CAN THE RIETVELD METHOD BE SUCCESSFULLY APPLIED TO THE SEVERELY SUPERIMPOSED DIFFRACTION PATTERNS OF TECHNICAL PRODUCTS CONTAINING SEVEN SOLID SOLUTIONS AND A STOICHIOMETRIC PHASE?

Burkhard Peplinski, Peter Köcher, Gerd Kley

Federal Institute for Materials Research and Testing (BAM),

Richard-Willstätter-Str. 11, D-12489 Berlin, Federal Republic of Germany

Many countries require the removal of phosphorus from wastewater before discharge to the ambient environment and the development of technologies for the reuse of the recovered phosphorus [1]. By doing this, local rivers and lakes are protected from eutrophication (phosphorus pollution), and the world’s limited resources of mineral phosphorus, which are estimated to be exhausted in about 250 years, can be used more economically. Although phosphorus recovery from sewage sludge is already practised on a larger scale, the economi-cal and ecologically sensible reuse of millions of tons p.a. of recovered phosphates is still being debated. The principal obstacle for the reuse of ashes from sewage sludge incineration as phosphate fertilizer in agriculture is their high content of ecologically harmful heavy metals, such as Zn, Cu, Cd, Hg, which exceeds the legal limits considerably. A new techno-logy [2] is being developed at the BAM that reduces the content of ecologically harmful heavy metals in ashes from sewage sludge incineration to a value, orders of magnitude below today’s legal limits. Ashes from sewage sludge incineration treated with this technology can be used as ecologically desirable, high-quality fertilizers. In addition, this treatment increases the bioavailability of the phosphorus content from a low original value of 50 to 100%, thus resulting in cost effectiveness, an essential advantage of the technology. The new technology has been developed on chemical intuition and decades of experience. A detailed explanation of the phase transformation processes accompanying this thermochemical treatment of ashes from sewage sludge incineration is the aim of this paper.

Ashes from sewage sludge incineration are nonequilibrium mixtures of a large number of interdispersed crystallographic phases. This complicates the applicability and the signifi-cance of results of many microscopical, spectroscopic, thermoanalytical and theoretical methods. Therefore, X-ray powder diffraction (XRPD) is the method of choice for studying i) the phase composition of ashes from sewage sludge incineration and ii) the solid-state reactions occurring during thermochemical treatment. Of course, the presence of a large number of solid solution phases in these materials also presents an exceptional challenge to the XRPD method, and qualitative (and semi-quantitative) phase analyses are not straight-forward. Instead, any solution found with the help of crystallographic databanks and search/match routines has to be regarded as a proposal only and must be validated. As there is no independent analytical method for the confirmation of the phase analysis of ashes from sewage sludge incineration and for the structure refinement of some of their constituents, procedures that can test the internal consistency of the XRPD results gain special significance here. The procedure serving this purpose best is the Rietveld method, especially if the profiles of the diffraction lines are described by the fundamental parameter approach. This method enables the fitting of the diffraction pattern calculated from the approximately determined phase composition of the sample to the experimentally observed diffraction pattern within a large 2q range. Thus, the reliability of the phase analysis results can be checked for internal consistency and the best alternative proposal selected. In doing so, the Rietveld method should by no means be used as a black-box method. Instead it is mandatory to consider the often ignored methodical problems of the Rietveld method, see for instance [3] and references therein, as well as the chemical and mineralogical specifics of the samples under investiga-tion. Making use of mineralogically well-founded restraints for the lattice parameters and the

Fig. 1. Observed (dotted line) and calculated (solid line) diffraction pattern, difference curve and agreement indices (small box) for one type of ashes from sewage sludge incinera-tion after being processed by the thermochemical treatment developed at the BAM (phase composition: seven solid solutions and a stoichiometric phase)

site occupancy factors of the multi-dimensional solid-solution series is often helpful, and sometimes even necessary. Besides this, in the present work, wherever possible, partial results of the Rietveld analyses were checked by the additional analyses of tailor-made model samples with simpli-fied chemical and phase compositions. A close collaboration between the specialist for the Rietveld method and the chemist synthesising the model samples was one of the key-factors for the successful completion of this challenging analytical task.

The subject of this investigation was ashes from sewage sludge incineration from several municipal wastewater treatment plants, each of which used a different precipitation process. These ashes were characterized both in their original state and after thermochemical treatment according to the technology developed at the BAM [2]. The diffraction patterns of these ashes were collected and analysed by the Rietveld method. Excellent agreement between the calculated and the observed diffraction patterns was achieved for all investigated samples, see Fig. 1. The results produced in this phase-analytical investigation provide an explanation for the repeatedly reported observation [4] that the bioavailability of phosphorus deteriorates with the presence of iron ions. At the same time, the solution to this relevant problem became obvious.

In summary, the title question of this paper can be answered with ‘Yes’.

1. http://www.nhm.ac.nk/mineralogy/phos/ (Natural History Museum, London)

2. Köcher, P., Kley, G., Brenneis, R. ‘Method for the separation of ecologically harmful heavy metals from ashes from sewage sludge incineration’, German patent 102 43 840.4; accepted 30.01.2004

3. Peplinski, B., Kleeberg, R., Bergmann, J. (2004) Materials Science Forum 443-444, 45-50

4. Römer, W., Samir I.S. (2002) J. Plant Nutr. Soil Sci. 165, 83-91