ARSENIC REMOVAL FROM DRINKING WATER BY MEANS OF LOW-COST BIOCHARS DERIVED FROM MISCANTHUS AND COCONUT SHELL

Arsenic is a toxic metalloid highly diffused in groundwater because of natural and anthropogenic factors (mainly deriving from mining activities and pesticides). Inorganic arsenic compounds are classified by the International Agency for Research on Cancer (IARC) in Group I (proven carcinogen). Groundwater concentration of As may vary from 0.5 to 5000 μg/L taking into account more than 70 Countries around the world. The most polluted groundwater of the planet was identified in late 1990s in Bengal delta (Bangladesh and Indian region West Bengal). Inorganic arsenic in natural water (pH 6-8) is mainly found as arsenite (H2AsO3−) and arsenate (HAsO42−), respectively species of As (III) and As (V). Due to the prevailing reducing conditions in deep aquifers of SE Asia, As mainly occurs there as oxyanion of As (III), which is also the most toxic form for humans. Quantitatively the most important human exposure route for As is ingestion. World Health Organization (WHO) set in 2003 a guideline value for maximum As concentration in drinking equal to 10 μg/L, while developing Countries refer to 50 μg/L, mainly because of the prohibitive remediation costs at lower concentrations. The use of biochar as sorbent for water depollution deserved a relevant attention in the recent years. In the specific case of Bangladesh the most critical aspects in order to choose the most efficient depollution method may be the following: high concentration of As (III), presence of numerous dissolved compounds in water and the necessity to find the cheapest and simplest solution considering the lack of instrumentation and technologies. Concerning these features, and also efficiency, cost and management complexity of the possible treatment technologies, adsorption could be considered as the most flexible and promising option for SE Asia. The aim of this work is the investigation of two biochars as low-cost sorbents to remove arsenic from drinking water. The performed laboratory tests involved As (III) and As (V), considering boundary conditions related to Bangladesh. Biochars were synthetized from Miscanthus and coconut shell, then characterized and compared as sorbents with a commercial Granular Activated Carbon (GAC). A novel reactor developed at ICFAR, the Jiggled Bed Reactor (JBR), was employed in slow pyrolysis mode to produce the biochars omitting the activation phase. Biochars deriving from Mischantus and coconut shell exhibited rather different features about surface area (respectively 428 m2/g and 208 m2/g), micro-pores volume (respectively 0.212 cm3/g and 0.098 cm3/g), and pore size (respectively 1.98 nm and 1.89 nm). Taking into account adsorption parameters, contact time, concentration of sorbent and of adsorbate were optimized. Particular attention was given to As (III), because of its higher abundance and toxicity. Isotherm and kinetic parameters in adsorption of As (III) were gathered. The best fitting of adsorption was observed with Freundlich model, while about kinetics with a pseudo-second order model, obtaining better results with Miscanthus biochar. A sensitive interference, due to the presence of sodium in aqueous phase, was quantified in As (III) adsorption (60% decrease). Miscanthus biochar, although un-activated, was found to have an adsorption capacity (0.112 mg/g) comparable to literature data about low-cost biosorbents and biochars already studied about arsenic adsorption. The tested substrates are not suitable for a quantitative removal of arsenic in the evaluated boundary conditions, although they may be considered a reliable option for the pre-treatment of heavily polluted water.