A comparison of solid wastes as low-cost adsorbents and immobilizing agents for heavy metals

Ya-feng Zhou (2011). A comparison of solid wastes as low-cost adsorbents and immobilizing agents for heavy metals PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

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Author Ya-feng Zhou
Thesis Title A comparison of solid wastes as low-cost adsorbents and immobilizing agents for heavy metals
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2011-02
Thesis type PhD Thesis
Supervisor R. J. Haynes
Ravi Naidu
Total pages 185
Total black and white pages 185
Subjects 07 Agricultural and Veterinary Sciences
Abstract/Summary The purpose of this research was to evaluate the use of inorganic and organic wastes of industry and municipalities as adsorbents and immobilizing agents of heavy metals from aqueous solutions and from a Pb/Zn-contaminated alkaline soil. The use of such materials as adsorbents and immobilizing agents would reduce the cost and need for their disposal as well as reducing pollution of ground and surface waters from contaminated effluent streams and reducing the bioavailability of metals in contaminated soils. In the first study, the adsorption capacity of seven inorganic solid wastes [air-cooled blast furnace (BF) slag, water quenched BF slag, steel furnace slag, coal fly ash, coal bottom ash, water treatment sludge, and red mud] for Cd2+, Cu2+, Pb2+, Zn2+ and Cr3+ was determined in batch adsorption experiments. All materials had the ability to remove metal cations from aqueous solution and their relative adsorption abilities increased greatly with increasing pH. The magnitude of adsorption at pH 6.0 followed the general order: Cr3+ ≥ Pb2+ ≥ Cu2+ > Zn2+ = Cd2+. The quantities of previously adsorbed Pb and Cd desorbed in 0.01 M NaNO3 electrolyte were very small but those removed with 0.01 M HNO3, and more particularly 0.10 M HNO3, were substantial. Water treatment sludge was shown to maintain its Pb and Cd adsorption capability over eight successive cycles of adsorption/regeneration using 0.10 M HNO3 as a regenerating agent. By contrast, for BF slag and red mud, there was a pronounced decline in adsorption of both Pb and Cd after only one regeneration cycle. This was explained in terms of residual surface alkalinity being a key factor contributing to the high adsorption capability of the latter two materials and that acid pre-treatment resulted in neutralization of much of this alkalinity. It was concluded that alum water treatment sludge showed greatest potential as an adsorbent. In the second study, the adsorption/desorption and regeneration capacity of two different samples of alum water treatment sludge for removal of Pb(II), Cr(III) and Cr(VI) (i.e. CrO42-) from aqueous solution was investigated. Adsorption characteristics of the two samples were generally similar. Adsorption isotherm data for all three ions fitted equally well to Freundlich and Langmuir equations. Maximum adsorption capacity and indices of adsorption intensity both followed the order: Cr(III) > Pb(II) > Cr(VI). Kinetic data correlated well with a pseudo-second-order kinetic model suggesting the process involved in adsorption was chemisorption. Adsorption was pH-dependant with percentage adsorption of Cr(III) and Pb(II) increasing from < 30% to 100% between pH 3 and 6 whilst that of Cr(VI) declined greatly between pH 5 and 8. Pb(II) and Cr(III) were effectively desorbed with HNO3 (at 0.10 and 0.50 M concentrations) and Cr(VI) by NaOH (at 0.01, 0.10 and 0.50 N concentrations). It was concluded that water treatment sludge is a suitable material for development of a low-cost adsorbent for removal of heavy metal cations and anions from wastewater streams. The metal (Cd2+, Cu2+, Pb2+, Zn2+ and Cr3+) adsorption capacity, from aqueous solution, of seven organic wastes (slash pine, red gum and western cypress bark, composted green waste, prawn exoskeletons, spent brewery yeast and mill mud from a sugar mill) was evaluated in the third study. All materials had substantial adsorption capacity for metal cations and spent yeast was the least effective. The order of selectivity of metals at pH 6.0, as evaluated by Langmuir qmax and Freundlich Kf values, was: Cr3+ > Cu2+ > Pb2+ > Zn2+ ≥ Cd2+. For pine bark, compost, mill mud and prawn shell, quantities of previously adsorbed Pb and Cd desorbed in 0.01 M NaNO3 electrolyte were negligible. However, HNO3 at concentrations of 0.10 M and 0.50 M HNO3 was effective at removing both adsorbed Pb and Cd. When 0.10 M HNO3 was used as the regenerating agent, pine bark and compost maintained their Pb and Cd adsorption capacity over 8 successive adsorption/regeneration cycles. By contrast, mill mud and prawn shell showed a noticeable decrease in adsorption capacity after only one regeneration cycle. This was attributed to their significant CaCO3 content (which is acid-soluble) that contributes substantially to their adsorption capacity. Compost, and to a lesser extent pine bark, showed great potential as a low-cost adsorbent. In the fourth study, five of the most effective adsorbents identified in the above studies (water treatment sludge, BF slag, red mud, compost and mill mud) were evaluated as metal immobilizing agents in an alkaline Pb/Zn-contaminated soil. Materials were incubated with the soil for a period of 12 months at rates of 5 and 10% w/w, after which, Rhodes grass was grown in the soils in a greenhouse study. Addition of BF slag, water treatment sludge and red mud markedly reduced acetic acid-, DTPA-, and EDTA-extractable Zn levels, while addition of mill mud, compost and red mud lowered acetic acid and DTPA-extractable Pb concentrations. Applications of red mud, mill mud and compost caused a significant increase in grass yield. Rhodes grass biomass was negatively correlated with both concentrations of extractable Pb in the soil and plant tissue Pb concentrations. Red mud was the most effective material for reducing the extractability of both Pb and Zn. From the research, it was concluded that in relation of both adsorption capacity and desorption/regeneration capability, water treatment sludge and composted green waste showed great potential as effective, low-cost adsorbents, and materials such as water treatment sludge, BF slag, red mud, and mill mud showed good potential as immobilizing agents. Where multielement adsorption or immobilization of metals is required, a combination of adsorbent materials is likely to be most effective.
Keyword solid wastes, heavy metals, adsorbents, adsorbent regeneration, immobilizing agents

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