- Email: [email protected]

Contents lists available at ScienceDirect

Water Research journal homepage: www.elsevier.com/locate/watres

Comment

Reply for comment on “Adsorptive removal of methylene blue by rhamnolipid-functionalized graphene oxide from wastewater” Zhibin Wu a, b, Hua Zhong a, b, c, **, Xingzhong Yuan a, b, *, Hou Wang a, b, Lele Wang a, b, Xiaohong Chen d, Guangming Zeng a, b, Yan Wu e a

College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China Key Laboratory of Environment Biology and Pollution Control, Hunan University, Ministry of Education, Changsha 410082, PR China State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, PR China d School of Business, Central South University, Changsha 410083, PR China e College of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China b c

a r t i c l e i n f o Article history: Received 20 October 2016 Accepted 24 October 2016 Available online 18 November 2016

We appreciate the questions that Nekouei et al. (Nekouei and Nekouei, 2016). raised on our paper (Wu et al., 2014) regarding the incorrect form of the equation for the pseudo ﬁrst-order adsorption kinetics in 3.6. In fact, in the original accepted manuscript of Wu et al. (2014), the correct form of the equation (Eq. (1)) was used:

logðqe qt Þ ¼ logðqe Þ

k1 t 2:303

(2)

We believe this mistake happened due to the problematic formatting of the division bar. We regret that we did not see this mistake in the proofreading stage before publication. All the values of the parameters listed in Table 1 are calculated based on the correct equation (Eq. (1)) and are not affected by the wrong typesetting. The conclusions in the paper (“Adsorptive removal of methylene blue by rhamnolipid-functionalized graphene oxide from wastewater”) are not affected as well. Other two questions raised by Nekouei et al. are associated with the BrunauereEmmetteTeller (BET) equation. One question is that

DOI of original article: http://dx.doi.org/10.1016/j.watres.2016.10.061. * Corresponding author. College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China. ** Corresponding author. College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China. E-mail addresses: [email protected] (H. Zhong), [email protected] (X. Yuan). http://dx.doi.org/10.1016/j.watres.2016.10.067 0043-1354/© 2016 Elsevier Ltd. All rights reserved.

Ce 1 ðK 1ÞCe ¼ þ b ðCs Ce Þqe Kb qm Kb qm Cs

(3)

We do not agree on this point and think it is not a problem to

(1)

After typesetting, the equation was wrongly printed as Eq. (2):

logðqe qt Þ ¼ logðqe Þ k1 =2:303t

the original literature regarding the BET theory, (Brunauer et al., 1938), should be cited in Section 3.7 in Wu et al., 2014, instead of a follow-up literature (Arami et al., 2006). This suggestion is correct. For correction, “Hussain et al., 2013” in Section 3.7 should be replaced by (Brunauer et al., 1938), and “the linear form of the BET equation is (Arami et al., 2006)” should be replaced by “the linear form of the BET equation in aqueous systems is”. The other question is that “To calculate the BET parameters, it Ce should be determined by the plots of ðCs C versus CCes (not versus e Þ$qe Ce mentioned by authors)” (the equation is shown as Eq. (3)).

determine BET parameters by plotting

Ce ðCs Ce Þ$qe

versus Ce. This is

because Cs is the saturation concentration of solute in water, which is a constant at a ﬁxed temperature for a given system (Juang and Shiau, 1999). The temperature was ﬁxed in the adsorption experiments (298, 308 or 318 K), and thus the Cs is a constant regardless of the methylene blue concentration in the solution. Therefore, the ﬁgure (Fig. 10d) is suitable and results obtained for BET model in Table 3 are valid. We respect the original paper. Thus, it is necessary for us to make a correction to the citation of pseudo-second-order equation in Section 3.6. In fact, Ho et al. developed a pseudo-second-order kinetic expression for the sorption systems and applied it to the kinetic studies of competitive heavy metal adsorption by sphagnum moss peat in 1996 (Ho et al., 1996). Then, a modiﬁed initial adsorption rate equation has also been made in the form of Eq. (4) due to a mistake was included in the paper published in 1996 (Ho, 2000; Ho and McKay, 1998a, b).

t 1 t ¼ þ qt kq2e qe

(4)

Where qe is the amount of dyes sorbed at equilibrium, mg g1; k

Z. Wu et al. / Water Research 108 (2017) 464e465

465

is the equilibrium rate constant of pseudo-second order sorption, g mg1min1. Therefore, the sentence “Pseudo-second-order equation can be expressed by the following equation after integration (Onal et al., 2006):” in Wu et al., 2014 should be replaced by “Pseudo-second-order equation can be expressed by the following equation after integration (Ho and McKay, 1998a, b):”. We request a notiﬁcation of corrections on Eq. (2) and on the citation for pseudo-second-order equation and BET theory in Wu et al., 2014. Table 1 Adsorption kinetics parameters of MB onto RL-GO (RL-GO dosage, 400mg/L; pH value, 7.0; temperature, 298 K) (Wu et al., 2014). Concentration Pseudo-ﬁrst-order kinetic (mg/L) k1 qe,cal R2 qe,exp (mg/g) (1/h) (mg/g) 100 150 200

242 305 365

0.19 0.28 0.25

Pseudo-second-order kinetic k2 qe,cal R2 qe,exp (mg/g) (g/mg h) (mg/g)

55.31 0.688 242 152.56 0.985 305 127.13 0.926 365

0.0208 0.0068 0.0093

241.55 0.999 309.60 0.999 364.96 0.999

Table 3 Isotherm parameters for the adsorption of MB onto RL-GO (Wu et al., 2014). Isotherms

Langmuir

Freundlich

Temkin

BET

Parameters

qmax (mg/g) KL (L/mg) R2 RL 1/n KF (L/mg) R2 KT (L/mg) BT R2 bT (J/mol) Kb qm (mg/g) R2

Temperature (K) 298

308

318

529.10 0.04 0.975 0.055 0.17 188.96 0.991 96.62 45.44 0.928 54.52 414.81 309.17 0.995

568.18 0.14 0.986 0.018 0.18 233.53 0.996 150.30 51.35 0.931 49.86 1148.75 392.11 0.999

581.40 0.16 0.987 0.016 0.16 244.84 0.995 348.03 48.77 0.925 54.21 17,184.23 406.82 0.996

References Brunauer, S., Emmett, P.H., Teller, E., 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60 (2), 309e319. Ho, Y.S., 2000. The kinetics of sorption of divalent metal ions onto sphagnum moss peat. Water. Res. 34, 735e742. Ho, Y.S., McKay, G., 1998a. A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents. Process. Saf. Environ. 76 (4), 332e340. Ho, Y.S., McKay, G., 1998b. Kinetic models for the sorption of dye from aqueous solution by wood. Process. Saf. Environ. 76 (2), 183e191. Ho, Y.S., Wase, D.A.J., Forster, C.F., 1996. Kinetic studies of competitive heavy metal adsorption by sphagnum moss peat. Environ. Technol. 17 (1), 71e77. Juang, R.S., Shiau, J.Y., 1999. Adsorption isotherms of phenols from water onto macroreticular resins. J. Hazard. Mater. B70, 171e183. Nekouei, F., Nekouei, S., 2016. Comments on the paper “Adsorptive removal of methylene blue by rhamnolipid-functionalized graphene oxide from wastewater”. Water. Res. http://dx.doi.org/10.1016/j.watres.2016.10.061. Wu, Z., Zhong, H., Yuan, X., Wang, H., Wang, L., Chen, X., Zeng, G., Wu, Y., 2014. Adsorptive removal of methylene blue by rhamnolipid-functionalized graphene oxide from wastewater. Water. Res. 67, 330e344.

Fig. 10. The equilibrium isotherms for MB adsorbed by RL-GO: (a) The Langmuir isotherm model; (b) The Freundlich isotherm model; (c) The Temkin isotherm model; (d) The BET isotherm model (Wu et al., 2014).