NATURAL ZEOLITES - REMEDIATION TECHNOLOGY FOR THE 21ST CENTURY?

Karen Stead1, Sabeha K.Ouki1 & Neil I. Ward*2

(1Centre for Environmental Health Engineering, School of Engineering in the Environment, University of Surrey, Guildford, Surrey, GU2 5XH, UK.  2ICP-MS Facility, Department of Chemistry, School of Physics and Chemistry, University of Surrey, Guildford, Surrey, GU2 5XH, UK) *n.ward@surrey.ac.uk

ABSTRACT

 

Recent methods to remove heavy metal contaminated effluents have involved physicochemical methods such as precipitation, activated carbon adsorption, ion exchange, reverse osmosis, foam flotation techniques and cementation.  Hydrated aluminosilicates (natural zeolites) are present in large quantities around the globe, and have been shown, through ion-exchange, to retain heavy metals found within contaminated effluents - thus acting as an in-situ method for remediating contaminated media.  Natural zeolites, formed by the reaction of fine-grained volcanic ash with pervasive ground, have many advantages and uses in the fields of environmental protection and remediation.  For the past 30-40 years, they have been used in agriculture, agronomy, aquaculture, animal husbandry, energy conservation, and most recently wastewater treatment and pollution control.  Natural zeolites reveal certain selectivities for heavy metal ions, and as such can be used to treat heavy metal contaminated effluents and solutions.  Apart from the treatment of contaminated media, they also play a large role in the application of essential nutrients to soils.  This paper will outline the mechanism in which zeolites are used to remove pollutants from the environment, namely their molecular sieve and ion exchange properties.  A review will also be presented of the research to date where natural zeolites have been used in pollution control and remediation.

 

INTRODUCTION

 

Natural zeolites were first discovered in 1756 by the Swedish mineralogist Freiherr Axel Fredrick Cronstedt (Mumpton, 1983).  However, it was not until the late 1950s that researchers showed their effectiveness for environmental protection and remediation.  With more than 2000 deposits found globally natural zeolites are a natural plentiful resource, and are inexpensive to mine, since the majority of deposits are found close to the earth's surface.  Natural zeolites constitute more than 90% of many sedimentary rocks of volcanic origin (Hawkins, 1983).  The production of synthetic zeolites began in the late 1800s for industrial use and are now perhaps more well known as phosphate replacements in laundry detergents.  Natural zeolites are less expensive than synthetic zeolites and as such are being increasingly used for agriculture, aquaculture, agronomy, animal husbandry, energy conservation, and wastewater treatment and pollution control.

 

ZEOLITE COMPOSITION

 

Natural zeolites are hydrated aluminosilicates, comprising of hydrogen, oxygen, aluminium, and silicon arranged in an interconnecting, open, three-dimensional structure. The primary building units are [SiO4]4- and [AlO4]5- tetrahedra linked together by oxygen atoms.  Their crystal structure allows water molecules to be held and removed within the channels and cavities within the lattice.  These cavities can also contain exchangeable cations, in particular K, Na, Mg, Ca, Sr and Ba.  The cavities and channels that exist within the zeolite framework can constitute as much as 50% of the total crystal volume (Passaglia & Galli, 1991), whilst water constitutes as much as 10-20% by weight of the natural zeolites (Mumpton, 1983).  The presence and occupation of cations, within the cavities and channels will largely rule the amount of water contained within the zeolite framework.  The general empirical formula, which represents a zeolite chemical structure, is shown below:

 

                                    M2nO . Al2O3 . xSiO2 . yH2O

 

M represents any alkali or alkaline earth cation, n the valence of the cation, x varies between 2 and 10, and y varies between 2 and 7 (Hawkins, 1983), with structural cations comprising Si, Al and Fe3+, and exchangeable cations K, Na and Ca.

 

Natural zeolites are not found as pure minerals.  They can often contain small percentages of quartz, feldspar, clay minerals, cristobalite, calcite, gypsum and untreated volcanic glass.

 

ZEOLITE PROPERTIES

Ion Exchange

The subsequent substitution of Si4+ by Al3+ leaves a net negative charge on the zeolite framework - known as Isomorphous Substitution.  These areas of negative charge are therefore ideal sites for adsorption of exchangeable cations in solution.  If there is no suitable site in the structure, or if it is already filled, the cations occupy the sites of water molecules upon ion exchange (Tsitsishvili et al., 1992).

 

Molecular Sieves

Zeolites also have the ability to exclude certain cations depending on their size; i.e. the size of the microporous channels and cavities within the zeolite structure can act to ‘sieve’ cations.  Those cations that are bigger than the internal cavities are excluded from all or part of the internal surface of the zeolite, whereas, cations that can ‘fit’ into the internal structure can be exchanged (through isomorphous substitution or ion-exchange) onto the structure and become part of the zeolite framework.  Hence, natural zeolites are renowned for their ‘molecular sieve’ properties.  Ion exclusion phenomena are frequently observed in zeolites in which a particular ion is excluded from the exchanger because of its size (Townsend 1984).  Ions can be partially exchanged because the volume the ion occupies may be too great, therefore occupying the intracrystalline space in the channels before complete exchange can be attained.  Tsitsishvili et al., (1992) detailed that zeolitic water molecules act as bridges for framework ions and exchangeable ions in large framework cavities.  This shows the mobility of these cations within the framework.               

 

ENVIRONMENTAL PROTECTION & REMEDIATION

 

Nuclear Energy

Natural zeolites, such as clinoptilolite and mordenite, have been used as 'buffer' materials to protect the environment against radioactive contamination.  They are used as a 'buffer' due to their high plasticity, high sorption capacity relative to the radionuclides, high thermal conductivity, chemical stability and mechanical strength (Pansini, 1996).  In the United Kingdom, one large nuclear power industry has utilised clinoptilolite for the removal of radionuclides in low-level radioactive effluent.  Once the zeolite has become exhausted the resultant mix of zeolite and radionuclides is compacted and sent for disposal.  There is no initial pre-treatment or regeneration of the natural zeolite after it has become exhausted. 

 

Wastewater Treatment

Research conducted by Kesraoui-Ouki & Kavannagh (1997) using natural zeolites in the treatment of mixed metal contaminated effluents, showed through the conditioning of clinoptilolite (with 2N NaCl), that 90% of heavy metals could be removed within a 15 minute contact time.  The selectivity sequence for clinoptilolite was shown to be Pb > Cu > Cd > Zn > Cr > Co > Ni.                        

 

Landfill Liner

Their ideal buffer characteristics have also led some researchers to ascertain the use of natural zeolites as a liner for landfills.  Kayabali & Kezer (1998) found using natural zeolites in place of a more traditional clay liner in landfills would reduce the thickness of the required liner and reduce leachate-based hazards for groundwater contamination.

 

Contaminated Land Remediation

The main research behind the use of natural zeolites as a remediation tool for contaminated soil has been conducted largely through laboratory and greenhouse trials.  There is very little evidence in the literature to support the long-term use of natural zeolites in real remediation projects.  Of concern within contaminated land remediation is the possibility of 'Na toxicity' to the surrounding soil, and hence uptake by plants and grazing animals.  Weber et al., (1983) and Campbell & Davies (1997) showed the effectiveness of heavy metal and radionuclide removal from soils, but also highlighted the agricultural problems when adding natural zeolites to soil.  They argued that an increase in Na to the soil solution could cause toxicity problems in plants.  One could further argue that grazing animals would also be affected since they consume a considerable amount of soil in their diet.  Campbell & Davies (1997) also highlighted that essential heavy metals, such as Zn, would be markedly decreased by the application of zeolite, which in turn could result in deficiency problems in cattle.

 

One must also consider the environmental conditions that may change the nature and effectiveness of the zeolite in the contaminated soil, e.g. change in pH, redox conditions, micro-organism activity, and the amount of clay minerals, or available electrolyte ions which may compete for the contaminants in the soil. 

 

Natural zeolites have been shown to increase the soil cation exchange capacity and soil moisture, improve hydraulic conductivity, increase yields in acidified soils, and reduce plant uptake of metal contaminants in soil (Allen & Ming, 1995).

 

FUTURE USE

 

At the University of Surrey a pilot project was undertaken to ascertain the use of natural zeolites as a filter for motorway stormwater run-off before the stormwater entered freshwater systems (Buckley, 2000).  Ideally the natural zeolites would help in the reduction of heavy metals present in the stormwater effluent.  However, there is one seasonal complication with this remediation method – the effect of de-icing salts entering stormwater during winter months.  In the UK, roads and motorways are sprayed with de-icing salt to limit ice formation on road surfaces.  An overburden of salt (NaCl) entering the zeolite filter would in effect regenerate the zeolite, i.e. return it back to a homoionic form.  In turn, heavy metals adsorbed by the zeolite would be removed from the ion-exchange sites, replaced by Na+, and fed back into the freshwaters - thus providing an immediate 'acid-flush' effect on the receiving ecosystem.  Natural zeolites used in this way would have to be applied as a seasonal filter, ideally removed in late summer, regenerated, and used the following spring.  The actual effect of the winter stormwater surges on the zeolite filter needs to be researched in more detail.

 

With the ban of biosolids to sea, the need to find an alternative safe disposal route is paramount.  The ideal disposal option is onto agricultural land, due to the high fertiliser content of the biosolids.  However, due to the amount of heavy metals present in biosolids, and the problems of accumulation of these metals with continued application, a remediation option needs to be applied.  Zorpas et al., (2000) found composting dewatered biosolids with the natural zeolite clinoptilolite decreased heavy metal content in the final compost.  Through fractionation studies the researchers were able to show that the clinoptilolite readily took up the metal content bound in the exchangeable and carbonate fractions.  It would therefore seem an ideal solution to perhaps utilise natural zeolites with sludge before the biosolids are spread onto agricultural land, either before dewatering at the wastewater treatment plant, or through composting.   

 

One of the main advantages highlighted for pollution control and remediation is the regeneration of the natural zeolite.  Once exhausted, regeneration not only allows for the continual use of the zeolite but produces a waste which is smaller in volume, easier to handle and in some cases the pollutants may be retrievable.

 

CONCLUSION

 

The future potential of using these minerals has not been fully appreciated, and there is an urgent need to undertake field trials and evaluate the in-situ efficiency for these remediation purposes.

 

REFERENCES

·        Allen E.R & Ming D.W 1995 In: Natural Zeolites '93. Int. Comm. Natural Zeolites (D.W Ming & F.A Mumpton, Eds), Brockport, New York, pp477-490.

·        Buckley A  2000 Motorway Stormwater Remediation Using Natural Zeolites.  MSc Spring Project, Department of Physics and Chemistry, University of Surrey. 

·        Campbell L.S & Davies B.E 1997 Plant & Soil. 189: 65-74.

·        Hawkins D.B 1983 In: Zeo-agriculture: Use of Natural Zeolites in Agriculture and Aquaculture (W.G Pond & F.A Mumpton, Eds.), Westview Press, Boulder Colorado, pp69-78.

·        Kayabali K & Kezer H 1998 Environmental Geology. 34, 2/3: 95-102.

·        Kesraoui-Ouki S & Kavannagh M 1997 Waste Management & Research. 15: 383-394.

·        Mumpton  F.A 1983 In: Zeo-agriculture: Use of Natural Zeolites in Agriculture and Aquaculture (W.G Pond & F.A Mumpton, Eds.), Westview Press, Boulder Colorado, pp33-43.

·        Passaglia E & Galli E 1991 European Journal of Mineralogy. 3: 637-640.

·        Pansini M 1996 Mineral. Deposita. 31: 563-575.

·        Tsitsishvili G.V, Andronikashvili T.G, Kirov G.N, Filizova L.D 1992 Natural Zeolites.  Ellis Horwood, Chichester.

·        Townsend R.P 1984 Chemistry & Industry. 2nd April: 246-251.

·        Weber M.A, Barbarick K.A & Westfall D.G 1983 In: Zeo-agriculture: Use of Natural Zeolites in Agriculture and Aquaculture (W.G Pond & F.A Mumpton, Eds.), Westview Press, Boulder Colorado, pp263-271.

·        Zorpas A.A, Constantinides T, Vlyssides A.G, Haralambous I & Loizidou M 2000 Bioresource Technology. 72: 113-119.