Sequestration of hazardous dye from aqueous solution using raw and modified agriculture waste materials

Abstract

World Bank reported in 2015 that dye effluent is responsible for 20% of water pollution problem globally. Dye consuming industries generate large volume of wastewater which is discharged into various environmental media (land and water body). This effluent is hazardous to both aquatic and human life. Human exposure to dye contaminated water has been attributed to risk of cancer development, skin irritation, eye inflammation, severe damage to brain, kidney, liver, reproductive system and central nervous system. Physical, chemical and biological methods of dye sequestration from wastewater have been employed. However, some of these methods are expensive, ineffective and are not applicable to real effluent. Adsorption process using commercial activated carbon as adsorbent has been widely used for effluent treatment. However, despite its efficient use, activated carbon remains a challenge. This has led to search for available and abundant agricultural waste materials as alternatives for expensive activated carbons. Litchi chinensis peel powder and Dicerocaryum eriocarpum seed powder in their raw (RL and RDE) and modified (CL and CDE) forms were used as potential adsorbents for adsorptive sequestration of congo red dye from simulated dye effluent. Biosorbents were characterised before and after adsorption of congo red dye by Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDS), Brunauer, Emmett and Teller (BET) surface area analysis and, X-ray diffraction (XRD). Determination of the point of zero charge (PZC) suggested congo red dye sorption from aqueous solution would be best in acidic pH. Operating parameters such as contact time, temperature, adsorbent dosage and dye concentration, adsorbate pH, adsorbent size, matrix effect, were all investigated for the best conditions for optimum congo red dye removal from aqueous solution using batch adsorption technique. Experimental parameters for optimum removal of congo red dye using raw biosorbents (RL and RDE) were 0.15 g mass of adsorbent, 90 min equilibrium time, 200 rpm agitation speed, temperature at 30 oC, pH of 2 and particle size of ˂125 μm. Also, experimental parameters for optimum removal of congo red dye using modified biosorbents (CL and CDE) were 0.05 g, equilibrium time of 15 min for CL, 90 min for CDE, 200 rpm agitation speed, temperature at 80 oC, pH of 2 and particle size of ˂125 μm. Optimisation of all these parameters achieved adsorption efficiency close to 100 % for raw and modified biosorbents. The kinetic of the adsorption processes for the biosorbents were ascertained by employing the pseudo first order, pseudo second order and intra-particle diffusion kinetic models. The pseudo-second order kinetic model best described the adsorption process. Equilibrium adsorption data was also evaluated using Langmuir, Freundlich and Temkin isotherm models and Langmuir model best described the adsorption process. Maximum adsorption capacities for biosorbents were RL- 55.56 mg/g, CL- 58.48 mg/g, RDE- 51.02 mg/g and CDE- 53.19 mg/g. Thermodynamic study revealed that the adsorption process was feasible and spontaneous. Adsorption of congo red dye from aqueous solution using raw biosorbents was exothermic while that of modified biosorbents was endothermic adsorption process. Desorption study found NaOH solution as the most suitable desorbing solvent. The five cycled regeneration/adsorption experiments showed that the regenerated biosorbents efficiently removed congo red dye from aqueous solution close to their virgin samples for the first three cycles. This research therefore establishes raw and modified Litchi chinensis peels and Dicerocaryum eriocarpum seeds as potential eco-friendly, affordable, abundant, and effective biosorbents for sequestration of hazardous dye from wastewater.