Adsorption of lead (ii) from aqueous solution using nano-papaya peel

Abstract

The presence of toxic heavy metals like lead (Pb(II)) in water resources due to industrialization is known to be a major environmental concern in many communities. Agrowaste has been the focus of studies as a reliable source of sustainable adsorbents for heavy metal removal from aqueous solutions. In this study, papaya peel has been introduced as a new source of agro-waste. The high annual papaya production in Malaysia potentially provides a good base to use its waste to develop an inexpensive adsorbent. Most of previous studies only consider the raw or carbon-active form of bioadsorbents. The present study aims to develop potential adsorption media for the removal of Pb(II) from contaminated water. While raw and carbon-activated adsorbents from papaya peel agro-waste are considered, a new nano adsorbent has been developed and evaluated in this research. The equilibrium sorptions of Pb(II) from an aqueous solution using synthesized adsorbents of activated carbon papaya peel (AC-PP) and nano-papaya peel (Nano-PP) were investigated. The synthesized adsorbents were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, field emission scanning electron microscopy, x-ray diffractometery and x-ray photoelectron spectroscopy. The batch experiments were carried out considering various Pb(II) concentrations of 10, 20, 50, 100, 200, and 400 mg/L, by changing variables of pH, adsorbent dosage, initial metal concentration, and temperature and contact time. The removal efficiency of the adsorbed amount of metal ions was considered relative to the equilibrium parameters. Desorption and regeneration studies were additionally conducted to evaluate reusability. The developed adsorbents showed excellent performance. Pb(II) was removed after 2 h of agitation, reaching optimal removal percentages of 82.6% using AC-PP (100 mg dosage) and 99.39 % using Nano-PP (50 mg dosage) after 3 h, at pH=5, in ambient condition. Equilibrium adsorption isotherms and kinetics were reviewed using the different isotherm models of Langmuir, Freundlich, and Temkin and kinetic models of the pseudo-first order, pseudo-second order, and intra-particle diffusion. The adsorption processes of Pb(II) onto Nano-PP and AC-PP were better described by the Langmuir isotherm model indicating monolayer Pb(II) adsorption onto the surface of the developed adsorbents and the adsorption kinetics was well fitted with the pseudo second-order kinetic model. Additionally, thermodynamic results confirmed the spontaneous adsorption processes with exothermic and endothermic nature onto surface of AC-PP and Nano-PP, respectively. The results obtained, especially for Nano-PP, confirm the capability of papaya peel adsorbents as a new, low-cost, efficient and environmentally friendly alternative for Pb(II) removal from contaminated water.