Evaluation of the use of agricultural waste materials as low-cost and eco-friendly sorbents to remove dyes from water: a review

Toxic pollutants, especially dyes, are the main source of chemical pollutants and pose a major challenge to conventional water treatment systems such as activated sludge. Therefore, there is a need to develop appropriate techniques to eliminate these toxic dyes from different media. There are several limitations to using different techniques such as coagulation, chemical oxidation, membrane separation process, electrochemical and aerobic and anaerobic microbial degradation to remove these pollutants from wastewater. Adsorption is a promising method for removing dyes due to its high efficiency and selectivity, simplicity, low cost and recyclability of the adsorbents. It has recently been demonstrated that using activated carbon is a highly efficient technology in removing dyes from wastewater. However, its extensive use is restricted because of its relatively high cost encouraging researchers to investigate alternative sources of unconventional, efficient, low-cost and environmental-friendly adsorbents. Agricultural waste materials are the most widely used bio-sorbents for removing dyes. These materials were characterized by different techniques such as adsorption-desorption of nitrogen at 77 K, Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy-energy-dispersive X-ray spectroscopy etc, and it was found that they have a loose and porous structure and contain many functional groups in addition to being low-cost, high adsorption capacity as well as efficiency and low energy demand. This made them materials capable of adsorption of various contaminants. But one of their main drawbacks is that they present a small surface area, often less than 50 m(2)/g. The dye adsorption efficiency of these adsorbents varies depending on the pH of the solution, initial dye concentration, adsorbent dosage and process temperature. It was found that the most efficient agricultural wastes for the adsorption of Methylene Blue and Malachite Green were papaya seeds (555.56 mg/g; similar to 80%) and orange peels (483.63 mg/g), respectively. This review highlights the evaluation of the use of various agricultural waste materials as low-cost adsorbents for dye removal from wastewater, which is a safer alternative to traditional adsorbents. Some of the fundamental principles of dye adsorption were also outlined. These adsorbents are recommended for removing dyes from real wastewater under a continuous design to achieve commercial objectives.