Immobilization of toxic element containing waste using cement based stabilization techniques

Presented by Dr. Nabajyoti SaikiaAssociate Professor, Department of Chemistry,

Kaziranga University, Jorhat-6 (Assam)

Various combustion wastes such as fly ash and various ashes generated from municipal solid waste incineration (MSWI) contain significant amounts of some toxic elements such as As, Cd, Cr, Hg, Pb.

These toxic elements are also present in various alkaline industrial and agricultural waste waters.

Cement based techniques can be used to stabilize these wastes and to immobilize various toxic contaminants.

The chemical properties of cement systems such as high pH, and various minerals, present in hydrated cement are ideal for the immobilization of many toxic elements.

Introduction

The major cement hydration products, calcium silicate hydrate (C-S-H) gel and calcium (sulpho) aluminate hydrates are the important minerals for sorption and substitution of various other species.

The ettringite (3CaO.Al2O3.3CaSO4.32H2O) and monosulphate or monosulphoaluminate (3CaO.Al2O3.CaSO4.12H2O) are the two important aluminates, which favour crystallochemical substitution reactions.

In cement pore water system, ettringite is the more stable form of calcium sulphoaluminate hydrate.

However, in cement based solidified product, ettringite is gradually converted into monosulphate and therefore both phases coexist in solidified products

Introduction

The structure of ettringite is channel-like.

The structure of monosulphate is lamellar.

Various anionic species such as AsO4

3-, B(OH)3-, CO3

2-, CrO42, NO3

-, OH-, SeO4

2-, SO32- and VO4

3- can substitute SO4

2-.

Cationic species can also substitute Ca2+ and Al3+ ions.

Structures of Ettringite and Monosulphate

Thus these minerals can be used for removal of some toxic components from waste waters or can be

used to solidify/stabilize some toxic wastes.

The study was therefore conducted to understand the removal behaviour of following toxic elements from alkaline wastewaters generated from fly ash and MSWI ash during precipitation of sulphoaluminate phases:

oxyanions of As (V), B(III), Cr (VI), Mo(VI), Se(VI) and

Cd (II) and Pb(II).

Modeled waste waters (with similar pH and in some cases similar conductivities) were also used to clarify the removal mechanisms of these elements.

Lime treatment was done to understand the possible effect of lime on the removal behaviour of some elements.

Aim and Experimental

Saturation indices of some mineral phases were also calculated to evaluate whether these minerals were formed during precipitation experiment.

ICP-AES was used for detection of elements in solutions. The detection limit of the instrument for determination of As was 1-10 ppb

XRD and SEM were used to characterize precipitated minerals

Aim and Experimental

SI > 0 The solution is oversaturated and therefore precipitation will occur

SI = 0 The saturated solution is in equilibrium with precipitate

SI < 0 The solution is undersaturated and no precipitation will occur

Lime

Ettringite

Results

Fig. 1: X-ray diffraction patterns and SEM images of solid, precipitated from FA-based wastewater after ettringite precipitation experiment and concentrations of As in the treated wastewater obtained at different time intervals after lime treatment and ettringite precipitation experiment.

1d 7d

Lime

Ettringite

Fig. 2: X-ray diffraction patterns and SEM images of solid precipitated from MSWI ash based wastewater after ettringite precipitation experiment and concentration of As in the treated wastewater obtained at different time intervals after lime treatment and ettringite precipitation experiment.

Results

1d 7d

Lime

Ettringite

Results

Fig. 3: Concentrations of B in the treated wastewaters at different time intervals during lime treatment and ettringite precipitation experiment.

Similar behaviour was also observed for Se and Cr.

Lime

Ettringite

Fig. 4: Concentration of Pb in two different wastewaters at different time intervals during lime treatment and ettringite precipitation experiment.

Results

Lime

Ettringite

Ettringite

Lime

Samples Concentrations of elements (mg/L or ppm)

Ca As Pb

MSWI ash-C3A-SO42-

Original 826.1 0.97 2.50

1d 121.0 0.15 1.80

3d 117.0 0.10 1.71

7d 115.5 0.10 1.79

FA-C3A-SO42-

Original 881.2 1.10 1.15

1d 275.1 0.50 1.03

3d 205.1 0.08 1.00

7d 120.0 0.06 0.98

ResultsConcentrations of As and Pb in the wastewaters-C3A extract-gypsum solution before

and after precipitation experiment

Results

Fig. 5: Saturation indices of Ca3(AsO4)2 and Pb(OH)2 and pH values of wastewaters after treatment at different time intervals.

pH

Pb(OH)2

Ca3(AsO4)2

pH

Pb(OH)2

Ca3(AsO4)2

SI > 0 The solution is oversaturated and therefore precipitation will occur

SI = 0 The saturated solution is in equilibrium with precipitate

SI < 0 The solution is undersaturated and no precipitation will occur

Implications of results for solidification of Arsenic containing waste and related studies

Our results suggests that the substitution of SO42- by oxyanions of As

(e.g. AsO42-) in ettringite or monosulphate and/or lime induce

precipitation as calcium arsenate are the major mechanisms of incorporation of As in the cement based solidified products.

However, it is necessary to evaluate the chemical stability of the As-incorporated mineral phases of the cement based solidified products in the normal environmental conditions.

To clarify such behaviour, leaching studies of cement stabilized products are normally done.

Several European and American tests are available for such evaluations.

Leaching of Arsenic and some other toxic elements from cement based products containing some waste materials

We have completed several studies on the use of some toxic or hazardous solid waste as a granular additive (e.g. as a partial substitution of natural aggregate) in the cement based product development i.e. cement mortar and concrete.

Following table shows the As concentrations in some such waste materials:

Using these materials, cement mortar samples were prepared, where 25% (weight/weight) of natural sand was replaced by these materials.

Waste Conc. of arsenic (As)

Metallurgical slag from lead blast furnace

4200 mg/kg

Residue from fluidized bed incineration of municipal waste

312 mg/kg

Leaching of Arsenic and some other toxic elements from cement based products containing some waste materials

Following leaching tests were performed to evaluate the environmental performance of waste based mortars:

Solutions were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Average of six analytical results are presented here.

Test Name Type of material; reacting solution; type Purpose

EN 12457/2 Powdered material; distilled water with initial pH at about 7; static

To classify the waste for disposal/recycling purposes (inert/reactive)

NEN 7345 tank leaching test

Monolithic body; acidic water at pH 4; semi-dynamic

Long term leaching information from waste recycled in monolithic preparation of e.g. cement mortar

pH dependent leaching test

Powdered material; alkaline, neutral and acidic water; range of pH: 0-14; static

For evaluation of geochemical behaviour of a species; can be used for thermodynamic modeling

Leaching of Arsenic and some other toxic elements from cement based products containing some waste materials: Results

The leaching results of a few toxic elements are presented below:

Types of test Elements Concentrations (mg/kg)

Conclusion

Slag Ash

From powdered waste Do not meet the specified standard for application in cement based product development; thus caution is necessary during handling of these wastes.

EN12457/2 As 0.11 16.4

Pb 49.1 0.26

Se 3.5 0.31

From powdered cement mortar

As nd nd The disposal of slag containing waste cement product may create problem due to the presence of high concentrations of some elements e.g. Pb in the wastewaters.

Pb 53.3 0.81

Se 0.06 nd

Leaching of Arsenic and some other toxic elements from cement based products containing some waste materials: Results

The results obtained from NEN 7345 can be used to predict the long term leaching scenario, leaching rate, leaching mechanisms etc.

Types of test

Elements Concentrations (mg/m2)

Slag Ash

NEN 7345 As nd nd Concentrations of all elements are significantly lower than limit values and therefore can be used for construction purposes if their engineering properties meets the specifications

Pb 9.3 0.3

Se nd nd

Cr 2.4 11.8

Cd nd nd

Table: Cumulative leaching (mg/m2) amount of a few toxic elements from cement mortars after 64-day immersion in a acidic water solution obtained according to NEN 7345 tank test

Our results suggest that the substitutions of SO42- present in

ettringite by the oxyanions of B, Cr and Se are the major removal mechanisms during precipitation of ettringite from the waste leachates ;

The removal of As is thought to be caused, in part by formation of Ca3(AsO4)2 and in part by incorporation into the ettringite structure.

Cement based solidification technique can be used to immobilize As containing various waste materials.

The research results presented here can be used to develop cement based processes for stabilization of As-containing waste, a product of As-containing water purification plant.

Conclusions

The results presented in this presentation were already published in following journals:

Saikia N., Cornelis G., Cisar O., Vandecasteele C., Van Gemert D., Van Balen K., Van Gerven T. (2012). Use of Pb blast furnace slag as a partial substitute for fine aggregate in cement mortar. Journal of Material Cycles and Waste Management, 14 (2) 102 – 112.

Saikia N., Cornelis G., Mertens G., Elsen J., Van Balen K., Van Gerven T., Vandecasteele C. (2008). Assessment of Pb-slag, MSWI bottom ash and boiler and fly ash for using as a fine aggregate in cement mortar. Journal of Hazardous Materials, 154: 766-777.

Saikia N., Kato S. and Kojima T., (2006). Behaviour of As, Pb, Cr, Se, B, Cd and Mo present in waste leachates generated from combustion residues during the formation of ettringite. Environmental Toxicology and Chemistry, 25: 1710-1719.