Presented by Dr. Nabajyoti Saikia Associate 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.Al 2 O 3.3CaSO 4.32H 2 O) and monosulphate or monosulphoaluminate (3CaO.Al 2 O 3.CaSO 4.12H 2 O) 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 AsO 4 3-, B(OH) 3 -, CO 3 2-, CrO 4 2, NO 3 -, OH -, SeO 4 2-, SO 3 2- and VO 4 3- can substitute SO  Cationic species can also substitute Ca 2+ and Al 3+ 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 > 0The solution is oversaturated and therefore precipitation will occur SI = 0The saturated solution is in equilibrium with precipitate SI < 0The 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. 1d7d

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 Lime

SamplesConcentrations of elements (mg/L or ppm) CaAsPb MSWI ash-C 3 A-SO 4 2- Original d d d FA-C 3 A-SO 4 2- Original d d d Results Concentrations of As and Pb in the wastewaters-C 3 A extract-gypsum solution before and after precipitation experiment

Results Fig. 5: Saturation indices of Ca 3 (AsO 4 ) 2 and Pb(OH) 2 and pH values of wastewaters after treatment at different time intervals. pH Pb(OH) 2 Ca 3 (AsO 4 ) 2 pH Pb(OH) 2 Ca 3 (AsO 4 ) 2 SI > 0The solution is oversaturated and therefore precipitation will occur SI = 0The saturated solution is in equilibrium with precipitate SI < 0The 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 SO 4 2- by oxyanions of As (e.g. AsO 4 2- ) 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. WasteConc. 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 NameType of material; reacting solution; typePurpose EN 12457/2Powdered 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 testElementsConcentrations (mg/kg) Conclusion SlagAsh From powdered wasteDo not meet the specified standard for application in cement based product development; thus caution is necessary during handling of these wastes. EN12457/2As Pb Se From powdered cement mortar Asnd 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 Se0.06nd

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 ElementsConcentrations (mg/m 2 ) SlagAsh NEN 7345Asnd 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 Send Cr Cdnd Table: Cumulative leaching (mg/m 2 ) 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 SO 4 2- 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 Ca 3 (AsO 4 ) 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: 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: