Comments on “Fast and efficient removal of Cr(VI) to ppb level together with Cr(III) sequestration in water using layered double hydroxide interclated with diethyldithiocarbamate”

Abstract

This paper primarily aimed to provide some concerns and continue discussion about the previous published paper in this journal. First, when the mechanism of Cr(VI) removal from solution involved in adsorption-coupled reduction was proposed, the X-ray photoelectron spectroscopy (XPS) of Cr 2p spectrum of laden adsorbent (i.e., DDTC-LDH after adsorption) needs to demonstrate the co-existence of Cr(VI) and Cr(III). The detection of reduced Cr(III) in solution after the completed adsorption of Cr(VI) only provides information on the mechanism regarding reduction, not adsorption-coupled reduction. Second, adsorption mechanism (chemisorption or physisorption) cannot be drawn only based on the best statistical fit between the time-dependent data of adsorption experiment and the kinetic model (i.e., the pseudo-second-order, Elovich, or Avrami model). Third, the constant KRP (liters per grams of adsorbent not adsorbate; L/g) of the Redlich–Peterson isotherm model is not equal to or used as the thermodynamic equilibrium constant KEqo. The application of the constant KRP for calculating the thermodynamic parameters of adsorption process (∆G°, ∆H°, and ∆G°) using the van't Hoff equation leads to a certain error in the values (sign and magnitude) of those parameters. Fourth, the pHPZC of adsorbent is significant different to its pHIEP on both meanings and analysis methods. The use of those terminologies in the fields of material and sorption (adsorption and absorption) must be correct. Finally, some important information needs to provide in the studies of adsorption isotherm and mechanism (i.e., solution pH) and characteristics of diethyldithiocarbamate intercalated-LDH (i.e., arrangement and orientation of diethyldithiocarbamate in the interlayer region of DDTC-LDH).