Study of the Digestate as an Innovative and Low-Cost Adsorbent for the Removal of Dyes in Wastewater

Sicong Yao,Oriella Aimone,Marco Race,Massimiliano Fabbricino,Alberto Ferraro,Ludovico Pontoni,Yao Chen

doi:10.3390/pr8070852

Abstract

Digestate, as an urban solid waste, was considered as an innovative adsorbent for colorant polluted wastewater. Batch adsorption experiments were carried out using digestate as an adsorbent material to remove various dyes belonging to different categories. The removal rate and adsorption capacity of dyes were evaluated and the dose of digestate, contact time, and initial dye concentration were studied. The maximum removal rate was approximately 96% for Methylene Blue. The equilibrium time for the Methylene Blue was 4 h, while for other dyes, a longer contact time was required to reach the equilibrium. The suspicion of colloidal matter release into the solution from solid fraction of the digestate led to the investigation of the consequence of a washing step of the digestate adsorbent upstream the adsorption experiment. Washed and not washed adsorbents were tested and the differences between them in terms of dye removal were compared. Moreover, experimental data were fitted by pseudo-first order, pseudo-second order, and intra-partial diffusion kinetic models as well as Langmuir, Freundlich, and Sips isotherm models. The results from fitted models showed that the adsorption of various dyes onto the digestate was mostly well fitted by the Langmuir isotherm and pseudo-second-order kinetic model.

Highlights

The textile industries’ wastewater pollution is becoming one of the environmental issues drawing more and more attention owing to the unlimited release of wastewater into the surface water body [1].With the expansion of synthetic dyes’ production scale, the dye industry has been in fierce competition of manufacturing technology in recent years owing to its advantages in high washability and isolation resistance [2]
It can be hypothesized that the dye adsorption phenomena through the involved digestate could be composed of the following steps: (i) the first initial and rapid colorants adsorption in the first 2 h~4 h of the reaction; (ii) the removal rate rapid increase, probably owing to the abundantly accessible sites on the adsorbent surface at the initial stage of the reaction; and (iii) the dye molecules fast combination with the active sites on the adsorbent [42]
The adsorption rate was controlled by the transmission rate from the surface of the adsorbent to the pore structure inside the adsorbent, until equilibrium was reached at 4 h for Methylene Blue (MB) and Acid Red 66 (AR66), while for Direct Yellow 27 (DY27), Reactive Violet 5 (RV5), and Reactive Black 5 dye (RB5), the equilibrium time lasted longer (8 h~24 h)

Summary

Introduction

The textile industries’ wastewater pollution is becoming one of the environmental issues drawing more and more attention owing to the unlimited release of wastewater into the surface water body [1]. With the expansion of synthetic dyes’ production scale, the dye industry has been in fierce competition of manufacturing technology in recent years owing to its advantages in high washability and isolation resistance [2]. The discharge of dyes into water only represented a small portion of the overall pollutants release if compared with other more hazardous wastewaters, the control of textile wastewater was considered as one of the most important environmental issues during recent decades, especially owing to the high visibility of dyes even discharged at very low concentrations [4]. Dyes in wastewater can absorb light, reduce the transparency of water bodies, consume a large amount of oxygen in water, Processes 2020, 8, 852; doi:10.3390/pr8070852 www.mdpi.com/journal/processes

Methods

Results

Conclusion