AU714496B2 - The soil filter method for removal of heavy metals from contaminated water - Google Patents

Description

A&I P 0

AUSTRALIA

Patents Act 1990 P100101:1 Regulation 3.2 Origial.

Compl ete lSpecification Standard Patent invention Title The Soil Filter Method for _Removal of Heavy Metals f rom Contam-inated-Water The foowingWf statemntf is a full descriptin of tiineioncung thebest method of promn known to me:seethe description of the inventiniciselod I- The Soil Filter Method for Removal of Radium and Uranium from Contaminated Water Inventor: Jiri Kvasnicka 10 Allendale Grove Sonyfell, SA 5066 A long-term fixation of uranium and radium 2 26 Ra) onto soil particles has been utilised to remove these heavy metals from uranium mine contaminated water by spray irrigation onto the surface of a native bush land [Kvasnicka, (1995). "Beta Radiation Dose Jo Above a Uranium Mine Contaminated Water Disposal Area", Int. J. of Applied Radiation and Isotopes, Vol. 46, No. 9, pp. 965 972].

In order to treat large volumes of effluent water contaminated by radium and uranium the spray irrigation technique has a number of limitations from which the most important ones are as follows: a significant contamination of a large surface area of land results from spray irrigation; it may be required to remove and dispose of soil from large surface areas after the treatment of contaminated water by spray irrigation is completed; spray irrigation has to be continuously monitored so the contamination of ground water by radium and uranium is minimised; and the operation cost of spray irrigation and the cost of the spray irrigation site remedial action program could also make this method less attractive.

This invention pertains to the method for removal of radium and uranium from contaminated water which utilises absorption of radium and uranium onto soil particles within a soil filter which is a part of a contaminated water treatment system. The present invention is a significant advancement and improvement in the field of radium and Suranium removal from contaminated water in the chemical industry, the mining industry 2 and any industrial activity which generates radium and uranium in effluent water. The method has a potential to treat large volumes of contaminated water at much lower cost in comparison with other methods- Considering that radium and uranium are trannpped within a relatively small volume of soil it is not difficult to safely dispose of contaminated soil when the absorption capacity of the soil layer filter is exhausted.

A typical soil filter system consists of a contaminated water treatment pond with the controlled supply of contaminated water and controlled output of filtered water. The soil filter consists of the following layers: gravel with clay punchbowl pipes; coarse sand; specially selected soil; and coarse sand. The coarse sand layer, which is constructed on the top of gravel, provides the base for a soil layer which minimum thickness is 20 cm. It would be possible to use a thinner soil layer but its construction in case of large soil filters could be technically difficult and variations in the relative soil layer thickness could affect the overall filtration efficiency. The layer of coarse sand on the top of the soil layer is to protect the physical integrity of the soil layer. The filtered water removal from the pond is achieved by clay punchbowl pipes which are placed within the gravel layer on the bottom of the pond. The clay punchbowl pipes empty water into the bottom of the filtered water collection pond or trench which is separated from the contaminated water treatment pond by a wall. The required contaminated water percolation rate through the soil layer is achieved by controlling filtered water removal rate from the filtered water collection pond or trench. A schematic presentation of the proposed commercial soil filter system for removal of radium and uranium from contaminated water is in Figure 1.

00A suitable soil type with good drainage is identified through a laboratory soil mixing experiment which determines if radium and uranium in contaminated water have a desired affinity to soil particles. 300 mg of a fine particle size fraction of soil (the soil 2- 5 particle fraction with particle diameters below 45 tm is used) is added to one litre of contaminated water and mixed for 24 hours. Soil particles are removed from contaminated water by filtration and the concentration of radium and uranium in the contaminated water is compared with the concentration of radium and uranium in filtered water which was treated by soil in the above soil mixing experiment. It has been established that the removal ratio (the ratio of the radium and uranium concentration in contaminated water to that in the filtered water) of some radium and uranium from Scontaminated water is under the above mixing conditions and for a certain concentration range of soil particles in contaminated water approximately a linear function of the concentration of fine soil particles added to contaminated water. Provided the radium and uranium removal has been measured as two or higher a soil filter column is built to further test the ability of soil to remove radium and uranium from contaminated water. A typical soil filter column is presented in Figure 2.

Contaminated water seeps through the soil column at a set flow rate and is analysed for the concentration of radium and uranium. Provided a desired radium and uranium removal efficiency is achieved a commercial system for removal of radium and uranium from contaminated water may be designed and constructed.

Results of the laboratory soil mixing and a soil filter column experiments, which were carried out to test soil for its suitability to remove soluble uranium and radium from contaminated uranium mine effluent water are summarised below. Beside uranium and radium contaminated water contained a number of other contaminants.

The surface absorption of radium and uranium is proportional to the ratio of the surface area to the volume of soil particles. Therefore, the highest radium and uranium absorption effect is achieved if a fine soil particle fraction is used in the laboratory :.experiment. The -45 lm soil particle fraction was subtracted from the bulk soil (bulk soil contained approximately 9% of the above fine soil particle fraction) and used in the laboratory soil mixing experiment.

70 Exactly 5, 15, 50, 150 and 300 mg of the above fine soil particle fraction was added to five glass beakers with 1 litre of a uranium mine effluent water which contained 1500 mBq/L of 226 Ra (radium) and 500 Vtg/L of natural uranium. Soil was mixed in the water for 24 hour at the laboratory temperature. The pH of the effluent water was approximately 7.7. After 24 hours soil particles were removed from water by filtration 2z and the radium and uranium concentrations were measured in the filtered water. The results of this experiment are summarised in Table 1.

In order to estimate the effect of the time of soil mixing on the removal of radium and uranium from contaminated water 300 mg of the fine soil particle fraction was added to three beakers with one litre of the contaminated water and beakers 1, 2 and 3 were mixed for 4, 12 and 24 hours respectively. The results are presented in Table 2.

Removal of radium and uranium from contaminated water was found to be proportional to the concentration of fine soil particles (to the ratio of the mass of soil particles and the Svolume of water used in the soil mixing experiment) and to the time of mixing.

Therefore, the product of the soil concentration and the time of mixing is also approximately proportional to the magnitude of removed radium and uranium from contaminated water. In case of 24 hour mixing of 300 mg of fine soil particles with one litre of contaminated water the above product was calculated as 0.3 1 x 24 (hour)= 7.2 (g hour/L).

The bulk soil contained approximately 90 g of the -45 pgm soil particle fraction per one kilogram (or approximately 158 g per one litre of bulk soil) and the soil porosity was *t approximately 30%. This implies that in case the pore space of the unit volume of bulk °soil is filled with contaminated water for 24 hours the above product is approximately 158 0.3 x 24 (hour) 12600 g hour/L, ie. 1760 times higher than the product in the above laboratory soil mixing experiment. The real product may be expected to be at least 10 times higher than the calculated product of 12600 g hour/L if one considers the absorption capacity of all soil particles in the soil filter. It also has to be pointed out that 300 mg of the fine soil particle fraction used in the soil mixing experiment did not have _0 its radium and uranium absorption capacity exhausted after the soil was mixed with one litre of contaminated water and therefore the same soil could be reused to treat more contaminated water.

The vertical soil filter column was built inside a plastic pipe which had an internal diameter of 3.2 cm. The column consisted of approximately 2 cm thick plastic fibre filter which was immediately above the column outlet tube and was to provide the porous base for the soil filter. The soil filter consisted of a 10 cm thick coarse sand layer, the 20 cm soil layer and the top 10 cm coarse sand layer (Fig. The column inlet tube was connected to a container with uranium mine effluent water containing soluble uranium and radium. The water flow rate through the column was regulated at 3 litres per 24 0 hours by applying a pressure onto the wall of an outlet plastic tube which had the internal diameter of about 2 mm. The product was calculated for the soil filter as approximately 811 g hour/L.

Filtered water was sampled several times during the experiment but only the analytical results of the first and the last two samples of filtered water are presented. The analytical Sf results of the filtered water sampled after 41 litres of contaminated water passed through the column are summarised in Table 3. The uranium and radium concentrations in the filtered water sampled after 265 and 340 litres of contaminated water passed through the column are summarised in Table 4.

The radium and in particularly the uranium concentration in the filtered water were significantly lower than in the contaminated water and the soil filter performed well even after 340 litres of contaminated water passed through the soil filter.

.9 The commercial water treatment system is constructed as per Figure 1. The size of the system depends on the required contaminated water treatment rate. The clay punchbowl pipe of the nominal internal diameter of 100 mm has the total area of holes in the wall per meter of the pipe of about 110 cm 2 This means that the clay punchbowl pipe can remove larger than required volumes of filtered water from the bottom of the soil filter.

Regular monitoring of filtered water is required to identify an optimum time for the replacement of the soil filter before its radium and uranium soil absorption capacity is exhausted.

120 Preferred embodiment of the present invention has been shown and described with a degree of particularity. It should be understood, however, that such description has been made by way of preferred embodiment and certain changes may be made without departing from the scope of the invention defined by appended claims.

Claims (4)

1. A method for removal of radium and uranium from contaminated water based on a soil filter consisting of a layer of gravel with clay punchbowl pipes, a layer of coarse sand, a layer of soil and the top layer of coarse sand being constructed on the bottom of a contaminated water treatment pond from which filtered water is drained by clay punchbowl pipes and is emptied into the bottom of the filtered water collection pond or trench.

2. Contaminated water percolation rate through the soil filter of claim 1, which depends on the difference of the water level in the contaminated water treatment pond and in I c the filtered water collection pond or trench, is regulated through the controlled output of filtered water from the filtered water collection pond or trench. o o

3. Soil used for the construction of the soil filter of claim 1 is selected through mixing a known amount of the -45 m soil particle fraction with 1 litre of contaminated water for a period of 24 hours, water is then filtered to remove soil particles, analysed and the radium and uranium concentration in contaminated water is compared with the radium and/or uranium concentration in filtered water to establish if radium and uranium absorb onto soil particles and thus if they could be removed form contaminated water.

4. The effect of the thickness of a soil layer in the soil filter of claim 1 and the effect of 2.0 the contaminated water percolation rate through the soil filter on the removal of radium and uranium from the contaminated water are established experimentally using the soil filter column through which the contaminated water is filtered, filtered water is analysed for its content of radium and uranium and the concentration of radium and uranium in filtered water is compared with the radium and uranium concentration in the contaminated water. 7 Table 1 Radium and uranium concentrations in contaminatfp w atPr mivA fir )4 h'nr i+kt 15, 50, 150 and 300 mg of the 45 pm soil particle fraction Added soil to contaminated water (mg/1) 150 300 Radium (mBq/L) 1430 1300 1020 800 590 Ratio 0.95 0.86 0.71 0.53 0.39 Uranium (Pg/L) 116 88 42 16 7 Ratio 0.22 0.17 0.08 0.03 0.01 S S. a S Note: The original radium and uranium concentrations in the contaminated water were 1500 mBq/L and 500 lg/L respectively. The ratio was calculated as the ratio of the radium or uranium concentration in water treated with the fine particle fraction of soil to the original radium or uranium concentration in contaminated water. 8 Table 2 Rsidiiimn v2ni iirnnhlm o~nnrpntrhtinine( in 1-nntfMin2f..I ,xItQ,-r rni .cA P,- 4, 12 and 24 hours with 300 mg of the 45 gim soil particle fraction Mixing time (hour) 4 12 24 Radium Ratio (niBq/L) Uranium (jig/L) 10 8 7 Ratio 0.019 0.015 0.014 750 660 590 0.50 0.44 0.39 a a a a. a a Note: The ratio was calculated as the ratio of the radium or uranium concentration in water treated with fine soil to the original radium or uranium concentration in contaminated water. 9 Table 3 Concentration of man~yanese- lithium ni rkeI zinc. And iiraniium in filtered wLater whic wa sampled after 41 litres of contaminated water passed through the soil filter column and concentrations of the same elements in the untreated contaminated water Concentration (ptg/L) Element Contaminated water Filtered water Mn >1000 348 Ni 6 2 Zn 53 14 U 520 3 Table 4 Concentration of uranium and radium in water which pasccse thrmoih +th kjxA soil filter column and in the contaminated water after 265 and 340 litres of waste water passed through the column Element 226Ra (mBq/L) U (gg/L) Concentration Contaminated water Treated water 1600 121* 520 1" 9Os* 00 *r 0 0* 40 0 226 Ra U (mBq/L) (gg/L) 1600 520 97** 1** Note: *analytical results of treated water after 265 litres of contaminated water passed through the column and **analytical results of treated water after 340 litres of contaminated water passed through the column. S S