Adsorbent Preparation

The GW used in this study was gathered from the community for a university’s teaching faculty in Zhengzhou, China. The GW is primarily composed of grass (Lolium perenne L.) and the leaves of osmanthus (Osmanthus fragrans (Thunb.) Lour.). The GW was dried in the open air for days and at 105°C for 2 h before being crushed and processed through 0.15 mm mesh. We prepared the GWBC by Pyrolysis, 15 g of GW powder was placed in a tube furnace and heated to 500- 1000°C (GWBC500-GWBC1000) at a rising gradient of 15°C/min for 2-6 h, respectively. By adding N₂ to the furnace at a rate of 20 mL/min, an atmosphere free of oxygen was formed. After cooling, the black particles were broken up and put through a 0.18 mm screen to create GW biochar (GWBC).

20 mL of the PEI and methanol combination (m:m = 1:100, 1:20, and 1:10) with the mass fractions of 1%, 5% and 10% was mixed with GWBC in a range of ratios (W:V=0.1:20, 0.5:20, 1:20, 1.5:20, and 2:20), and the mixture was vigorously stirred for 5 h at 30°C. To get the PEI modified GWBC (PGWBC), the solids were filtered, washed with deionized water until the pH was constant, and then vacuum-dried for 4 h at 60°C.

Adsorbent Characterization

A Fourier transform infrared spectroscopy (FTIR, Nicolet IS50, Thermo fisher, USA) was used to analyze the functional groups of PGWBC before and after Cr(Ⅵ) adsorption. An automatic specific surface area analyzer (BELSORP-max, MicrotracBEL, Japan) was utilized to measure the specific surface area and pore features of PGWBC.

Artificial Wastewater Synthesis

The wastewater containing Cr(Ⅵ) was synthesized by dissolving a certain amount of K₂Cr₂O₇ into the deionized water. The pH value of the working solution was adjusted using dilute HCl and NaOH solutions.

Batch Adsorption Experiment

0.02 g PGWBC was added into 50 mL Cr(VI)- bearing wastewater, and the mixture was incubated in a shaker at 120 rpm, reacting at different time spans and temperatures. The residual Cr(VI) in the supernatant was determined by a UV-vis spectrophotometer (UV- 5100, Yuanxi, China) using the 1,5-diphenylcarbazide method at 540 nm [29]. All the experiments were carried out in triplicate.

Fig. 1.

Investigation of the optimal conditions for PGWBC preparation based on the Cr(Ⅵ) adsorption: determination of the optimal temperature (a) (T=298 K, t=4 h,C₀=50 mg/L); determination of the optimal pyrolysis time for GWBC preparation (b) (T=298 K, C₀=50 mg/L); determination of optimal GWBC:(PEI/methanol) (m:m) (c) (T=298 K, t=4 h,C₀=50 mg/L); determination of the optimal mass ratio of PEI:methanol (m:m) (d) for PGWBC preparation (T=298 K, t=4 h, C₀=50 mg/L).

Investigation of the Optimal Condition for PGWBC Preparation

As shown in Fig. 1a, 800 ℃ was determined as the optimal temperature due to the fact that GWBC800 had the highest Cr(Ⅵ) uptake of 3.23 mg/g in the temperature range of 500-1000 ℃. Cr(Ⅵ) uptake rose from 0.85 mg/g to 3.18 mg/g as the pyrolysis time increased from 2 to 4 h, and continued to increase to 3.22 mg/g marginally when the pyrolysis time increased to 6h (Fig. 1b). Hence, 4h was determined as the best pyrolysis time for GWBC preparation considering the energy conservation. The ratio of GWBC:(PEI/ methanol) (m:m) =1:20 was best since PGWBC prepared at the condition had the largest Cr(Ⅵ) uptake of 32.42 mg/g (Fig. 1c), which was better the other two composites prepared using the mass fractions of 1% and 5% (Fig. 1d). According to the above description, the optimal condition for PGWBC preparation was that GWBC created at 800°C for 4 h was reacted with a combination of PEI and methanol (m_PEI:m_methanol=1:10) at 1:20 (m_GWBC:m_PEI/methanol) , which was used to prepare the PGWBC for the following studies.

Characterization

As shown in Fig. 2, both GWBC and PGWBC possessed type Ⅳ adsorption-desorption isotherm with type H₄ hysteresis loop, indicating the two materials contained mainly narrow slit mesopores. GWBC and PGWBC had pore size distribution in the range of 0.91-1.86 nm and 0.68-1.99 nm, respectively, and their corresponding peaks were located at around 1.30 nm and 1.12 nm, respectively, indicating the two biochars were mesoporous materials.

As shown in Table 1, PGWBC had a much larger BET specific area and total pore volume than those of GWBC due to the PEI modification, offering more active sites for better Cr(Ⅵ) adsorption [30], whereas, PGWBC had less average pore diameter than GWBC, which might have been caused by the blocking of the pore by PEI [31].

Fig. 2.

Nitrogen adsorption-desorption isotherms and pore size distribution of GWBC and PGWBC.

In contrast to that of GWBC (Fig. 3a), the FTIR spectra of PGWBC (Fig. 3b) showed the NH₂ stretching at 3432 and 1636 cm^-1 [32, 33], respectively, and CN stretching at 1384 cm^-1 [34], indicating the surface of GWBC was successfully modified by PEI.

Static Adsorption

Kinetic Study

As shown in Fig. 5, Cr(VI) uptake rose rapidly with the increase of reaction time during the first 240 min for both of the two different initial Cr(VI) concentrations due to there being enough free active sites on the surface of PGWBC for swift Cr(VI) adsorption, and about 76.7% and 85.6% of total Cr(VI) were removed during this stage for the two reactions, respectively. In the subsequent stage, the reaction was slowed down as the active sites decreased.

The pseudo-first-order model, the pseudo-second-order model, and the Elovich model (1-3) were used to further investigate the experiment data [36-38].

(1)

log(qe−qt)=logqe−k12.303t

(2)

tqt=tqe+1k2qe2

(3)

qt=1βln(αβ)+1βlnt

Fig. 4.

Influence of the initial solution pH on the adsorption of Cr(Ⅵ) (a), Zeta potential analysis of PGWBC (b).

Fig. 3.

FTIR spectra of GWBC (a) and PGWBC (b).

Fig. 5.

Effect of contact time on Cr(Ⅵ) adsorption (m=0.02 g, pH=4, T=298 K).

Fig. 6.

The isotherms of Cr(Ⅵ) adsorption by PGWBC at different temperatures (m=0.02g, pH=4, t=48h).

Table 2.

Kinetic parameters of Cr(Ⅵ)adsorption by PGWBC.

C0 mg/L	Pseudo-first order				Pseudo-second order			Elovich
	qe (mg/g)	k1 (min-1)	R2	qe (mg/g)		k2 [g/(mg·min)]	R2	α [g/(mg·min)]	β (g/mg)	R2
50	19.53	0.0027	0.8113	44.33		0.000178	0.9951	94.36	0.113	0.8788
80	22.33	0.0019	0.7928	49.65		0.000173	0.9974	81.98	0.106	0.8831

Table 3.

Isotherm parameters of Cr(Ⅵ) adsorption by PGWBC at different temperatures.

Temperature (K)	Langmuir					Freundlich
	qm (mg/g)		KL (L/mg)	R2	RL	KF (mg/g)	n	R2
288	51.1771	0.2719		0.9911	0.0275	28.4085	8.3724	0.9573
298	52.1105	0.3028		0.9917	0.0248	29.9659	8.8394	0.9605
308	56.0852	0.2919		0.9895	0.0257	31.8654	8.7207	0.9176

Where q_e (mg/g) and q_t (mg/g) were the Cr(VI) uptake at equilibrium and time t, respectively; k₁ (min^-1) and k₂ (g/mg·min) were the rate constants for the pseudo-first-order model and the pseudo-second-order model, respectively; α (mg/g·min) was the Elovich adsorption rate, and β (g/mg) was the Elovich constant.

Table 2 showed the kinetic parameters of the three kinetic models. The R² of the pseudo-second-order model was much higher than those of the other two models, and the calculated Cr(VI) uptake at equilibrium (44.33 mg/g for C₀=50 mg/L, 49.65 mg/g for C₀=80 mg/L) was close to the experimental values, indicating the pseudo-second-order model was best to describe the adsorption process.

Isotherm and Thermodynamic Study

Fig. 6 showed the residual Cr(VI) in the solution and the Cr(VI) adsorption by PGWBC at equilibrium. Two isotherm equations, namely, the Langmuir and Freundlich equations (4-5) were used to explore the equilibrium data [39, 40].

(4)

ceqe=1qmb+ceqm

(5)

logqe=logkf+1nlogCe

where q_m (mg/g) was the theoretical saturated Cr(VI) adsorption, and b (L/mg) was the Langmuir constant. Both k_f (Lⁿ∙mg^1–n∙g⁻¹) and n were the Freundlich constants.

Fig. 7.

The relationship between 1/T and lnK0 for the adsorption of Cr(Ⅵ) by PGWBC.

Fig. 8.

Penetration curves of Cr(Ⅵ) adsorption by PGWBC under three different influent velocities (T=298 K, pH=4, C0=50.2 mg/L).

Table 3 showed the isotherm parameters of Cr(Ⅵ) by PGWBC at three different temperatures, which indicated that all the three R² of the Langmuir model were higher than those of the Freundlich model, indicating the Langmuir model was better at describing the adsorption, and monolayer adsorption of Cr(Ⅵ) by PGWBC took place during the process.

Three thermodynamic parameters, including ΔG⁰, ΔS⁰, and ΔH⁰, which were calculated from the following equations (6-8) were used to analyze the thermodynamic feature of Cr(VI) adsorption by PGWBC [41].

(6)

ΔG0=−RTlnk

(7)

ΔG0=ΔH0−TΔS0

(8)

lnk=−ΔG0RT=ΔS0R−ΔH0RT

where ΔG⁰, ΔH^0, and ΔS⁰ were the Gibbs free energy change (kJ/mol), the standard enthalpy change (kJ/mol), and the standard entropy change (J/mol·K), respectively. R was the ideal gas constant (8.314 J/mol·K). k was the thermodynamic constant (L/g), which could be calculated from the intercepts of the fitted line shown in Fig. 7.

As shown in Table 4, the ΔG⁰ values were all negative and declined with rising temperature, implying that the adsorption of Cr (VI) by PGWBC was a spontaneous process, meanwhile, the positive ΔH⁰ and ΔS⁰ indicated the endothermic nature of the adsorption process.

Dynamic Adsorption

For further study, the influence of flow rates on Cr(Ⅵ) adsorption was investigated by a dynamic adsorption experiment. As shown in Fig. 8, the removal rate and adsorption capacity of PGWBC for Cr(Ⅵ) were decreased with the increase of flow rates, meanwhile, the time to reach saturation was also shortened due to the fact that the contact time between Cr(Ⅵ) and PGWBC got shorter with the increasing flow rate, leading to the decrease of adsorption capacities [42, 43]. Under the different flow rates of 1.09 mL/min, 2.11 mL/min, and 3.18 mL/min, the maximum Cr(Ⅵ) uptakes were 29.53mg/g, 27.76 mg/g, and 23.01 mg/g, respectively, as shown in Table 5.

The Thomas model [44, 45] and Yoon-Nelson model [46, 47] (Eqs. 9-10) were used to fit the maximum adsorption capacity in the dynamic process, the parameters were listed in Table 6 and Table 7.

where C₀ (mg/L) and C_t (mg/L) were the Cr (VI) concentration at initial and time t, q₀ (mg/g) was the theoretical saturated Cr (VI) adsorption uptake, m(g) was the mass of PGWBC, Q (mL/min) was the flow rate, k_Th (mL/(min·mg)) was the Thomas constant, K_YN (min^-1) was the Yoon-Nelson constant, τ (min) was the time required for 50% adsorbent penetration, respectively.

As shown in Table 6, the values of R² of Thomas model were higher between the two equations. The theoretical Cr(VI) uptakes calculated from the Thomas equation at varied flow rates were 30.45, 27.49 and 22.27 mg/g, respectively, which agreed with the experimental data, with the maximum relative error being 3.22%. Table 7 showed Cr(VI) uptakes calculated from the Yoon-Nelson equation were 36.48 mg/g, 34.86 mg/g, and 29.37 mg/g, which had the maximum relative error of 22.08% compared with the experimental data. Thus, the Thomas model was more suitable to represent the dynamic Cr(VI) adsorption by PGWBC.