Journal of colloid and interface science article

Pergamon Wat. Sci. Tech. Vol. 38, No. 2. pp. 17-23, 1998.

IAWQ

0 1998 Published by Elsevier Science Ltd.

PII: SO273- 1223(98)00427-2 0273-1223/98 $19fKl+ OCG

CHROMIUM SPECIATION IN MUNICIPAL

SOLID WASTE: EFFECTS OF CLAY

AMENDMENT AND COMPOSTING

Goen Ho and Liang Qiao

Institute for Environmental Science, Murdoch University, Murdoch 6150.

Western Australia

ABSTRACT

The addition of clay in the form of bauxite refining residue (red mud) prior to composting has been

suggested as a way to control heavy metal mobility in compost. Leachability and plant availability of metals

in a mixture of grass clippings and sawdust spiked with metal solution was markedly reduced during the

composting process. The fate of metals in municipal solid waste compost applied to land was examined by

using a sequential step extraction to investigate metal speciation (into exchangeable and bound to carbonate

forms, to Mn & Fe oxides, to organic matter and in residue phase) in red mud amended compost. The effects

of red mud and the composting process on metal speciation in the compost for Cd, Cr, Cu. Ni, Pb and Zn

were investigated, and a comparison of some effects with biosolids compost was made. Addition of red mud

reduced the metal mobility and the potential hazard of releasing metals from compost through promoted

precipitation, adsorption and complexation of free metal cations to red mud. Red mud however, was not able

to desorb metals bound to organic matter. Since most of the metals in the municipal solid waste were. not

usually bound to organic matter, the addition of red mud prior to composting fixed the free metal ions before

they bound to this fraction. Results for Cr speciation are reported in this paper. 0 1998 Published by Elsevier

KEYWORDS

Bauxite refining residue (red mud); chromium; composting; metal mobility; metal speciation; municipal

solid waste.

INTRODUCTION

Compost produced from mixed MSW can be contaminated by heavy metals derived from contamination of

domestic waste by dry cell batteries, metal coatings, paints, solvents, cosmetics, dyes, pesticides, lubricants

and metals in electronic equipment and other discarded domestic appliances. One way of overcoming the

potential heavy metal problem of compost produced from mixed MSW is to add clay particles (< 2 pm)

which have a large surface area and capacity to adsorb heavy metals, thus reducing the leachability and plant

availability of heavy metals in the compost.

Hofstede (1994) investigated the immobilisation of heavy metals in the compost of artificial MSW using

bauxite refining residue (red mud) amendment. A mixture of grass clippings and sawdust was spiked with a

metal solution to make artificial MSW, amended with red mud and then composted in controlled laboratory

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