The adsorption of MB dye from aqueous solution onto HCl acid treated waterhyacinth (HWH) was investigated by carried out batch sorption experiments. The effect of process parameters such as pH, adsorbent dosage, concentrations and contact time, and ionic strength were studied. Adsorption of MB onto HWH was found highly pH dependent and ionic strength shows negative impact on MB removal. To predict the biosorption isotherms and to determine the characteristic parameters for process design, Langmuir, Freundlich, Temkin, and Halsey isotherms models were utilized to equilibrium data. The adsorption kinetics was tested for pseudofirstorder (PFO), pseudosecondorder (PSO), intraparticle diffusion (IPD), and Bangham’s kinetic models. The Langmuir isotherm model showed the goodnessoffit among the tested models for equilibrium adsorption of MB over HWH and indicated the maximum adsorption capacity as 63.30 mg/g. Higher coefficient of determination (
Adsorption is one of the most widely applied techniques for removal of certain classes of chemical pollutants from waters, especially those that are hardly demolished in traditional watertreatment plants [
The extent of pollutants uptake by aquatic plant has been extensively tested [
Low et al. [
Kaur et al. [
In this study, the waste WH, after treatment with HCl acid, was used and evaluated as a possible biosorbent for the removal of a MB from aqueous solution. The pretreatment of WH biomass with HCl acids causes the loss of biomass weight by removing the lignin [
Live WH was collected from the local ponds. The collected WH were cleaned thoroughly with water for several times to eliminate earthy matter and all the soil particles followed by boiling in water for 30 min. Live WH consists of 9495% water and barely contains 50–60 g total solid per kilogram [
Methylene blue (C_{16}H_{18}N_{3}
To study the effect of important parameters like pH, adsorbent mass, initial concentrations, and contact time on the adsorptive removal of MB, the kinetic adsorption experiments were carried out. The experimental procedure was as follows:
The batch equilibrium studies were carried out by adding 0.25 g HWH adsorbent to
The procedures of kinetic experiments were basically identical to those of equilibrium tests. The effect of adsorbent dosage was investigated by contacting
Fourier transform infrared spectroscopy of the adsorbent was done by using an FTIR spectrophotometer (Model: FTIR 2000, Shimadzu, Kyoto, Japan). Spectra of the samples were recorded in the range from 500 to 4000 cm^{−1}. Approximately 3% of dry samples were taken to prepare about 150 mg KBr disks shortly before analysis of the FTIR spectra.
The effect of ionic strength on the amount of MB adsorbed by HWH was performed over the NaCl concentration range from 0 to 0.18 mol/L. MB solutions of 100 mg/L were agitated with 0.25 g/L of HWH for 4 hours.
The adsorption isotherm indicates how the adsorption molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. Langmuir isotherm [
To determine whether the MB adsorption process by HWH is favorable or unfavorable for the Langmuir type adsorption process, the isotherm shape can be classified by a term
The Freundlich isotherm [
Temkin and Pyzhev [
The Halsey isotherm model [
The kinetic behavior of MB removal by using HWH was studied to evaluate the rate of adsorbate uptake from aqueous solution, which controls the mechanism of dye adsorption. Several twoparameter kinetic models, namely, pseudofirstorder (PFO), pseudosecondorder (PSO), and intraparticle diffusion (IPD), are applied to evaluate the dynamics of the adsorption of MB from aqueous solution onto HWH. These models can be expressed as follows.
PFO model [
PSO model [
IPD model [
WH is a natural fiber, which is primarily composed of cellulose, lignin, and wax. The FTIR spectrum of WH would therefore contain many bands at the different absorption regions. The WH FTIR spectrum cannot be accurately interpreted to identify its functional groups. It can, however, be used as one of the tools to differentiate the modified WH. Figure
FTIR analysis before and after adsorption of MB onto WH.
The interaction between dye molecule and adsorbent is basically a combined result of charges on dye molecules and the surface of the adsorbent [
Effect of pH on adsorption capacity for MB onto HWH.
The adsorption of MB onto adsorbent surface is influenced by the surface charge on the sorbent and the initial pH of the solution [
Determination of pH_{PZC} of HWH adsorbent.
Adsorbent dose is representing an important parameter due to its strong effect on the capacity of an adsorbent at given initial concentration of adsorbate. Effect of adsorbent dose on removal of MB was monitored by varying adsorbent doses from 0.50 to 3.0 gm/L. The adsorption of dye decreased with the adsorbent dose and the percentage of dye removal increased (24.20–96.80%) with increasing HWH adsorbent dosage from 0.50 to 3.0 gm/L [
Adsorbent dosage function of adsorption capacity for MB over HWH at pH of 6.9 and
Figure
Adsorption kinetics of MB on HWH for different initial concentration at pH of 6.9 and
The extent of MB adsorption was sharply attributed by the concentration and nature of the electrolyte ionic species added to the dyebath [
Effect of ionic strength on MB removal over HWH adsorbent.
The wellestablished Langmuir isotherm suggests the presence of monolayer coverage of the adsorbate at the outer surface of the adsorbent; once an adsorbate molecule occupies a site, no further adsorption can take place at that site. The linearized equation (
Parameters and correlation coefficient of the studied isotherm models.
Model name  Evaluated parameters 


Langmuir isotherm 

0.9938 
Freundlich isotherm 

0.9851 
Temkin isotherm 

0.9873 
Halsey isotherm 

0.9851 
Langmuir isotherm model for MB adsorption onto HWH at pH of 6.9 and
Separation factor for MB onto HWH.
From Figure
According to (
Freundlich isotherm model for MB adsorption onto HWH at pH of 6.9 and
The experimental equilibrium data for MB adsorption over HWH adsorbent, calculated from (
Temkin isotherm model for MB adsorption onto HWH at pH of 6.9 and
The Halsey isotherm model describes the multilayer adsorption and the fitting of the experimental data to this equation validates the heteroporous nature of the adsorbent. According to (
Halsey isotherm model for MB adsorption onto HWH at pH of 6.9 and
The experimental kinetic data of MB, calculated from (
Adsorption rate constant and coefficient of correlation associated with kinetic models.
Model name 



Identified parameters 

NSD  SSE  EABS 

PFO  50  33.00  25.50 

0.8675  7.0160  2.6508  1.1687 
100  48.39  21.55 

0.8996  4.9911  4.0881  1.2191  
150  53.10  8.131 

0.9073  7.9788  7.8205  2.1378  


PSO  50  33.00  34.36 

0.9963  2.2246  2.6310  1.3700 
100  48.39  49.26 

0.9996  3.2251  4.0524  1.2670  
150  53.10  54.00 

0.9992  5.2038  4.1585  1.3942  


IPD  50  33.00 

0.9576  5.5710  7.0520  2.8170  
100  48.39 

0.8366  2.7590  2.7340  6.9580  
150  53.10 

0.9376  2.390  3.4840  7.1093 
The representation of PFO model for MB adsorption on HWH for different initial concentration at pH of 6.9 and
By analyzing the
The experimental kinetic data of MB were further validated by using PSO model of (
The representation of PSO model for MB adsorption on HWH for different initial concentration at pH of 6.9 and
It may be observed from Table
For a PSO type adsorption process it is necessary to investigate the kinetic curve’s characteristics by means of an approaching equilibrium factor value in order to determine whether the MB adsorption by HWH approaches equilibrium or not. The approaching equilibrium factor can be written as displayed in the following equations [
Adsorption kinetic behavior in the PSO model and equilibrium approaching factor (

Type of kinetic curve  Approaching equilibrium level 


Linear  Not approaching equilibrium 

Slightly curved  Approaching equilibrium 

Largely curved  Well approaching equilibrium 

Pseudorectangular  Drastically approaching equilibrium 
Characteristic curves of PSO kinetic model.
The curvature of the adsorption curve decreases as
Intraparticle diffusion (IPD) equation was used to study diffusion mechanism. Broadly speaking, the initial adsorption usually occurs on the adsorbent surface during batch experiments. Additionally, there is a high probability of the adsorbate to diffuse into the interior pores of the adsorbent and, hence, IPD emerges as the dominant process [
Thus the
Representation of IPD kinetic model for MB adsorption on HWH for different initial concentration at pH of 6.9 and
As the double nature of intraparticle diffusion plot confirms the presence of both film and pore diffusion, in order to predict the actual slow step involved, the kinetic data were further analyzed using the Boyd kinetic expression. This kinetic expression predicts the actual slowest step involved in the sorption process for different sorbentsorbate systems. The linearized Boyd kinetic expression is given by [
Representation of Boyd plots for MB adsorption on HWH for different initial concentration at pH of 6.9 and
To correlate the experimental findings evidently, sorption data were further utilized to identify the slow step occurring in the present adsorption system based on the equation proposed by Aharoni et al. [
Representation of Bangham’s plots for MB adsorption on HWH for different initial concentration at pH of 6.9 and
The present study shows that the HCl acid treated waterhyacinth (HWH) can be used as an adsorbent for the removal of MB from its aqueous solutions. Upon comparing all the isotherm models, the isotherm results predicted by the Langmuir model coincide with the experimental values with a high correlation coefficient. The equilibrium data fitted very well in a Langmuir isotherm equation, confirming the monolayer sorption of MB onto HWH with a monolayer sorption capacity of 63.30 mg/g. However, Freundlich, Temkin, and Halsey isotherm model equations were used to express the adsorption phenomenon of MB. The kinetics of MB adsorption onto HWH was examined using PFO, PSO, IPD, and Bangham’s kinetic model. As is evident from the adsorption profiles the PSO equations provide a best fit description for the sorption of MB onto the HWH adsorbent amongst several kinetic models, due to its high correlation coefficient. The adsorption of MB via the HWH adsorbent may be controlled by external mass transfer followed by IPD.
Adsorption capacity at equilibrium (mg/g)
Adsorption capacity at time
Approaching equilibrium factor
Bangham’s constants
Concentration of solution at time
Cooperative binding constant
Dimensionless factor
Dimensionless separation factor
Effective diffusion coefficient (
Equilibrium MB concentration (mg/L)
Freundlich constants related to adsorption capacity
Freundlich constants related to adsorption intensity
Generalized isotherm constants (mg/L)
Halsey isotherm constant
Halsey isotherm constant (L/g)
Hydrochloric acid treated WH
Initial MB concentration (mg/L)
Intraparticle diffusion coefficient
IPD rate constant
Langmuir isotherm constants (L/mg)
Mass of dry adsorbent (g)
Mathematical function of
Maximum adsorption capacity (mg/g)
Methylene blue
Normalized standard deviation
Number of data points
PFO rate constant
pH at the point of zero charge
Pseudofirstorder kinetic model
Pseudosecondorder kinetic model
PSO rate constant
Regression coefficient
Sum of absolute errors
Sum of the errors squared
Temkin constant related to heat of adsorption
Temkin isotherm constants (L/mg)
Volume of solution (L)
Waterhyacinth.
The authors declare that there is no conflict of interests regarding the publication of this paper.