Fly ash adsorbents for multi-cation wastewater treatment

Abstract

Class “F” fly ash (FA), collected from the Central Heat and Power (CHP) Plant Brasov (Romania), with oxides composition SiO₂/Al₂O₃ over 2.4 proved good adsorbent properties, and was further used for obtaining a new substrate with good adsorption capacity for heavy metals from multi-cation wastewater treatment. Firstly, the new adsorbent was characterized by AFM, XRD, DSC, FTIR and the surface energy was evaluated by contact angle measurements. The experimental data suggested that the new type of substrate is predominant crystalline with highly polar surface.

The substrate was used for removing the Pb²⁺, Cd²⁺ and Zn²⁺ cations from mixed solutions. The results show high efficiency and selective adsorption the Pb²⁺ and Zn²⁺ cations.

The optimized adsorption parameters were further used in thermodynamic and kinetic studies of the adsorption processes. The Langmuir and Freundlich models were used to describe the processes. The pseudo-second order kinetics could well model all the processes, indicating a surface concentration of the adsorption sites with the same order of magnitude as the cation concentrations.

Introduction

Combined heavy metals cations result in wastewaters from many industrial processes, such as electroplating, inorganic pigment manufacturing, wood processing, photographic operations and petroleum refining. The heavy metals are also utilized in rubber, pesticides and plastics. Many compounds of heavy metals are easy soluble in water and can be adsorbed by living organisms part of the food chain. Some heavy metals (copper, cobalt, iron, manganese, vanadium, strontium and zinc) are accepted in small concentration for living organisms but excessive levels of essential metals can be detrimental to the organisms. Non-essential heavy metals (including cadmium, lead and copper) are dangerous for living organisms because they tend to bio-accumulate and in time they cause serious health effects [1] (cancer, liver damage, renal disorder, visceral cancers, insomnia, depression, lethargy, vomiting). Particularly children will be at risk to consume more food with heavy metals resulting in a reduction of growth, and a damage of the nervous system and the brain.

Although Zn²⁺ is considered to be relatively nontoxic (is a co-factor in over 100 enzymes reactions) its excessive inhalation (as fumes) may cause the diseases. The toxicity symptoms – nausea, vomiting, skin, eyes yellow, “Zinc chills”, epigastria pain, or, lethargy, neuropathy, low blood pressure and fatigue – appear at high Zn intakes.

To minimize the human and environmental exposure to these hazardous heavy metals the US Environmental Protection Agency (US EPA) established the limits of cadmium, lead and zinc that may be discharged into wastewater at: 0.01 mg/L, 0.006 mg/L and 0.80 mg/L, respectively.

The heavy metals can deactivate the active sludge (poisoning the bacteria) from the secondary treatment plants [2]; therefore the chemical treatment must remove heavy metals before the biological step. The most common and widely used method for removing heavy metals from wastewater is chemical precipitation using caustic soda or lime [3]. This method is not expensive but requires a large amount of chemicals, resulting in a high quantity of sludge that requires a supplementary treatment. Moreover, according to the precipitate's solubility, the residual amount of dissolved heavy metal may exceed the discharge limit.

Other methods for advanced heavy metal removal are ion exchange, reverse osmosis, ultra filtration, electrochemical deposition and adsorption.

Although largely used in industry, adsorption, particularly ion exchange, presents several disadvantages like pH sensitivity, non-selectivity, and cost. Replacing synthetic substrates with low-cost adsorbents is therefore intensively studied and there were reported materials for heavy metals removal obtained from agriculture and forest wastes such as bagasse fly ash [4] sugar beet pulp [5] activated carbon derived from bagasse [6], maple saw dust [7], clay [8], [9] volcanic ash bone char [10] humus [11] or bituminous coal.

Heavy metals (cadmium, copper, zinc, nickel) removal was reported on scrap rubber, bituminous coal, peat [12], natural zeolite [13] while 4A zeolite synthesized by dehydroxylation of low grade kaolin is reported to remove Cu(II) and Zn(II) ions at neutral and alkaline pH [14].

Fly ash has potential applications in wastewater treatment because of its chemical composition, low cost and good adsorbent properties (porosity, large surface area and particle size distribution.

Table 1 summarizes some of the recent results of Cd(II), Zn(II), Pb(II) ions removal from wastewater using fly ash.

The large majority of the studies report on mono- or bi-pollutant systems but industrial wastewaters have usually a more complex composition, thus occupying the adsorption sites depends on the components’ affinity and substrate energy, and may lead to completely different efficiencies.

The coal combustion products (CCP) are: fly ash (FA) (>60% of CCP), bottom ash, boiler slag and flue gas desulfurization materials. Fly ash is a waste mainly used in cement manufacturing (due to the large amounts of silica and alumina oxides) but fly ash can also be used as adsorbent material, solving two major environmental problems, by reducing the air pollution threats and cleaning the wastewater. The adsorption capacity of raw fly ash is rather low but can be improved, usually by chemical treatment. Previous studies proved that conditioning by alkali treatment (1–3 N) can be a viable path for enhancing the adsorption efficiency of heavy metals [16] or multi-component systems of heavy metals and dyes [17]. This concentration is significantly lower compared with the 5–8 N usually reported.

This paper presents the results of adsorption on a new substrate, obtained from fly ash, used as adsorbent for removing Pb²⁺, Cd²⁺ and Zn²⁺ from three-cation solutions. The results are compared with those obtained when using macro- and microporous cationic exchangers.

Based on the results obtained in testing this new substrate in removing heavy metals from aqueous solutions, the next step will be to test the adsorption capacity of the substrate on real/industrial wastewaters, where the composition includes also other cationic and non-ionic components.