Numerical modelling and laboratory studies on the removal of Direct Red 23 and Direct Red 80 dyes from textile effluents using orange peel, a low-cost adsorbent

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Abstract

Wastewater containing direct dyes discharged from various industries, in particular, textile industry often causes many environmental problems. Considering the possibility of the adsorption process of synthetic dyes and their removal from aqueous solutions using orange peel as an eco-friendly and low-cost adsorbent before discharging to the receiving environment is the most important task. This paper presents a numerical finite element model to simulate the removal of dissolved textile synthetic dyes from wastewater taking into consideration both linear and the Langmuir isotherms to describe adsorption process. The CTRN/W model was integrated with SEEP/W model and modified to solve mathematical equations describing the problem at hand. The modelling accuracy was first verified with an analytical equation for a reactive mass transport problem including advection, dispersion and linear adsorption processes through groundwater flow medium. The modelling results were then compared to those results obtained from experimental tests for the removal of Direct Red 23 and Direct Red 80 dyes from textile wastewater using orange peel adsorbent. The model predictions somewhat agree with the experimental measurements. It was found that the adsorption process by orange peel adsorbent could be well described by the Langmuir isotherm. Furthermore, a pseudo-second order kinetics showed to be well-suited with the rate of sorption. Although the generation of such wastewater by the different industries is nearly unavoidable, the results of model presented here can help to design an appropriate environmental management strategy to minimise the adverse impacts caused by industrial wastewater.

Introduction

Textile industries produce huge amounts of polluted effluents that are normally discharged to surface water bodies and groundwater aquifers. These wastewaters cause many damages to the ecological system of the receiving surface water [1], [2] and create a lot of disturbance to the groundwater resources.

Most dyes used in textile industries are stable to light and are not biologically degradable [3]. Furthermore, they are resistant to aerobic digestion [4]. In order to reduce the risk of environmental pollution from such wastes, it is necessary to accurately treat them before discharging to the receiving environments.

Considerable efforts have been made by many researchers to find appropriate treatment systems in order to remove pollutants and impurities of wastewaters emanated from different industries, in particular, textile industry [1], [2], [3], [4], [5], [6], [7]. Many chemical and physical methods of dye-removal, including photocatalytic degradation [7], membranes [8] and adsorption techniques [3], [4] have been used from time to time. Adsorption process is noted to be superior to other removal techniques because it is economically cost effective, simple [2] and it is capable to efficiently treat dyes in more concentrated form [3].

Although many experimental works have been conducted to assess the capability and the performance of various adsorbents for the removal of dyes from the textile industry, little research has been done to model dye-removal process from the textile wastewaters and to evaluate the significance of the effect of major parameters on the percent of dye adsorption. Numerical models are valuable tools to provide insight into the adsorption process. A numerical model enables the prediction of future events. It may indicate which factors in a real system are most important from a sensitivity point of view. A model can help to design, optimise and predict the performance of field remediation and polluted waters treatment programs.

A numerical finite element model using CTRN/W software has been presented to simulate removal of dissolved textile dyes from wastewaters taking into account that adsorption is the main mechanism for the removal process. The adsorption efficiency of two different dyes was considered in the model. Furthermore, the effects of the initial concentrations of both dyes on the quantity of dye removal were evaluated. The results obtained from the model presented here can help to design an appropriate wastewater management strategy to minimise the socio-economic and environmental impacts of textile effluents.

A commercial numerical finite element software called CTRN/W [9] was modified to simulate the removal of direct dyes from wastewater. This software can be used to model the transport of contaminants through porous media such as soil and rock. The main processes incorporated in CTRN/W are diffusion, dispersion, adsorption, radioactive decay and density dependencies.

CTRN/W is coupled with SEEP/W, another finite element software [9] that creates the finite element grid and calculates the flow velocity for a transport problem in which the movement of the contaminants is also considered. CTRN/W utilises the SEEP/W finite element grid to model adsorption process of the direct textile industry wastewater. In all CTRN/W simulation, the transport equations solved by CTRN/W are expressed as:(θ+ρdSC)Ct=θD2Cxj2UjCxjKSρdKθCwhere

  • θ = volumetric water content (dimensionless);

  • C = concentration (mg/l);

  • D = hydrodynamic dispersion coefficient (mm2/s);

  • xj = Cartesian coordinates (mm);

  • Uj = Darcian velocity in the xj direction (mm/s);

  • ρd = bulk density of the medium (1/1000 mg/mm3);

  • S = concentration in the solid phase (mg/g);

  • t = time (s);

  • K = decay constant (1/s).

The decay constant K can be related to the half-time T (s) for decaying substance as follows [10]:T=0.693KEq. (1) was modified to model the removal of dissolved textile synthetic dyes from wastewater by assigning appropriate initial and boundary conditions to the model.

Section snippets

Experimental

To consider the capability of the numerical model for the simulation of the removal of pollutants from the industrial effluents in order to reduce environmental pollution risk, the results obtained from an experimental test were selected for the comparison purposes. In this laboratory test, the potential for the removal of Direct Red 23 and Direct Red 80 dyes from textile effluents by sorption process using orange peel adsorbent was investigated.

Adsorption isotherms

Reactions between solutes and the surfaces of solids play a crucial role in controlling the chemistry of industrial effluents. Sorption isotherms are often used to describe interactions between solutes and solid matrix [12]. The adsorption isotherms illustrate the relationships between equilibrium concentrations of adsorbate in the solid phase (S), and in the liquid phase (C) at constant temperature [5]. The distribution of pollutant such as dye between the adsorbent and the dye solution under

Conclusion

A numerical finite element model has been developed to simulate the removal of the direct dyes from textile effluents taking into consideration both linear and the Langmuir isotherms to describe adsorption process. To solve the problem numerically, the CTRN/W software was coupled with the SEEP/W model. The parameter values in the model were obtained from an experimental test for the removal of dyes Direct Red 23 and Direct Red 80 from textile wastewaters using orange peel as a low-cost

Acknowledgments

The authors appreciatively acknowledge the financial support provided by the Iran Colour Research Centre and continuous encouragements of Shahrood University of Technology.

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