Simple processing technology of leaching water using CO2 microbubbles☆

Kenzi Suzuki*,Chunshan Li*,Suojiang Zhang *

1 Nagoya Industrial Science Research Institute,Nagoya 464-0819,Japan

2 Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

Keywords:CO2 microbubble Carbonate Final land fill site Leaching water Removal of Ca2+ions

ABSTRACT The effectiveness of CO2 microsize bubbles for removal of Ca2+ions in the leaching water discharged from the final land fill site was evaluated using imitation water.For the important parameter in the Ca2+ion removal,it was found that the treatment in alkaline region above pH=10 was optimum.The possibility of using CO2 microbubbles was examined by substitution of chemical Na2CO3,which is used in large quantities today.In addition,the excellence of CO2 microbubbles was demonstrated by comparing with CO2 millimeter size bubbles.

1.Introduction

When the size of a gas bubble in water is small to microsize(described as“micro bubble”),the new feature that is not presented in millimetersize bubbles(described as “milli-bubble”)will appear[1-6].For example,its rise velocity is extremely low and its residence time is very long.Moreover,the gas dissolution in water advances efficiently by self pressurization effect.Since a microbubble has negative charge on the surface,the combination of bubbles is difficult with the electrostatic repulsion.In recent years,the application of microbubbles in the fields of environment,agriculture,medical treatment,and others is active[7-14].

In Japan,about 1 kg/person of garbage is discharged every day[15].After separate disposal,for the garbage unsuitable for recycling or reuse,combustion processing is mainly performed in the municipal waste incinerator,150 tons garbage per day.To remove harmful components HCl,SOx,NOx,CO,dioxins,etc.in the combustion gas,slaked lime[Ca(OH)2]and activated carbon are blown into it.Slaked lime mainly removes HC land activated carbon removes dioxins.They are collected by bag filter after processing.Slaked lime reacts to HCl and is collected as CaCl2,which is taken out with fly ash and solidified in the cement.It should be safely kept in a fin all and fill site without leaking to environment in the long term.

When it rains,the rain water containing water soluble substances will be collected at the bottom of the final land fill site,which is called leaching water.A schematic of leaching water processing in the final land fill site is shown in Fig.1.The bottom of the final land fill site is covered with the high density polyethylene sheet,so that the water does not leak into environment directly.The main water soluble substance is Ca2+ion from CaCl2.The concentration of Ca2+ion is about 2000 mg·L-1.These Ca2+ions will form scale in the drainage pipe,causing the blockage of the pipe.Fig.2 shows an example of the drainage pipe with the scale.The scale in the pipe is very hard and very difficult to remove.Therefore,the removal of Ca2+ions from leaching water is performed before discharge by using chemicals such as sodium carbonate(Na2CO3).With Na2CO3added,Ca2+ions in the water are removed as calcium carbonate(CaCO3).The amount of Na2CO3consumed in one year is not less than 200 tons for a fin all and fill site.Therefore,reducing the amount of chemical used is strongly required from the financial side.In such a situation,development of new Ca2+ion removal technology without using Na2CO3is strongly needed.In this study,we will establish a new technology for removal of Ca2+ions in leaching water with CO2microbubbles.

2.Experimental

The water with CO2microbubbles was prepared using MA3-type microbubble generator(Asupu Co.,Japan).Fig.3 shows the preparation equipment of CO2microbubble water.Distilled water of 500 ml was circulated at the flow rate of 150 ml·min-1and CO2gas supplied from CO2gas cylinder was crushed in the circulating water.Bubbling time was determined by pH value of bubbling water.The relationship between pH of CO2microbubble water and bubbling time is shown in Fig.4.Measurement conditions are as follows.Initial pH of distilled water is 6.5 and temperature of water is 22.2°C.The pH value of water moved to acidity side with bubbling time,changed from 6.5 to 4.6 after bubbling for 10 min and kept at 4 after 20 min.Therefore,the bubbling time was determined as 30 min.

Fig.1.A schematic of leaching water processing in the final land fill site.

Fig.2.A drainage pipe blockaded by scale formation.

Fig.3.The preparation equipment of CO2 microbubble water.

The water with CO2millimeter bubbles was prepared by bubbling CO2gas directly using a vinyl tube with spherical porous material.The bubbling time was also determined as 30 min as shown in Fig.4.The color of CO2microbubble water was white by stagnation of fine bubbles for a long time,while CO2millibubble water was transparent because the bubbles dispersed for a short time.

Fig.4.The relationship between pH of CO2 bubble water and bubbling time.

The bubble size distribution from MA3-type microbubble generator is shown in Fig.5,with bubble diameter from 10 to 80 μm and average diameter about 40 μm.

Fig.5.Microbubble size distribution.

In this study,the experiment was carried out using two kinds of imitation water,CaCl2solution,and elution water of fly ash.CaCl2(Wako,JIS Special Grade)was dissolved in distilled water and CaCl2solutions with various concentrations were prepared.The fly ash obtained from a municipal waste incinerator was added in distilled water and the elution water of fly ash containing Ca2+ions was prepared by filtration after stirring.Ca2+ion concentration in the elution water was controlled by changing the amount of fly ash added in distilled water.Ca2+concentration of elution water was 250 to 1070 mg·L-1and pH was around 12.5.The main components of fly ash were SiO2,Al2O3,Ca(OH)2,and CaCl2.

The CO2microbubble water of 500 ml was added into 500 ml of CaCl2solution or elution water of fly ash with different Ca2+concentrations,and the pH value of the mixture was adjusted by adding 1.0 mol·L-1NaOH solution.The mixture was filtered after stirring for 10 min.And then Ca2+concentration was measured using a Ca2+ion electrode.

The Ca2+extraction ratio is calculated as follows.

where Cbeforeand Cafterare Ca2+concentrations before and after reaction,respectively.

3.Results and Discussion

3.1.Reaction of CaCl2 solution and CO2 microbubble water

The chemical equilibrium of CO2gas in the water is shown in Fig.6[16].The CO2dissolved water is prepared(i.e.,carbonated water)by dissolution of CO2gas in the water,forming carbonic acid(H2CO3).Carbonic acid dissociates to form bicarbonate()and carbonate().The equilibrium constant between carbonic acid and bicarbonate,K1,is 2.5×10-4,and that between bicarbonate and carbonate,K2,is 4.7×10-11.The proton concentration reaches an equilibrium state with the carbonate concentration as the dissolution ofCO2reaches equilibrium[17].Moreover,the amount of carbonate is strongly affected by proton concentration.Changes of molar ratio of carbonic acid,bicarbonate,and carbonate with pH are shown in Fig.7[16].In acidic region,carbonic acid is the main component and bicarbonate increases with pH value.Then carbonic acid decreases rapidly in the alkaline region,while bicarbonate is in large quantities.At the same time,carbonate appears in alkaline region and increases with pH value.

Fig.6.Chemical equilibrium of CO2 in the water.

CO2microbubble water was added into the CaCl2solution(initial Ca2+concentration=650 mg·L-1)and then Ca2+concentration was measured.The Ca2+extraction ratio calculated by Eq.(1)is shown in Fig.8.The formation of CaCO3is acceptable in alkaline region over pH=9.This result can be easily understood from Fig.7.That is,carbonate(CO32-)hardly exists in the acidic region,but it is abundant in the alkaline region.

Fig.7.The relationship between chemical equilibrium and pH of the carbonated water.

Fig.8.The relationship between Ca2+extraction ratio and pH of mixture.

The CO2micro-ormillibubble water was added in the CaCl2solution.The mixed solution(initial Ca2+concentration=650-1050 mg·L-1)was filtered after stirring for 10 min.And then Ca2+concentration in the filtrate was measured using a Ca2+ion electrode.The result is shown in Fig.9.For the millibubble water,Ca2+removal did not change much,about 620 mg·L-1.For the microbubble water,Ca2+removal increased with Ca2+concentration,with the maximum removal of 880 mg·L-1.From these results,it is concluded that the microbubble water has moreions,1.4 times those in millibubble water.The reason is the “residence time”of CO2microbubbles in the water.For CO2microbubble water,moreions are supplied from dissolved CO2for reaction with Ca2+ions.

3.2.Reaction of elution water and CO2 microbubble water

Fig.9.The relationship between Ca2+removal and initial Ca2+concentration of CaCl2 solution.

Table 1 The removal of Ca2+ions in elution water of fly ash with CO2 microbubble water

With the elution water of fly ash and CO2microbubble water,the result is shown in Table 1.Ca2+concentration changed from 250 to 1070 mg·L-1.The pH of the elution water without CO2microbubble water was around 12.5.The Ca2+extraction ratio with CO2microbubble water showed a small value less than 0.7.For the elution water containing Ca2+ions above 550 mg·L-1especially,extraction ratios were smaller than 0.06.The pH value shifted from about 12.5 to acid side,around 6.Then the pH value was adjusted to 10 by adding 1.0 mol·L-1NaOH solution.As a result,the Ca2+extraction ratio was largely improved.For the elution water of550 mg·L-1,the extraction ratio was improved rapidly from 0.0091 to 0.996.From these results,it is confirmed that CO2microbubble water can be used for treatment of leaching water discharged from a final land fill site instead of using Na2CO3.

4.Conclusions

The following phenomena have been confirmed by experiments.

(1)CO2microbubble water carbonates Ca2+ions,so the removal of Ca2+ions is possible.The reaction equation is Ca2++→CaCO3.Thus the CO2microbubble technology could reduce the use of chemical Na2CO3.

(2)Ca2+extraction ratio is greatly influenced by pH of the mixture.The pH needs to be adjusted above 10 to remove Ca2+ions effectively.

It is demonstrated that the CO2microbubble method is promising as a new Ca2+removal technology instead of using chemical Na2CO3.When bubble size is smaller,a lot of bubbles with longer residence time will be realizable.From such a viewpoint,using CO2nanobubbles will be beneficial.

Acknowledgments

We would like to thank Mr.Masayuki Yamada of Nagoya University,Mr.Toshihito Yamada and Mr.Tomokatsu Takahashi of Gifu City,and Mr.Yosimi Goto of Yabashi Industries Co.,Ltd.for their important contribution.