Executive Summary
The beneficial use of ash from coal consumption is an important requirement for the sustainable use of coal. The CCSD research program on By-products and Waste Utilisation was initiated to focus on research that offered prospects to use large tonnages of fly ash.
This study is a final report on the Centre’s research to develop low grade zeolites from fly- ash for use as fertilizer in bulk agricultural applications. It provides information on the production of zeolite product prepared from flyash and includes some preliminary tests of its application to enhance the efficacy of plant growth. A summary of the doctoral thesis that established a chemical basis to produce the zeolite from ash is also provided as an appendix to the report.
In this work, fly ash was converted into zeolite using an improved hydrothermal method with optimised H2O/Al molar ratio. It was found that the optimum ratio is 57.4 where the main product is analcime zeolite. The zeolite, in the forms of both K-and Na-zeolite, was subjected to trials of plants (canola, spinach and wheat) growth in pots using two type of soils, namely grey Bassendean sand and yellow gravely loamy sand (Collie soil). When planted in Bassendean sand, Spinach and wheat grow better at 1 and 2 wt% of K and Na-zeolite addition sand while canola only grow better at 1 wt% K-zeolite addition. When the zeolites were added to a more fertile Collie soil, there is an apparent beneficial effect on canola, spinach and wheat at all 3 levels of additions of both types of zeolite. Cadmium and Mercury were detected on the dried shoots of Spinach and Wheat planted on zeolite added soils but their concentration still within FAO and WHO recommended level. The Cadmium and Mercury level decreases as higher percentage of zeolites were added to the soils.
1. INTRODUCTION
A promising new approach to the utilisation of fly ash is to convert it into a value-added product like zeolites. Zeolitic minerals are hydrous aluminosilicates that in the soil environment can be considered very stable. Zeolite considerably raises the Cation Exchange Capacity (CEC) of soils thus making it suitable for agricultural/horticultural applications. The zeolite is able to freely exchange certain plant nutrients, acting to significantly improve fertiliser utilisation. Cationic nutrient ions, such as those of calcium, magnesium, potassium, sodium and particularly ammonia are exchanged in this way.
Coal fly ash has a similar chemical component to zeolite but they have different crystal structure. Majority of fly ash components are amorphous phase with small amounts of crystalline phases which are generally mullite and quartz. Therefore, there is a possibility of synthesising zeolite-like materials from the fly ash.
There are three methods to convert fly ash into zeolite [1]. The first method is conventional direct hydrothermal conversion from a mixture of fly ash and alkaline solution (normally sodium hydroxide or potassium hydroxide solution). The method was also used by Inada et al. [2] on two different types of Japanese fly ashes and the main product was zeolite P1 and hydroxosodalite. Many other researchers [3-7] also employed this method. This is a single step and the simplest method but suffers from low fly ash conversion to zeolite as pointed by Singer and Berkgaut [8]. Only about 50% of fly ash converted into zeolite P and/or hydroxysodalite.
The second method is carried out through fly ash fusion followed by a hydrothermal reaction as reported by several researchers [5, 9, 10]. In the fusion step, fly ash powder is mixed with fusion chemicals (NaOH or Na2CO3) and fused at high temperature (typically 550 °C) to convert fly ash into soluble sodium silicate and sodium aluminate. Then it is followed by dissolving the soluble fused products in an alkaline solution and reacted under hydrothermal condition. The method has high fly ash conversion into zeolite and can be directed to a certain type of zeolite by adjusting the hydrothermal reaction condition. However, the use of the high temperature fusion step makes this method very energy intense.
Both methods were regarded as only partially convert fly ash into zeolite by Hollman et al [11]. For this reason, Hollman et al developed a two steps procedure, the third method, which starts with silicone extraction from fly ash followed by hydrothermal reaction of the extracted silicone solution with the addition of aluminate solutions in certain ratio. This method produced high purity zeolitic products and can produce certain phase of zeolite by controlling the chemical composition and reaction conditions. A similar method to that of Hollman et al with membrane extraction process was reported by Tanaka et al. [12, 13].
Although the direct method was regarded as the worst in terms of zeolite purity produced from the fly ash, the simplicity and single step method makes it more favourable in converting fly ash into low grade zeolites but suitable for use in agricultural areas such as control released fertiliser and soil amendment. The method and the proposed application in agricultural areas have been thoroughly investigated by Elliot [14] using various Australian fly ashes as well as bottom ashes. While Elliot reported a successful conversion of various fly ashes and bottom ashes into agricultural grade zeolite, the reaction conditions used in the preparation are far from economical due to very large water to aluminum (H2O/Al) ratio and the zeolites were never tested as soil amendments or controlled release fertiliser.
Water plays very an important role in hydrothermally zeolite synthesis. It is a medium for zeolite components such as aluminium, silicone, cations and templates to react and crystallised under certain temperature and pressure. To quantify the amount of water in the synthesis, it is commonly reported as a relative proportion to other chemicals involved in the synthesis. For example Elliot [14] used the ratio of water to aluminium (H2O/Al) to quantify the water. Others may use silicate to water (SiO2/H2O) or water to sodium oxide (H2O/ Na2O) ratio.
Elliot used high H2O to Al ratio (H2O/Al = 400) to ensure that high crystalline zeolite was formed from fly ash. Although it can produce zeolites, the high H2O to Al ratio will restrict the production rate because the maximum amount of liquid to be placed into any given hydrothermal reactor should be less than 2/3 of its total volume to avoid an excessive pressure build up due to phase change of liquid (hence is water) into gas. Excessive use of water may also create problem due to the large amount of wastewater that requires treatment before disposal. Therefore, it is important to minimise the amount of water used in the zeolite synthesis from fly ash.
This paper reports our latest attempts to reduce the H2O/Al ratio, yet being able to produce high yields of zeolite. The zeolite thus produced was then tried in a series of pot tests on canola, spinach and wheat as a soil amendment as well as controlled release fertiliser.
2. EXPERIMENTAL
2.1. Fly Ash Sample
The fly ash was collected from Collie Power Station in Western Australia. Physico-chemicals properties of the ash are given in Table 1.
Table 1: Chemical and Physical properties of Collie coal - fly ash [14]
Based on the mineral composition, it has been calculated that the amorphous phase of the fly ash contains 49.1 wt % SiO2, 19.6 wt % Al2O3 and 16.8 wt % Fe2O3.
2.2. Zeolite Syntheses
The zeolite syntheses were conducted using two types of reactors, namely, a teflon® lined stainless steel reactor (300 mL total capacity) for trial syntheses to optimise the process conditions and a 1.8 L teflon® lined Parr® autoclave for large scale synthesis. The trial syntheses were carried out at 140°C for 24 hours without agitation, except for trials No. 1 to 3.The H2O/Al ratios were varied between 25 and 420 while keeping the amounts of other ingredients constant. The experiment conditions for the trial syntheses are given in Table 2.
The large scale zeolite syntheses were carried out under the optimum conditions found from the trial syntheses under which a high zeolite yield was achieved at the lowest H2O/Al ratio in the trials. All zeolite products were separated from the reaction solution by filtering using No. 41 Whatman filter paper straight after the reaction was terminated. The solid residue left on the filter paper was washed with double distilled water until the pH of the filtrate was between 8 and 9. The solid was then dried in an oven at 105 °C for 12 hours. The zeolite thus synthesised was called Na-Zeolite.
Table 2: Experiment conditions
2.3. Preparation of K-Zeolite
K-Zeolite was prepared from the Na-Zeolite by ion exchange. The Na-Zeolite was stirred in 0.5 M KNO3 solution for 6 hours with the ratio between the mass of zeolite to the volume of KNO3 solution set at 1:5. The zeolite was filtered without washing, and dried at 105 °C for 12 hours.
2.4. Characterisation
The zeolite products were characterised using X-ray diffraction for phase analysis and Scanning Electron Micrsocopy for morphology analysis. All X-ray analyses were carried out using a Siemens D 500 diffractometer with Cu KšŖ radiation generated at 30 mA and 40 kV. The cation exchange capacity of both raw fly ash and zeolite products were determined using the ammonium chloride method [15].
2.5. Pot plant trials
The pot plant trials were carried out using three plant species, namely, canola, spinach and wheat, and two Zeolite products, namely, Na-zeolite and K-zeolite, applied at 0, 1, 2 and 5% to two soil types, namely, a yellow gravelly loamy sand from the Collie region (Boyup Brook, Western Australia) and Bassendean sand. The top 10 cm of the respective soil types were used. Gravel and debris were removed by sieving the soils through a 2 mm stainless steel sieve and the < 2 mm soil fraction used in the experimentation. All pot trials were conducted in a glasshouse facility located at the Muresk Institute, Northam, Western Australia. The temperature was maintained close to 25°C ±3°C to maximise growth.
2.5.1. Pots and basal nutrients
A total of 1.8 kg of air-dried soil was added to free-draining plastic pots (measuring 15 cm deep by 15 cm diameter). For each crop, basal amounts of N, P, K, Cu, Zn, B and Mo was applied per kg soil to all pots, for example as follows: 100 mg K as K2SO4, 1.2 mg Cu as CuSO4.5H2O, 1.0 mg Zn as ZnSO4.7H2O, 0.24 mg B as H3BO3, 0.05 mg Mo as Na2MoO4.2H2O. All treatments received a total of 160 mg N/kg for each of the three crops with the N applied in solution as ammonium nitrate (NH4NO3) and split over three applications; prior to sowing and at 10 and 20 days, respectively. Monocalcium phosphate [MCP; Ca(H2PO4)2 H2O] was used as the inorganic source of P and was thoroughly mixed dry into the soil at a rate of 150 mg P/kg.
2.5.2. Sowing
Soils were watered to a gravimetric soil water capacity (GSWC) as determined for each soil type (using deionised water) and then allowed to incubate in the glasshouse for 7 day prior to sowing. The GSWC was determined as the amount of water remaining after 24 h in the top 5 cm of soil in a 50 cm high free draining soil column covered with plastic wrap to prevent evaporation. The moisture content is determined by drying soil samples at 105°C for 48 h.
For each consecutive crop, where canola or wheat was used, ten seeds of each species were sown in each pot. The seeds were pre-germinated two days prior to sowing. Plants were thinned to five plants per pot at the 2.5 leaf stage, leaving the five most median sized plants. Spinach was purchased as seedlings and 3 plants planted in each pot and thinned to 2 healthy plants after the first week. Pots were made up to GSWC using deionised water at least every second day and the position of pots re-randomised.
2.5.3. Plant measurements and harvest
For each crop, plants were harvested at 30 DAS (Days after Sowing) by cutting the shoots at ground level and then drying at 70°C for 48 h in a forced draught oven. The dried plant matters were weight to measure the growth.
3. RESULTS AND DISCUSSIONS
3.1. Zeolite synthesis
The zeolite syntheses were performed in three parts. The first part of the syntheses was aimed at refining the reaction condition to obtain zeolite from fly ash based on our previous method [14]. Including in the first part is trials 1 to 4. The second part was aimed at optimising H2O/Al ratio at the best conditions found from the first part. Including in this part is trials 5 to 10. Then, the optimised H2O/Al ratio and reaction condition were applied to the large scale zeolite production.
X-ray diffactograms of products from the first part of syntheses are shown in Figure 1. No zeolite peaks appear on the diffractograms under the trials 1 and 3 conditions. Meanwhile, peaks of zeolite P appeares on the diffractograms of the products of trials 2, 4 and 5.
In trial 1, the reaction was carried out at H2O/Al = 400 and using a lower NaOH concentration. In this trial, the Collie fly ash was practically not converted into any types of zeolite as shown by diffractogram of trial 1 in Figure 1. The diffractogram only shows peaks of mullite and quartz, which are the main crystalline phases found in the fly ash. The same results were also obtained in trials 3.
Figure 1: X-ray diffractograms of zeolite products from Collie coal fly ash in various H2O/Al ratio. P = Zeolite Na-P, M= Mullite and Q= Quartz.
In trials 1, the zeolite phase was not formed probably because of the alkalinity of the solutions was not strong enough to dissolve silicone and aluminium components from the fly ash particles due to their relatively lower initial pH. Even though trial 1 was carried out at higher temperature, there was no indication that the ash particles were dissolved in the solution system. On the other hand, trial 3 was carried out at higher initial pH but the low reaction temperature (120 °C) did not seem to be high enough to initiate the dissolution of silica and alumina components.
The low solubilisation of silica and alumina from the ash particles is evidenced by the results of electron microscopy analysis of trial 1 product as shown in Figure 2. The figure shows a fly ash particle with an initial stage of crystal growth on its surface. Crystals growth on an ash particle where its components were formerly dissolved in the solution and then followed by crystal precipitation and growth on the left over surface.
Figure 2: (a) An early stage of zeolite crystal formation on a surface of fly ash particle. (b) A trace of partly dissolved fly ash particle where new crystals (zeolites) being grown on it
As discussed by Iler (page 56 of [16]), the dissolution of silica will be altered when there is aluminium or iron ions in the solution. Both elements are the second major components in Collie coal fly ash. As a result, although the high pH favours the silica dissolution, the coexistence of aluminium and iron ions decreases the solubility of silica and thus reduces the possibility of zeolite formation. A high H2O/Al ratio was ineffective since the high water content also favours the solubilisation of aluminium and iron.
To improve the solubility of silica and overcome the effect of aluminium and iron ions in the solution, a higher pH and temperature are needed to provoke the zeolite formation. For these reasons, trials 2 and 4 were run with a high initial pH but at a lower H2O/Al ratio. It was very interesting that these trials produced zeolite. Furthermore, trial 4 only left a minor amount of un-reacted mullite and quartz with no evidence of amorphous phase to exist (see Figure 1, Trial 4).
In trial 5, where the H2O/Al is between those in trials 3 and 4, peaks of zeolite phase (Zeolite P) are clearly seen although peaks of mullite and quartz are still dominant. The diffractogram of trial 5 shows that reducing the amount of water by a factor of two increases the amount of zeolite produced. The trend was followed when water amount is halved of that in trial 5 as shown by diffractogram of trial No. 4. The diffractogram shows very little residue of mullite and quartz, and zeolite P peaks becomes very dominant. The high fly ash conversion into zeolite in trial 4 can also be seen on its electron microscope image as shown in Figure 3.
Figure 3: Highly crystalline phase of zeolite formed under trial 4 condition.
3.1.1. Effect of reducing H2O/Al ratio
The effect of reducing H2O/Al ratio was investigated systematically in the second part of the zeolite syntheses (trial 6 through to trial 10). Figure 4 shows the difractograms of the products of these trials, together with those of trials 1, 4, and 5 for comparison. Trial 6 was carried out at exactly the same composition as that of Elliot [14] while trials 7 to 10 at decreasing H2O/Al ratio.
Figure 4: X-ray diffractograms of zeolite products from Collie coal fly ash in various H2O/Al ratios. P = Zeolite Na-P, G= Gismondine, M= Mullite and Q= Quartz.
All trials in this part produced zeolite P, some amounts of zeolite P1 (Gismondine type) and un-reacted mullite and quartz. There was no evidence of amorphous phase left in each trial indicated by no broad background intensity from 15 to 35o of 2 theta as those found in diffractogram of trial 1.
Figure 4 shows an increasing amount of zeolite P produced in the trial and reaches a peak in trials 7 and 8, as evidenced by characteristic peaks of zeolite P at around 13.5o and 28o 2š”. In trial 9 in which water was further reduced, there was less zeolite product than in trial 8 and more unreacted fly ash material. It is shown by lower intensity of characteristic peaks of zeolite P and higher intensity of peaks of mullite and quartz. It means that under this condition, there was not enough water available to induce the dissolution of Si and Al from the fly ash and crystallised into zeolite.
A semi quantitative analysis of the product using š°-Al2O3 (corundum) as an internal standard in the diffraction analysis gives a relative phase composition of the products of these trials (Figure 5). The figure shows that more zeolite products were produced when H2/Al ratio was reduced from 419 to 105, then the zeolite productivity decreased at H2O/Al = 26 (trial 10)
Figure 5: Composition of crystallisation product as a function of H2O/Al ratio
3.1.2. Large scale zeolite synthesis
Results from trials 1 to 10 reveal that the optimum H2O to Al ratio is that falling between trials 7 and 9 (H2O/Al between 209.5 and 41.9). However, one should remember that no agitation was given during the hyrdrothermal reaction. As the reaction/crystallisation occurs via dissolution of Si and Al from the fly ash particles followed by crystallisation, agitation will alter the reaction either by increasing the solubility of silicone and aluminium or delaying the crystallisation process. The Parr 1.8 L vessel that will be used to produce kilograms of zeolite has an agitator while being able to maintain high pressure condition. For this reason, further optimisation need to be done using the Parr reactor.
To maximise the zeolite produced while maintaining high fly ash conversion into zeolite, a reaction composition in between trials 8 and 9 was chosen, i.e. the ratio of H2O to Al was 57.4. Due to the low water content in the reaction, an agitator was used throughout the hydrothermal process with a speed of 200 rpm. At the end of the reaction, the speed was reduced to 50 rpm. Reaction without agitation using the Parr reactor was also investigated to replicate the condition applied to the 300 mL reactor without agitator.
X-ray diffraction analysis of the resulting zeolite synthesised using the Parr reactor is given in Figure 6. When the reaction was carried out without agitation, the product is similar to the product in trial 8, i.e it contains zeolite P and some amounts of un-reacted mullite and quartz. A dramatic improvement in the zeolite product was shown when the reaction was carried out with agitation. Under exactly the same reaction conditions and reactant composition, those with agitation produced a single phase of analcime, one of the high temperature phases of zeolite [17].
Figure 6: Zeolite product from optimised reaction condition using the 1.8 L reactor shows a high conversion of fly ash materials into Analchime and zeolite P. Batch 1 was carried out without any agitation while batch 3 with agitation.
It has been shown that when the reaction condition between trials 8 and 9 was used together with agitation, single phase zeolite, i.e analcime, is the only zeolite product found in the product. The same product as well as XRD crystallinity was also produced at trial 8 with agitation. However, when in trial 10 with agitation, the product started to change as shown in Figure 7. In this run, zeolite P becomes the only zeolite phase produced and there were some un-reacted fly ash particles represented by mullite and quartz peaks.
Semi quantitative analysis of zeolite products with the X-ray diffractograms in Figure 7 is given in Figure 8. It is again shown that the synthesis condition between trials 8 and 9, i.e synthesis with H2O/Al ratio of 57.4, is the optimum reaction condition to make zeolite from Collie coal fly ash and will be used throughout in this research.
Figure 7: Comparison of zeolite produced from reaction under agitation using condition in trial 8 (T-8), trial 10 (T-10) and condition between trial 8 and 9 (Na-Zeolite).
Figure 8: Composition of crystallisation product as a function of H2O/Al with agitation. XRD phase composition search/match result is given in Figure 9.
Figure 9: Diffractogram of products using trial 10 condition with agitation. Al2O3 was added into the product as an internal standard.
3.1.3. Effect of reaction time on the zeolite product
In order to further optimise the large scale zeolite synthesis, the optimised reaction condition mentioned in the previous section was tested at shorter reaction time, i.e 12 and 4 hours. Figure 10 shows diffractogram while Figure 11 shows the relative composition of zeolite produced at 4, 12 and 20 hours of hydrothermal reaction.
Extra peaks shown in the diffractograms at the 12 and 4 hours runs (compared to the 20 hours) belong to the zeolite P phase. Small peaks at 2 theta between 26 and 26.2o are specific peaks of mullite phase, which, together with quartz, are the main crystalline phases found in the fly ash. No quartz peaks and amorphous phase are visible in the diffractogram. The Figures show almost all fly ash particles were converted to zeolite even when the crystallisation was run for only 4 hours. It means that to make zeolitic phase from fly ash suitable for low grade zeolite utilisation such as for controlled release fertiliser, a crystallisation time of 4 hours is sufficient.
Figure 10: Effect of hydrothermal reaction time on the formation of zeolite from fly ash.
Figure 11: Quantitative analysis on the effect of reaction time to the zeolite formation
3.2. Pot test trial results
3.2.1. Characteristics of the zeolite, sand and soil
All zeolite used in the pot plant trial was synthesised according to the optimum condition aforementioned. They are highly crystalline and in the form of very fine powder where the particle size is similar to the parent fly ash as shown in Figure 12. Other characteristics of the zeolite, fly ash and the the soils, the yellow gravely loamy sand (hence call as soil or Collie soil) and Bassendean sand used in the pot plant experiments are given in Table 3
Figure 12: Electron microscope image of zeolite made from fly ash using the optimum synthesis condition.
Table 3: Characteristics of zeolites, Bassendean sand, Collie soil and fly ash
Table 3 shows that Bassendean sand and Collie soil have very low electrical conductivities (EC) and cation exchange capacity (CEC), and neutral to slightly acid pH. Electrical conductivities (EC) of Bassendean sand and Collie soil are about one order of magnitude lower than the fly ash and both Na- and K-zeolites. Their toxic heavy metal contents are also very low except for the lead concentration in Collie soil. Bassendean sand has the lower exchangeable cations while Collie soil has similar exchangeable Ca and Mg to the fly ash and the Na and K-zeolites and higher exchangeable K than the fly ash. The low CEC, EC and exchangeable cations of Bassendean sand indicate that the sand is infertile.
In contrast to Bassendean sand and Collie soil, the pH, CEC and EC of the Na and K-zeolites are very high. The high pH of the zeolite suggests its ability to exchange its Na+ or K+ with H+ from the water, since the zeolites have high cation exchange capacities, being 31 and 35 for Na and K-zeolite, respectively. The exchange capability also contributes to the electrical conductivity (EC) because the zeolites can introduce cations to the water being used to measure the EC. Exchangeable Ca and Mg of the zeolites are also high, similar to those of Collie soil. They also have very high exchangeable Na and K due to the nature of the synthetic Na and K-zeolites. While the zeolites have everything to improve the soil properties, they also contain one order of magnitude higher concentration of toxic heavy metals than Bassendean sand and Collie soil, which have been carried forward from the fly ash. The toxic heavy metals in the Na and K-zeolites are very similar to those in the fly ash with As and Se seeming to have been leached out during the zeolite synthesis.
3.2.2.The growth of the crops
The pot trial results are featured in Figure 13. In the figure, the pots were arranged from left to right in the order of increasing the amount of zeolite added into the soils so as to show the effect of zeolite addition into the soils on the growth of canola, spinach and wheat.
The effect of zeolite addition was clearly shown on spinach and wheat when they were planted in Bassendean sand. The crops grow better at 1 and 2 wt% of zeolite additions for both K- and Na-zeolite. However, apparent improvement to the growth of canola has only been found at 1 wt% K-zeolite addition and an adverse effect on the growth was found when adding Na-zeolite at any dosage and K-zeolite at more than 2 wt % to the soils. The zeolites also improved the growth of canola, spinach and wheat when they were added to the more fertile Collie soil as clearly shown in the photos.
Figure 13: The pot plant trial results of K- and Na-zeolite. For each set of photos, the pots were arranged (from left to right) as 0% (control pot), 1%, 2% and 5%.
Figure 14: The mass of dried shoots of (a) Canola, (b) Spinach and (c) Wheat at different levels of zeolite added into the soils.
The growth of the crops in the soils with different zeolite additions can be represented by the mass of the crop dried shoots. Figure 14 shows that both K- and Na-Zeolites improve spinach and wheat growth when they were added at a dosage between 1 and 2 wt % to the sand. Canola also grows better in K-zeolite added sand but it is restricted only at 1 wt% addition. An observable beneficial effect can be seen when the zeolites were added to the Collie soil at any level on canola, spinach and wheat.
Canola and spinach require greater nutrient (such as nitrogen, phosphorus and sulphur) input than other crops [18]. Nitrogen is one of macronutrient greatly needed by canola and spinach. When soil’s nitrogen is low, nitrogen fertiliser is added to the soil. The nitrogen fertiliser added is in the form of ammonia nitrate. Zeolite added to the soil can adsorbs the ammonium ion from the fertiliser via an ion exchange reaction, releasing its cation (either Na or K, depends on the zeolite being used). When the zeolite added is Na-Zeolite, sodium will be released to the soil in exchange of ammonium ion resulting in a nitrogen-deficient state and excessive sodium content in the soil. The same process can also occur when using K-Zeolite. However, potassium ion released from the zeolite is required by canola while sodium is not.
3.2.3. Heavy metals in the soils and crops
Heavy metal pollution is a serious problem for agriculture and health when the metal concentrations are high. Some crops such as wheat, soy bean and peanuts can highly accumulate the heavy metal in their shoots [19].
Zeolites in this research were made from fly ash. It is known that fly ash may contain trace amounts of toxic heavy metals such as Lead (Pb), Mercury (Hg), Arsenic (As), Cadmium (Cd) and Selenium (Se). The metals might still exist in the zeolite although it is believed that these toxic metals are not incorporated into zeolite [2].
As listed in Table 3, the concentrations of the toxic heavy metals in the zeolites were carried forward from the fly ash and they are one order of magnitude higher than those in Bassendean sand and Collie soil. Therefore, when the zeolites were added into the soils, the increase of its concentration is expected.
Although the concentrations of the toxic heavy metals in the zeolites is relatively high, zeolite addition to the soils has been shown not to increase the heavy metal concentrations in the dry biomass of the plants investigated. The analysis results of the concentrations of the toxic heavy metals in the zeolite added soils show that the heavy metal concentration is undetectable except for Pb (lead), which is shown to increase as the zeolite added increases. Table 4 and Figure 15 show the lead concentrations in zeolite added soils.
Table 4: Lead in soil samples
Figure 15: Heavy metals concentration in the soil and sand after plants’ harvest. Only Lead (Pb) was detected while other heavy metals were below the detection limit.
Figure 15 shows that zeolite addition to the sand and soil increases the Lead concentration. The increase linearly correlates with the amount of zeolite added into the soils. However,
although a high lead concentration was found in the zeolites and zeolite added soils, Lead was not found in the dried shoot mass of all crops tested in this work as shown in Table 5.
In contrast to the soils, the toxic heavy metal concentrations in dried shoots of canola, spinach and wheat did not follow the zeolite addition and is apparently independent of the heavy metal concentrations in the soils. For example, although there were relatively high Lead concentrations in the zeolite added soils, no Lead was detected in the dry mass of the crop shoots. Arsen (As) and Selenium (se), the two of toxic metals found to be below the detection limit in the zeolite added soils, were not found in the crops either. However, Cadmium (Cd) and Mercury (Hg) were found in the dry shoot biomass of canola, spinach and wheat even when no zeolite was added into the soils and their concentrations in the soil are very low.
Table 5: Heavy metals found in Canola, Spinach and Wheat
The Cadmium level in spinach found in this test is within the range reported on vegetable grown in Metropolitan Boston and Washington DC [20]. The Cadmium level was reported in the range of 0.9 to 2.6 mg kg-1. It is also still within permissible level by WHO and FAO (0.3 mg kg-1) as reported by Bahemuka and Mubofu [21].
The zeolite addition to either sand or soil apparently reduces the heavy metal contents in spinach and wheat except for spinach planted in the K-zeolite added Bassendean sand. This is contrary to the fact that the heavy metal (Lead) concentration in the soils increases with increasing the amount of zeolite added. This phenomenon can only be explained if the zeolite retains the heavy metals inside its lattice structure.
4. CONCLUSIONS AND RECOMMENDATIONS
Water content, expressed as the H2O/Al molar ratio, plays a very important role in hydrothermal conversion of Collie coal fly ash to zeolite. It is found that the optimum H2O/Al ratio is 57.4. The main zeolite product produced under the optimum condition is analcime. It is also found that four hours hydrothermal reaction is sufficient to produce fertiliser grade zeolite which contains a mixture of analcime, Zeolite P and Zeolite P1, from Collie fly ash.
Addition of up to 5 wt% zeolite into yellow gravelly loamy sand (Collie soil) apparently showed some observable beneficial effect on the growth of canola, spinach and wheat, except on spinach at 5 wt% Na-zeolite addition. On the other hand, when the zeolite was added to grey Bassendean sand, spinach and wheat grow better at 1 and 2 wt% zeolite additions, both in sodium and potassium form. Overall, K-zeolite addition gives better spinach and wheat growth than Na-zeolite.
In contrast to spinach and wheat, zeolite addition at almost all levels dwarves the canola planted on Bassendean sand. Only at 1 wt% K-zeolite addition the canola grows better.
The zeolite might adsorb and retain Nitrogen which is highly required by the canola and spinach, causing nitrogen deficiency in the soil. The nitrogen was added into the soil in the form of ammonium nitrate fertiliser where the ammonium ion can be adsorbed by the zeolite added by ion exchange mechanisms.
Only Lead was found at an insignificant figure in the soil added with zeolite and its concentration increases as the zeolite addition increases. However, Cadmium and Mercury were detected in the dried shoots of spinach and wheat even without zeolite addition. Nevertheless, their concentrations are within FAO and WHO recommended levels. It is also interesting that Cadmium and Mercury levels decrease as higher percentage of zeolite was added into the soils.
ACKNOWLEDGEMENTS
The project was supported by the CRC for Coal in Sustainable Developments (CCSD). We would also like to express our gratitude to Collie power station for supplying the fly ash sample and Mr. Adam Hyland for technical assistance in some zeolite preparations.
Source: Hamzah Fansuri 1, Deborah Pritchard 2, Dong-ke Zhang 1
1 Centre for Fuels and Energy, Curtin University of Technology
2 Muresk Institute, Curtin University of Technology
The 10 largest coal producers and exporters in Indonesia:
- Bumi Resouces
- Adaro Energy
- Indo Tambangraya Megah
- Bukit Asam
- Baramulti Sukses Sarana
- Harum Energy
- Mitrabara Adiperdana
- Samindo Resources
- United Tractors
- Berau Coal
