Speciation of metals in soil solutions – the concept of forced – shift – equilibrium: quantification of the complexing ability of soil solutions

K.E. Ødegård, K.H. Karstensen, W. Lund

The concept of forced-shift-equilibrium makes it possible to model the degree of metal complexation in solutions without knowing the actual ligands in the sample. The experimental basis for the concept is the addition of increasing amounts of a chelating cation exchanger to a sample and a reference solution. By treating the ion exchanger as a reactant in the calculations, the complexing ability of the sample solution can be estimated. The concept is tested on two laboratory made solutions to determine the complexation of Cu by humate and of Cd by chloride.

Seven years of experiences with lysimeter leaching of pulverised fuel ash

R. Meij, B.H. te Winkel

In the Netherlands the leaching behaviour obtained by the Dutch Column test is decisive if and in what way the by-products can be used as a building material or in what way the waste has to be dumped. Accordingly, the translation of results of the Dutch standard leaching tests into field conditions is important and therefore a major research area at KEMA.

In the leaching process two steps can be considered: percolation and dissolution of various water-soluble compounds. Only through the combination of these two processes compounds can be leached out of the waste and enter into the environment. In order to study these processes under field conditions in detail, lysimeters with a height of 1-m and 4-m were built and filled with pulverised fuel ash produced by Dutch coal-fired power plants. In these lysimeters the moisture content was monitored and the vaporisation of the moisture determined. The hydrology was modelled. In order to gain more insight into the chemical processes, the pore water in the lysimeters and the percolate was analysed as well.

In this paper the results of the first seven-years of the experiment are summarised. The L/S depends on the height of the lysimeter and amounts after six years to 1,5 L· kg-1 for the low lysimeter and 0,5 L· kg-1 for the high lysimeter. The leaching of the lysimeters is compared with the laboratory leaching tests for the same L/S value. It appears that the leaching of Si and B in the lysimeters is comparable as in the column test. The same observations are also found for Mo, Cr and Se, if only the low lysimeter is taken into account. However, the leaching for Mo, Cr and Se is higher in the high lysimeter with respect to the column test. The leaching of Ca, Fe, Mg, Ba, Cu and Sb in the lysimeters is lower than in the column test. The leaching of Al, V, K and Na in the lysimeters is higher than in the column test, besides the leaching in the high lysimeter is higher than in the low lysimeter. The differences between laboratory tests and lysimeters are ascribed to the different time scale. Most probably the observed differences are kinetically determined.

Due to meteorological circumstances in the upper part of the lysimeters dry-wet cycles were observed. It appears that when the PFA became wet again, every time peak leaching of Ca, S, Mo and Se was observed.

Utilization of wastes in ground improvement works

G. Rajasekaran, S. Nagan

A viable solution to waste disposal problem would be the reuse of wastes either by recycling or modify their properties to suit for various geotechnical applications. New cost effective technologies should be developed to provide a comprehensive solution to address the by-products disposal problem. In this paper, the various processing technologies involved in utilizing or treating wastes, and the product suitability for construction works have been discussed. Also the use of construction wastes such as excavated spoils and concrete debris for land reclamation and subsequent ground improvements have been proposed. The above methods serve as solution scenarios for various geo-environmental problems.

Leaching processes in cement-stabilised municipal incinerator air pollution control residues

I. Baur, Ch. Ludwig, C.A. Johnson

The geochemical and hydrological factors controlling field and laboratory leachate composition of cement stabilised air pollution control (APC) residues (41% APC residues, 22% cement, 3% Na2CO3, and 32% water, w/w) have been investigated. Field samples were taken as a function of hydrological conditions from a field lysimeter (3 x 5 x 1.5 m) containing cubic blocks (edge length, 0.5 m) during 3 rain events. It was assumed that the field leachate composition was probably not strongly influenced by diffusion processes because in the short time scale of the experiment (7 years), it was estimated that less than a millimeter of the block surfaces was leached. Laboratory experiments were carried out on ground samples at solid/liquid ratios of 0.05, 0.1 and 0.2 with an equilibration time of 28 days. The major components Ca, SO4, Al and Si could be partially modelled by assuming calcium silicate hydrate (C-S-H), portlandite and ettringite to be the solubility controlling phases. There were obviously additional minerals that could not be taken into account in calculations because of the lack of data. Heavy metal concentrations were generally lower in the field leachate than in laboratory experiments and again could only be partially modelled. It is probable that cement minerals play an important role in their immobilisation. There are also indications that for some heavy metal anions, Ca metallates could be dominant solubility-limiting phases.

Using rice husk ash as a cement replacement material in concrete

M. Anwar, T. Miyagawa, M. Gaweesh

Many countries have the problem of shortage of conventional cementing materials. Recently there are considerable efforts worldwide of utilizing indigenous and waste materials in concrete. One of such materials is the rice husk which, under controlled burning and if sufficiently ground, the ash that is produced can be used as a cement replacement materials in concrete. This research has been undertaken to study the main characteristics of the Rice Husk Ash (RHA), properties of fresh concrete and development of the fundamental properties of hardened concrete. Also, the research involves developing a comprehensive engineering data-base on RHA concretes including durability aspects such as long term performance in artificial sea water (5% NaCl solution). The chloride ions permeability and chloride diffusion coefficient were measured using potentiometric titration analysis. Moreover, the porosity and pore structure of concrete were performed using Mercury Intrusion Porosimetry (MIP). The obtained test results showed that using RHA improved the different studied properties of concrete. In this paper, experimental program, test results and analysis as well as conclusions are presented.

Modelling the effects of waste components on cement hydration

R.J. van Eijk, H.J.H. Brouwers

Ordinary Portland Cement (OPC) is often used for the Solidification/Stabilization (S/S) of waste containing heavy metals and salts. These waste componenents will precipitate in the form of insoluble compounds onto unreacted cement clinker grains preventing further hydration. In this study the long term effects of the presence of contaminants in solidified waste is examined by numerically simulating cement hydration after precipitation of metal salts on the surface of cement grains. A cement hydration model was extended in order to describe porewater composition and the effects of coating. Calculations were made and the strength development predicted by the model was found to agree qualitatively with experimental results found in literature. The complete model is useful in predicting the strength and leaching resistance of solidified products and developing solidification recipes based on cement.

Leaching behaviour of synthetic aggregates

H.A. van der Sloot, D. Hoede, D.J.F. Cresswell, J.R. Barton

In the framework of EU project Utilising innovative kiln technology to recycle waste into synthetic aggregate (BRST-CT98-5234), the leaching behaviour of synthetic aggregates has been studied to assess its environmental compatibility in the various stages of its use. Since the conditions are very different for the different uses, the assessment calls for a variety of different leaching conditions. The pH dependence test is used to cover important differences in pH environment to which the materials are exposed to as well as for an assessment of the buffering capacity of the material. Synthetic aggregate features a low buffer capacity, which makes it sensitive to externally imposed pH conditions. Utilisation and storage exposed to acidic conditions needs to be avoided. The pH dependence test and column leaching test are consistent. The CEN TC 154 method appears provide systematically low values due to the arbitrary selection of test conditions. Synthetic aggregate studied to date will not adversely affect the concrete in its service life. The main issue for aggregate use is the recycling and the "end of life" condition, when the material becomes construction debris. Not metals, but oxyanions, such as Cr VI and Mo are most relevant under these conditions. A concise test has been applied to assess crucial aspects of leaching for different production mixes.

Leaching of heavy metals from soils – an analysis

T. Honders, M. Gadella, P. de Wilde, C. Zevenbergen

The leachabilities for As, Cd, Cr, Cu, Hg, Pb, Ni, Zn of ca. 1000 lots of (untreated and treated) soils have been measured. The leachabilities are largely independent on concentration over a broad concentration range. From an evaluation against legal leaching standards, a decision support system is derived. Thermal treatment and soil washing generally yield an improved soil quality in terms of leachability.

The development of a code of practice for the environmental sound use of PFA as a fill

R. Coombs, L.K.A. Sear

Although pulverised fuel ash (PFA) has been successfully used in a range of applications for many years there has always been need to give consideration to its environmental impact. The increasing awareness of environmental issues has had an impact on sales, with the need to assess its use on a site by site basis, the time taken to get approval for the use of PFA can exceed the time scales imposed by site operations. There has therefore been a need for an alternative approach to the problem.

This paper details the approach at present being developed. This is a three-stage one with an initial assessment of the process, a review of existing data and what further data is required and finally a code of practice that recommends a rational approach to the use of PFA as a fill.

Leaching behaviours of heavy metals from MSWI residues and Pb adsorption onto the residues under alkaline condition

S. Mizutani, S. Sakai, H. Takatsuki

A small-scale column test simulating a monofill landfill site, which is composed of MSWI residues alone, was performed. The pH of the leachate was higher than 12 for 2 months and the adsorption of Pb and Zn onto the residues took place. The Pb adsorption was Freundlich type, based on batch experiments to study adsorption behaviour. Adsorptive ability of MSWI residues were different for each residues, and a positive correlation between adsorptive ability and total contents of "Fe + Si + Mn + Al" of MSWI residues was observed.