdez et al. 2012). The pH dependence of Cr(VI) removal can be largely attributed to the surface charge of biosorbent and chemical speciation of hexavalent chromium in aqueous solution (Golie & Upadhyayula 2017). There are various forms of Cr(VI) in the solution, such as chromic acid (H2CrO4), dichromate (Cr2O72−) and hydrogen chromate (HCrO4), depending on the solution pH and total concentration of chromate (Bhattacharya et al. 2008). In the pH range of 2–6, there are two soluble Cr(VI) species which are Cr2O72− and HCrO4 (Barrera-Díaz et al. 2012; Hou et al. 2019). With increasing pH (2 < pH < 6), the decrease of positive surface charge and deprotonation of functional groups like amino and carboxyl could decrease the electrostatic force of attraction between PEI-USAH and metal anions. At lower pH (pH < 2), the major species of chromium in solution was H2CrO4. And the positive charge on the surface of adsorbent can restrict the approach of metal cation Cr3+ and its binding to the adsorbent, and the presence of H+ and H3O+ in the solution may compete with metal cation Cr3+. The results suggested a possible adsorption mechanism for uptake of Cr(VI) by PEI-USAH. Protonated amines and carboxyl on the surface of the biosorbent could adsorb metal anion HCrO4 by electrostatic attraction under acidic condition, then the Cr(VI) with high redox potential was reduced to Cr(III) at low pH. Finally, there was the chelation formed by Cr(III) and amine nitrogen on PEI.

Effect of biosorbent dose

Effect of PEI-modified hydrochar dose on Cr(VI) biosorption was investigated by different amounts (1, 2, 4, 6 and 10 g/L) of biosorbent under optimal acidic condition. The result is shown in Figure 4. It was observed that Cr(VI) removal efficiency using PEI-USAH increased with the increase of biosorbent dose due to larger surface area and more active sites. And the maximum removal was observed with an adsorbent dose of 6 g/L of PEI-USAH in different concentrations (100 and 500 mg/L). It was also found that the PEI-USAH showed a better absorption performance in high concentration than in low concentration. However, as shown in Figure 4, the adsorption capacity was decreased with increasing adsorbent dosage. As the adsorbent dose was increased from 1 to 10 g/L, the adsorption capacity by different concentrations (100 and 500 mg/L) was decreased from 67.68 mg/g to 10.72 mg/g and 378.26 mg/g to 58.01 mg/g, respectively. It was suggested that excessive adsorbent particles resulted in an overlapping of active sites.

Figure 4

Effect of biosorbent dose on the uptake of Cr(VI) onto PEI-USAH (temperature 298 K, initial concentration 100 mg/L and 500 mg/L).

Figure 4

Effect of biosorbent dose on the uptake of Cr(VI) onto PEI-USAH (temperature 298 K, initial concentration 100 mg/L and 500 mg/L).

Effect of contact time and kinetics study

As shown in Figure 5, the Cr(VI) adsorption capacities as a function of contact time for PEI-USAH at different initial concentrations (100 mg/L and 500 mg/L) were studied. For Cr(VI) biosorption, there was a sharp increase in Cr(VI) uptake in the first 10 min, which may be related to rapid transfer of Cr(VI) by the high concentration of the solution and the abundance of surface active sites on the biosorbent at the beginning of the biosorption process. And then the biosorption processes followed a moderating stage until the adsorption equilibrium (about 30 min for 100 mg/L and 50 min for 500 mg/L), which can be attributed to the decrease of remaining vacant active sites and aggregation of Cr(VI) surrounding the biosorbent surface, restricting the free movement of Cr(VI) anions.

Figure 5

Effect of contact time on the uptake of Cr(VI) onto PEI-USAH (temperature 298 K, biosorbent dose 1 g/L, initial concentration 100 mg/L and 500 mg/L).

Figure 5

Effect of contact time on the uptake of Cr(VI) onto PEI-USAH (temperature 298 K, biosorbent dose 1 g/L, initial concentration 100 mg/L and 500 mg/L).

In this study, the adsorption kinetics was analyzed using the pseudo first order model and the pseudo second order model. The kinetic parameters from the nonlinear fitting curves of Equations (3) and (4) are listed in Table 3. According to the experimental data, the correlation coefficients (R2) showed the pseudo second order model at different concentrations to be a better fit than the pseudo first order model. The values of qe from the pseudo second order model were closer to qexp than were those from the pseudo first order model. The smaller NSD from the pseudo second order model indicated the better suitability of the pseudo second order model to predict the biosorption kinetics using PEI-USAH for the Cr(VI) removal process, and the Cr(VI) removal rate by PEI-USAH may be mainly controlled by the chemisorption process. The same conclusion has been found in some earlier studies (Kwak & Lee 2018; Liang et al. 2018; Shi et al. 2018; Zhang et al. 2018; Geng et al. 2019).

Table 3

Simulation parameters of Cr(VI) adsorption dynamics on PEI-USAH

Cr(IV) (mg/L)Pseudo first order
Pseudo second order
qexp (mg/g)
R2NSDk1qe (mg/g)R2NSDk2qe (mg/g)
100 0.940 0.239 0.587 91.700 0.963 0.232 0.382 96.527 94.380 
500 0.856 0.177 0.331 287.792 0.929 0.173 0.230 299.911 316.477 
Cr(IV) (mg/L)Pseudo first order
Pseudo second order
qexp (mg/g)
R2NSDk1qe (mg/g)R2NSDk2qe (mg/g)
100 0.940 0.239 0.587 91.700 0.963 0.232 0.382 96.527 94.380 
500 0.856 0.177 0.331 287.792 0.929 0.173 0.230 299.911 316.477 

The adsorption isotherm

The experimental data from adsorption isotherms were analyzed with the Langmuir isotherm model and Freundlich isotherm model as mentioned above. The results are presented in Table 4. The values of R2 (0.9955, 0.9435, 0.9885 and 0.9729) from the Langmuir isotherm model were closer to 1 than those from the Freundlich isotherm model. The experimental data at different temperatures (288–318 K) were fitted better by the Langmuir isotherm model than by the Freundlich isotherm model because of the smaller RMSE and χ2 from the Langmuir isotherm model. Therefore, the adsorption process of Cr(VI) by PEI-USAH was monolayer adsorption and there was a homogeneous distribution of PEI on the surface and in the pores of PEI-USAH (Owlad et al. 2010; Liu & Huang 2011). The values of RL at different temperatures were between 0 and 1, which indicated the favorable Cr(VI) adsorption by PEI-USAH.

Table 4

Parameters of Freundlich and Langmuir isotherm models

T (K)Freundlich
Langmuir
R2RSMEχ2KF1/nR2RSMEχ2KLRLqm
288 0.973 11.723 3.329 50.298 0.365 0.996 3.520 0.279 0.023 0.299 416.667 
298 0.915 10.814 9.428 35.455 0.348 0.944 5.643 7.973 0.015 0.405 481.284 
308 0.886 8.298 4.448 31.354 0.485 0.989 7.183 2.674 0.0127 0.442 559.845 
318 0.916 3.803 3.165 15.827 0.617 0.973 2.945 2.321 0.007 0.602 707.695 
T (K)Freundlich
Langmuir
R2RSMEχ2KF1/nR2RSMEχ2KLRLqm
288 0.973 11.723 3.329 50.298 0.365 0.996 3.520 0.279 0.023 0.299 416.667 
298 0.915 10.814 9.428 35.455 0.348 0.944 5.643 7.973 0.015 0.405 481.284 
308 0.886 8.298 4.448 31.354 0.485 0.989 7.183 2.674 0.0127 0.442 559.845 
318 0.916 3.803 3.165 15.827 0.617 0.973 2.945 2.321 0.007 0.602 707.695 

Thermodynamic study

The spontaneity of Cr(VI) removal from aqueous solution using PEI-USAH was investigated by thermodynamic parameters. Based on the experimental data at different temperatures (288, 298, 308 and 318 K), ΔG, ΔH and ΔS were evaluated by using Equations (10) and (11). The values of thermodynamic parameters are listed in Table 5. The values of ΔG for Cr(VI) uptake on PEI-USAH were negative, which indicated the thermodynamically feasible and spontaneous nature of the Cr(VI) adsorption (Gupta & Rastogi 2009; Al-Othman et al. 2012; González Bermúdez et al. 2012). And the ΔG values decreased with the rise in temperatures. It confirmed that high temperature favored the Cr(VI) adsorption process. The positive values of ΔH specified the endothermic process of Cr(VI) adsorption. The values of ΔS revealed the increase of randomness at the solid–solution interface during biosorption of Cr(VI) by PEI-USAH (Avila et al. 2014).

Table 5

Thermodynamics parameters for Cr(VI) adsorption on PEI-USAH

Temperature (K)ΔG (kJ/mol)ΔH (kJ/mol)ΔS (J/mol·K)
288 −1.4850 12.7872 495.5625 
298 1.9460   
308 2.4458   
318 −2.9684   
Temperature (K)ΔG (kJ/mol)ΔH (kJ/mol)ΔS (J/mol·K)
288 −1.4850 12.7872 495.5625 
298 1.9460   
308 2.4458   
318 −2.9684   

Cr(VI) recovery

In order to recycle chromium from Cr-laden PEI-USAH, Cr-loaded samples were treated with 0.1 mol/L HCl (desorbing agent) for 48 hours, followed by washing three times with deionized water until the pH value of the filter solution was neutral. Finally, the recycled biosorbent was placed in an oven at 333 K for 24 hours. After desorption of Cr(VI), the recovery efficiency of chromium on PEI-USAH was 90.43%. It indicated that hydrogen ions from HCl solution could break the electrostatic interactions between Cr(VI) and adsorption sites, and result in the release of Cr(VI) from the Cr-laden PEI-USAH.

SEM analysis

SEM images were useful for characterizing the surface morphology of PEI-USAH. As shown in Figure 6, there was a clear difference in the morphology of hydrochar, USAH and PEI-USAH. After pretreatment by HCl under ultrasonic condition, it was found that the development of cracks and cavities increase the volume of pores in USAH. The rough surface and grooved structure might allow PEI to crosslink with functional groups on the suface of USAH effectively. Also, it was observed from Figure 6(c) that PEI was attached to the surface of adsorbent, comparing with the USAH image. Crosslinking reaction decreased the surface area for PEI-USAH because some cracks and cavities were blocked. It agreed with the BET result. The surface areas of hydrochar and PEI-USAH were 44.85 m2/g and 14.12 m2/g, respectively.

Figure 6

SEM micrograph of (a) hydrochar, (b) USAH, (c) PEI-USAH.

Figure 6

SEM micrograph of (a) hydrochar, (b) USAH, (c) PEI-USAH.

FTIR spectra

In order to investigate the changes between the functional groups of hydrochar, USAH and PEI-USAH, the samples were characterized using FTIR spectroscopy (Figure 7). As can be seen from Figure 7, the characteristic peak at 3,408 cm−1 for hydrochar was due to the overlap of O-H and N-H stretching vibrations, and became boarder and stronger after modification (Daneshvar et al. 2017). The peaks at around 2,924 cm−1 were assigned to stretching vibrations of aliphatic C-H group (Ahmad et al. 2018). The bands at around 1,425 cm−1 represented C-O stretching vibrations of carboxyl groups. The peak at 1,109 cm−1 corresponded to C-N stretching from amine groups. After modification, the shift of the C=N stretching vibration of imine groups from 1,618 cm−1 to around 1,654 cm−1 proved the presence of the crosslinking reaction by glutaraldehyde (Wang et al. 2019). Specifically, one aldehyde group of glutaraldehyde and hydroxyl groups on the surface of hydrochar can generate an ether bond; the other aldehyde group of glutaraldehyde and an amine group of PEI can generate an imine bond (Figure 7, spectra (a) to (c)) (Shi et al. 2018). The FTIR spectra of Cr-loaded biosorbents also showed some distinct changes (Figure 7, spectra (d) to (f)). After adsorption, the shift observed at 3,300–3,400, 1,100–1,200 and 1,500–1,700 cm−1 confirmed that hydroxyl, amine and carboxyl directly impacted the Cr(VI) biosorption.

Figure 7

FTIR spectra of (a) hydrochar, (b) USAH,(c) PEI-USAH, (d) Cr-loaded hydrochar, (e) Cr-loaded USAH and (f) Cr-loaded PEI-USAH.

Figure 7

FTIR spectra of (a) hydrochar, (b) USAH,(c) PEI-USAH, (d) Cr-loaded hydrochar, (e) Cr-loaded USAH and (f) Cr-loaded PEI-USAH.

XPS analysis

The composition of the PEI-USAH surface before and after Cr(VI) adsorption was analyzed using XPS. The elemental compositions of the PEI-USAH surface from the XPS analysis were 69.28% C, 13.45% N, 15.52% O, and 1.37% Cl. Nitrogen content was slightly higher than the result (12.30% N) from elemental analysis. It confirmed that PEI-USAH had a well-distributed chemical component and PEI crosslinking reaction mainly occurred on the surface of PEI-USAH. Figure 8 shows the XPS spectra of PEI-USAH before and after Cr(VI) adsorption. In the O1 s spectrum, there was only one peak at around 535 eV that could be fitted with two peaks at 532.2 eV (C-OH) and 531.1 eV (C=O) (Tangtubtim & Saikrasun 2019). After Cr(VI) adsorption, the binding energy (BE) and area of the peak at 532.2 eV decreased confirming that oxygen-containing groups were involved in Cr(VI) adsorption. And the increased BE of the peak at 531.1 eV could be attributed to the overlap of the new peak (CrO42−) at 531.4 eV after the Cr(VI) adsorption process (Shi et al. 2018). The BE values of N1 s spectra were found fitted with two components at 399.6 eV (–N=) and 398.3 eV (–NH–/–NH2). It suggested the successful grafting of PEI on the adsorbent surface (Ma et al. 2014). After Cr(VI) adsorption, a slight shift was observed in the peaks of the N1 s spectra and a new peak appearing at 400.9 eV could be attributed to –NH2+–/–NH3+. It showed that the amine groups were protonated at acidic conditions (Tian et al. 2015; Zhang et al. 2018). The changes confirmed that nitrogen-containing groups were the main active sites for Cr(VI) adsorption by electrostatic interaction. Compared with the Cr2p region of XPS spectra at high resolution before and after adsorption, there were two new peaks (Cr2p1/2 and Cr2p3/2) which can be deconvoluted into two peaks, respectively. The ones at 587.6 eV and 586.4 eV corresponded to Cr(VI) and Cr(III) in the Cr2p1/2, while others at 577.6 eV and 576.6 eV corresponded to Cr(VI) and Cr(III) in the Cr2p3/2 (Bertagnolli et al. 2014; Sillerova et al. 2014). The result confirmed that the reduction of Cr(VI) to Cr(III) occurred by the electrons of primary amine groups on PEI.

Figure 8

XPS spectra of PEI-USAH and Cr-loaded PEI-USAH.

Figure 8

XPS spectra of PEI-USAH and Cr-loaded PEI-USAH.

CONCLUSION

In this work, a novel biosorbent, PEI-USAH, was prepared for Cr(VI) removal from aqueous solution. Ultrasonic irradiation technique was used as a new surface modification method to reduce the crosslinking reaction time for hydrochar and PEI. Operation parameters (pH, contact time, and biosorbent dose) were investigated for the Cr(VI) biosorption process. It was also inferred from the results that ultrasonic acidic pretreatment significantly improved the PEI-grafted performance for hydrochar. The maximum Cr(VI) removal capacity of PEI-USAH was 316.477 mg/g for initial concentration of 500 mg/L. And it was found that the equilibrium had a good fit to the Langmuir isotherm model. The kinetics was well described by the pseudo second order model. It indicated that Cr(VI) biosorption on PEI-USAH from aqueous solution was the chemisorption process. Thermodynamic parameters demonstrated that Cr(VI) removal by PEI-USAH was spontaneous and endothermic under studied conditions. Therefore, the above conclusions and high absorption capacity for Cr(VI) removal from aqueous solution proved that PEI-USAH could be used as a promising adsorbent for treating Cr(VI)-containing wastewater in the future.

ACKNOWLEDGEMENTS

The research was supported by ‘Transformation and industralization of regional demonstration results of marine economic innovation and development in Zhejiang province’ under grant 201583. And the research was supported by Biodiesel Laboratory of China Petroleum and Chemical Industry Federation and Zhejiang Province Key Laboratory of Biofuel.

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