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The retention of heavy metals in Spearwood sand: A theoretical, empirical and waste management study

Marks, Peter John (1980) The retention of heavy metals in Spearwood sand: A theoretical, empirical and waste management study. PhD thesis, Murdoch University.

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Land disposal of waste requires a more scientific understanding of how pollutants are removed by soil. This is of particular concern on the Swan Coastal Plain of Western Australia where waste disposal must occur adjacent to ground water that is used for Perth's water supply.

A theoretical framework for understanding the removal of heavy metals in soil is developed by considering first the aqueous solution chemistry of divalent heavy metal cations, second the equilibrium thermodynamics of cation exchange and adsorbtion in soils, and third the miscible displacement of a solution flowing through a soil column. A General Retention Model is constructed which suggests that there are two modes or compartments by which heavy metal cations are adsorbed onto the mineral oxide components of soils - cation exchange (a fast reaction) and mineralisation (a slow reaction). Mineralisation is defined as specific adsorbtion (or ligand penetration) and/or surface precipitation,both of which are better understood in terms of a Trilamina Lattice forming on the interface between soil and solution. The model also shows how Hard-Soft Acid-Base Theory, Ionic Mobility, Free Energy of Solvation, and the Degree of Complexation by Anionic Ligands should aid in the prediction of the retention of various heavy metal cations.

The empirical study examined the interaction of solutions containing the heavy metals Pb 2+, Cu 2+, Zn 2+, Co 2+ and Cd 2+ as they each flowed through columns of Spearwood Sand, using a technique which measured pH, conductivity and Ca 2+ concentration right through an adsorbtion and desorbtion phase. These results were viewed against a background of retention patterns established with the alkali earth metals Ca2+, Ba2+, Sr 2+ and Mg 2+ which showed that only cation exchange was occurring due to their stable solvation and negligible complexation characteristics. The order of retention Ba 2+ > Sr 2+ Ca 2+ > Mg 2+ is consistent with their AG solvation and Limiting Ionic Mobilities.
The single heavy metal runs showed that the major removal mechanism was cation exchange (compartment 1) but that mineralisation (compartment 2) began to occur simultaneously and especially in the case of Cu 2+ became highly significant as time went on so that desorbtion was much reduced. A mixed metal run showed that the order for removal was Pb2"+' > Cu 2+ ~ Zn 2+ ~ Ni2 + > Ca 2+ > Co 2+ > Mn 2+ > Cd 2+ which indicated, along with other supporting data, that mineralisation was more important in this experiment (due possibly to the higher Cl concentration and hence greater complexation). The anomolous position of Zn 2+ is understood in terms of lattice configurations assisting mineralisation.
The flow patterns showed a number of electrokinetic interactions may be occurring in the soil column and the increase in dispersion with increasing flow rate indicates that the solution flow is as in capillary flow through a non-unimodal soil.

Waste management conclusions were made based on the theoretical framework and the empirical aspects of the study. The mineral oxide component of soils is the key to pollutant removal but no simple measurement e.g. C.E.C. can determine potential removal capacity; the development of techniques to estimate coulombic energy in a soil-waste system would greatly assist this. As mineralisation is a preferred removal process because it is not as easily reversible and because bulk precipitation can cause channelling, the scientific adjustment of pH could be a powerful tool in the land treatment of waste. As a general rule it can be predicted that the higher the atomic weight the greater the retention of metals in soils, however some Transition elements are unusual because of complexing; the Irving Williams order of reactivity characterises the ability of the 1st Transition series metals to be retained in soils. When mixed with other anions such as PO43-,C032- and s2- in leachate heavy metals such as Cu2+ and Hg 2+ may be more easily retained by soils; on the other hand Ni 2+ may be mobilised by amino complexes. Desorbtion of heavy metals may occur if acids, brines or complexing anions are added to former waste sites.
A simple model predicts that 2 metres of Spearwood Sand should be sufficient for the removal of most heavy metals in wastes except for anions such as Cr^O. 2- which should first be chemically converted to a cation before land treatment.
The use of column techniques as developed in this study are suggested to provide more important information for waste management than the traditional batch techniques.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental and Life Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Newman, Peter
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