Removal of Zn(II) ions from aqueous solution using Moringa oleifera Lam. (horseradish tree) biomass

Abstract

The removal of Zn(II) ions from aqueous solution using pure and chemically pretreated biomass of Moringa oleifera was investigated at 30 ± 1 °C in this study. The experimental results explored that the maximum pH (pH_max) for efficient sorption of Zn(II) was 7 ± 0.1 at which evaluated biosorbent dosage and biosorbent particle size, were 0.5 g/L, <0.255 mm, respectively. The cellular Zn(II) concentration increased with the concentrations of Zn(II) in solution. Pretreatment of M. oleifera biomass affected the sorption process and the uptake capacity (mg/g) of biomass for Zn(II) uptake was in following order: NaOH (45.76) > H₂SO₄ (45.00) > CTAB (42.80) > Ca(OH)₂ (42.60) > Triton X-100 (42.06) > H₃PO₄ (41.22) > Al(OH)₃ (41.06) > SDS (40.41) > HCl (37.00) > non-treated biomass (36.07). There was significant increase in uptake capacity of M. oleifera biomass, which suggested that affinity between metal and sorbent can be increased after some sort of pretreatment. Both Langmuir and Freundlich isotherm model fitted well to data of Zn(II) biosorption as represented by high value of their correlation coefficient (i.e. R² ≈ 1). Kinetic studies revealed that Zn(II) uptake was fast with 90% or more of uptake occurring with in 40 min of contact time and the equilibrium was reached in 50 min of contact time. The sorption rates were better described by a second order expression than by a more commonly applied Lagergren equation. Finally it was concluded that pretreatment of M. oleifera biomass can achieve superior Zn(II) uptake capacity in comparison to non-pretreated biomass.

Introduction

Heavy metals are released into the aqueous environment through a variety of sources such as metal smelters, effluents from plastics, textiles, microelectronics, wood preservatives-producing industries, usage of fertilizers and pesticides [1], [2], [3], [4]. Natural waters also contain toxic metals depending upon the bed rock [5]. Increased consciousness for safeguarding the aqueous environment has prompted a search for alternative technologies for the removal of toxic metal ions from aqueous solutions. Conventional methods for removing heavy metals include chemical precipitation and ion exchange [6], [7], [8]. These become inefficient or expensive especially when the concentration of the heavy metal ion is low, of the order of 1–100 mg/L. Dead biological materials are known for their efficient potential to adsorb heavy metals at low concentrations with very high uptake capacity [9], [10]. In this regard, a wide variety of dead biomass is being considered as adsorbents of heavy metals for treatment of industrial and domestic wastewaters as well as natural waters, including drinking water. However, uptake capacity of pretreated biomasses has not been extensively studied till present. Zn(II) may be found in wastewater discharges from acid mine drainage (AMD), galvanizing plants, as a leachate from galvanized structures and natural ores, and from municipal wastewater treatment plant discharges. Zn(II) travels through the food chain via bioaccumulation. Hence, there is significant interest regarding Zn(II) removal from wastewater streams. Traditional methods for removal of Zn(II) ions from solution are often expensive and ineffective at low metal concentrations [11]. Therefore, there is a need for a cost effective treatment method that is capable of removing low concentrations of Zn(II) from solution.

The Moringaceae is a single genus family with 14 known species. Of these Moringa oleifera Lam. (syns. Moringa pterygosperma Gaertn.) is the most widely known and utilised species. This species is one of the world's most useful plants. M. oleifera: a native of the sub-Himalayan regions of north west India, is now indigenous to many countries in Africa, Arabia, South East Asia, the Pacific and Caribbean Islands and South America, cultivated for its leaves, fruits, and roots for a variety of food and medicinal purposes. Some research has been focused on the use of M. oleifera seeds and fruits in water purification. It is commonly known as the ‘horseradish’ tree (arising from the taste of a condiment prepared from the roots) or ‘drumstick’ tree (arising from the shape of the pods), M. oleifera has a host of other country specific vernacular names, an indication of the significance of the tree around the world [12], [13], [14], [15], [16], [17], [18], [19].

The main objectives of this study include identifying the comparative maximum theoretical Zn(II) uptake capacity of the native and modified M. oleifera biomass, determining the reaction kinetics and evaluating the importance of solution pH, particle size, sorbent dose and initial metal concentration on Zn(II) uptake. This paper reports the results of an evaluation of the parameters important for the biosorption of Zn(II).