Article pubs.acs.org/IECR
Development and Characterization of Biosorbents To Remove Heavy Metals from Aqueous Solutions by Chemical Treatment of Olive Stone M. A. Martín-Lara, G. Blázquez,* A. Ronda, A. Pérez, and M. Calero Department of Chemical Engineering, University of Granada, 18071 Granada, Spain S Supporting Information *
ABSTRACT: Olive stone is an agroindustrial waste, which has been used as precursor for the development of new biosorbent materials for lead ions removal. Chemical treatments of biosorbent were studied to analyze their effects on physical-chemical properties and lead removal. Chemical treatment of olive stone can remove lignin and holocellulose and increase the porosity or surface area. A detailed description of the changes on chemical, physical, and textural properties is presented in this study. Acid treatment improved cellulose hydrolysis, increased surface area and average pore volume, and decreased pore diameter of the olive stone. Also, the acid−base properties of the solids were well described by simplified chemical equilibrium models. Concretely, treatment of olive stone by acids developed negatively charged groups in the acidic pH range. Consequently, olive stone chemically modified with acids has a much lower pHZPC value indicating that the biosorbent surface becomes more negative due to dissociation of weakly acidic oxygen-containing groups. Lead biosorption properties of chemically modified olive stone were improved compared to a native one. For example, after basic treatment with NaOH, the maximum biosorption capacity of lead increased from 6.32 to 38.02 mg/g.
1. INTRODUCTION The presence of heavy metals in the environment can be detrimental to a variety of living species including humans. Metals can be distinguished from other toxic pollutants, since they are not biodegradable and can be accumulated in living tissues, causing various diseases and disorders.1 Lead is a hazardous heavy metal, since it does not take part in biological processes, it tends to accumulate in ecosystems. It also may enter the body via inhalation, ingestion, and skin adsorption, causing effects on children and adults, even at low concentrations.1−4 Many procedures have been applied in order to remove heavy-metals from aqueous streams. Among the most commonly used techniques are chemical precipitation, chemical oxidation and reduction, ion-exchange, filtration, electrochemical treatment, reverse osmosis (membrane technologies), evaporative recovery, and solvent extraction.5 These classical or conventional techniques have given rise to several problems such as unpredictable metal ions removal and generation of toxic sludges which are often difficult to dewater and require extreme caution in their disposal.5 Besides that, most of these methods also present some limitations, only being economically viable at high or moderate concentrations of metals but not at low concentrations, meaning diluted solutions containing from 1 to 100 mg/L of dissolved metal(s).6 Another disadvantage of using these classical techniques for heavy-metal removal is the extremely expensive cost due to the high reagent or energy requirements.5 For these reasons, particular attention has been paid to the use of biological systems as a promising alternative method for heavy-metal removal from industrial waste waters. Biosorption is an emerging biological method, which poses several advantages over the conventional method. Among these © 2013 American Chemical Society
changes are the following: the process does not produce chemical sludge, hence nonpolluting, it is easy to operate and very efficient for removal of pollutants from very dilute solutions. A major advantage of biosorption is that it can be used in situ and with proper design, it may not need any industrial process operations, and it can be integrated with many systems.7 Besides, agricultural and forestry byproducts can be used as biosorbents, so that biosorption could be considered as an eco-friendly technology.8 The advantages of using the agricultural and forestry byproducts as biosorbents are that they are byproducts or wastes from agricultural or forestry processes, and they are already available in large quantities.9 In recent years, a high number of agricultural and forest byproducts are being used for heavy metal removal from wastewaters; some of them are as follows: rice husk,10 olive waste,11−13 mangrove polyflavonoid tannins,14 orange peel,15 pine bark,4,16 date pits,17 and cork biomass18 have been used for heavy metal removal from waters and wastewaters. In this particular research, the olive stone has been the investigated waste. On the other hand, treatment of wastes can extract soluble organic compounds and enhance chelating efficiency, improving their biosorption capacity. Treatment methods using different kinds of modifying agents (such as base solutions, mineral and organic acid solutions, organic compounds, oxidizing agent, etc.) for the purpose of increasing efficiency of metal adsorption have been performed by many researchers.19−29 Received: Revised: Accepted: Published: 10809
April 19, 2013 July 9, 2013 July 20, 2013 July 20, 2013 dx.doi.org/10.1021/ie401246c | Ind. Eng. Chem. Res. 2013, 52, 10809−10819
Industrial & Engineering Chemistry Research
Article
was determined by adding the volume of each of the pore sizes and the average pore diameter from values of the area surface and the total volume of pores. 2.4.3. SEM. The SEM analysis was used to observe the particle size and the porosity of each one and to study the morphological changes in the surface area after chemical treatments. This analysis was performed on raw and chemically modified olive stone. Samples were previously assembled on an aluminum holder of 12.5 mm of diameter, using silver glue. These were covered with a thin layer of gold to improve their conductivity. Finally, prepared samples were introduced into the microscope chamber with a high vacuum. 2.4.4. Loss of Biomass. Furthermore, during the chemical treatment of OS a loss of biomass was produced. The percentage of loss is critical and has to be accounted. This loss is due to the attack of chemical agents, the solubility of the components in the chemical solutions, or mass loss in washing and filtration process, among others factors. The percentage of loss is determined by the difference between the initial sample weight (before treatment) and the final sample weight (after completed treatment process) according to eq 1
Olive stone is one of the most popular agroindustrial wastes in Spain, the olive being the fifth most cultivated product in the country. Furthermore, Spain is the number one producer of olives in the world. The Spanish olive production in 2011 was nearly 7 million tons.30 Olive stone remains available as a waste product, for which no important industrial use has been developed, so it is normally incinerated or dumped without control. Although in the last times olive stone is used as fuel, a lot of this follows remained as waste. Therefore the utilization, the study of other alternative uses, and the environmental concerns it presents are all extremely important. The aim of this research is to screen and characterize new biosorbents to be used for removal of Pb(II) from aqueous solutions. In particular, in this work, raw and chemically modified olive stone were characterized to study their properties as biosorbents and to study how treatments can increase the removal capacity of native olive stone.
2. MATERIALS AND METHODS 2.1. Olive Stone (OS). Olive stone (OS) was provided by an oil extraction plant “Cooperativa Nuestra Señora del Castillo” located in Vilches, province of Jaen (Spain). The stones were obtained from the separation process of the olive cake with an industrial pitting machine. The solid was milled with an analytical mill (IKA MF-10), and
