Feedstocks and regents
The EPRs were obtained from a pharmaceutical factory in Shaanxi Province, China. Generally, the main compositions of EPRs were mycelium, nutritional substance like starch, maize slurry, protein, and amino acid (Xiao et al. 2015).A novel moderate oxidation drying technology and electron beam has been applied for eliminate the potential ecological risks. There were no residual antibiotic and antibiotic residence genes detected in EPRs according to our previous studies (Zhao et al. 2018). After collection, samples were air-dried followed by oven-drying at 65°C for 72 h. Samples were then crushed and sieved to pass through a 0.15 mm nylon sieve, and stored in a desiccator until further experiments.
All chemicals used for analysis were of analytical grade and used without further purification. Acetone (CH3COCH3), 1,5-diphenylcarbazide (DPCI), HCl, HNO3, H3PO4, H2SO4, K2Cr2O7, CaCl2, and NaOH were obtained from Xinong Chemical Co., Ltd. (China). Stock solutions of Cr(VI) were prepared by dissolving K2Cr2O7 in 1000 mL of deionized water (DW, 18.2 MΩ). The working solutions were obtained by diluting the stock solution with a different ratio of DW.
Biosorbent preparation and characterization
SEPRs were prepared following methods described by Montazerrahmati et al. (2011) and Abdolali et al. (2015). Briefly, EPRs were added into 1 mol L-1 CaCl2 at a ratio of 1:20 (w/v). The mixture was then stirred at 200 rpm for 24 h at room temperature. After that, the solid phase was centrifugated and washed with DW until solution pH reached to 7.0 ± 0.1. Samples were then oven dried at 65°C, ground and stored in a desiccator until further utilization in the sorption experiments.
Elemental composition (i.e., C, H, N, and S) of EPRs and SEPRs were determined by a CE 440 elements analyzer (Agilent, US). The surface morphology of samples was characterized using a scanning electron microscopy (S-4800, Hitachi, S4800). BET surface area (SBET), total pore volume, and pore size distribution of adsorbents were analyzed by a Surface Area and Porosity Analyzer (V-sorb 2800 P, App-One, China). The zeta potential of each sample was characterized using a zeta potential analyzer (Nano ZS90, Zetasizer, England). Additionally, surficial functional groups of samples before and after Cr(VI) sorption were observed using a Tensor 27 Fourier transform infrared (FTIR) spectrometer (Bruker, Germany).
Batch adsorptive removal experiments
The removal of Cr(VI) from aqueous solution using EPRs and SEPRs were examined in a series of batch experiments. Firstly, the effect of biomass dosage on Cr(VI) removal was investigated by adding different amounts of EPRs/SEPRs (i.e., 0.1, 0.2, 0.25, 0.3, 0.4, 0.5, 0.75, 1.0, 1.25 g (dry weight)) in 100 mL polycarbonate tubes that contained 50 mL of 50 mg L−1 Cr(VI) solution (pH = 4.0). Tubes were then shaken for 4 h at room temperature using a horizontal shaker. Additionally, the effect of solution pH was investigated by adjusting solution pH to be in the range of 1.0 to 9.0 using 0.1 M NaOH/HCl solutions. Moreover, adsorptive removal isothermal experiments were carried out with Cr(VI) concentration ranged from 0 to 500 mg L−1 (pH = 1.0). Besides, adsorptive removal kinetic experiments were carried out in 250 mL flasks with 0.5 g EPRs/SEPRs and an aliquot of 100 mL 50 mg L−1 Cr(VI) solution (pH = 1.0). At each interval of 0, 10, 20, 30, 40, 50, 60, 90, 120, 150, 180, 210, 240, 270, 300, 360 and 480 min, 1 mL of suspension was withdrawn and then filtered using cellulose filter membrane (0.45µm). Furthermore, thermodynamic studies were also conducted to evaluate Cr(VI) removal efficiencies by adjusting temperature into 25°C, 30°C, 40°C, and 50°C.
Cr concentration determination
The total Cr concentration in the filtrate was analyzed by a flame atomic absorption spectrophotometer (FAAS Z-5000, Hitachi, Japan). Simultaneously, the concentrations of Cr(VI) in the filtrate was analyzed by complexing with 1,5-diphenylcarbazide in an acidic medium, using a UV-visible spectrophotometer at 540 nm (Muthusamy et al. 2014). The level of Cr(III) was then calculated by the difference between the total Cr content and the content of Cr(VI) (Xiao et al. 2018).
All the experiments were triplicated, and the efficiency of Cr(VI) removal (%) was calculated from equation (1). Meanwhile, the adsorption capacities of each sample were calculated based on the difference in the total Cr concentration in filtrate before and after adsorption according to the equation (2)
Removal (%) = (C0 ‒ Ce)×100/C0 (1)
qe = (C0 ‒ Ce)×V/m (2)
where C0 and Ce are the initial and the final concentration (obtained by FAAS) of the Cr(VI) in solution (mg L−1), respectively. V is the volume of solution (L) and m is the sorbent mass (g).
Model simulation of Cr(VI) biosorption
Widely applied Langmuir model (eq. 3) and the Freundlich model (eq. 4) in linearized forms were used to fit the equilibrium data(Wang 2017; Li et al. 2017).
qe = KcqmCe/(1 + KcCe) (3)
lnqe = lnKf +1/n × lnCe (4)
Where qm (mg g−1) is the maximum adsorption capacity and Kc (L mol−1) is the Langmuir constant, Ce is the equilibrium concentration of Cr(VI) in the solution (mg L−1), qe is the adsorption capacity of Cr(VI) at equilibrium (mg g−1), Kf (L mg−1) is a Langmuir binding constant related to the energy of adsorption.
To investigate the controlling mechanism of biosorption process, the pseudo-first-order (eq.5) and the pseudo-second-order (eq.6) kinetic models in linearized forms were used to analyze the sorption process (Li et al. 2019).
ln(qe – qt) = lnqe – k1t (5)
t/qt = 1/(k2qe2) + t/qe (6)
where, qe and qt (mg g−1) are the biosorption capacity at equilibrium and at time t, respectively. k1 (min−1) and k2 (g (mg·min)−1 are the rate constants of pseudo-first-order and pseudo-second-order adsorption, respectively.
Moreover, thermodynamic parameters associated with chemical reactions in isothermal processes, such as changes in Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), were estimated according to the following Equations (Li et al. 2017; Hlihor et al. 2017). The equilibrium constant Kc (L mol−1) derived from the Langmuir modeling of isotherm data, T is the sorption temperature (K) and R is the ideal gas constant (8.314 J (mol·K) −1).
ΔG° = - RTlnKc (7)
ΔS° =(ΔH° –ΔG°) / T (9)