Stabilization and Solidification of Metal-Laden Wastes by Compaction and Magnesium Phosphate-Based Binder

Abstract

Bench-scale and full-scale investigations of waste stabilization and volume reduction were conducted using spiked soil and ash wastes containing heavy metals such as Cd, Cr, Pb, Ni, and Hg. The waste streams were stabilized and solidified using chemically bonded phosphate ceramic (CBPC) binder, and then compacted by either uniaxial or harmonic press for volume reduction. The physical properties of the final waste forms were determined by measuring volume reduction, density, porosity, and compressive strength. The leachability of heavy metals in the final waste forms was determined by a toxicity characteristic leaching procedure (TCLP) test and a 90-day immersion test (ANS 16.1). The structural composition and nature of waste forms were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.CBPC binder and compaction can achieve 80-wt % waste loading and 39-47% reduction in waste volume. Compressive strength of final waste forms ranged from 1500 to 2000 psi. TCLP testing of waste forms showed that all heavy metals except Hg passed the TCLP limits using the phosphate-based binder. When Na 2 S was added to the binder, the waste forms also passed TCLP limits for Hg. Long-term leachability resistance of the final waste forms was achieved for all metals in both soil and ash IMPLICATIONS Hundreds of millions of dollars are spent by businesses and the government for the treatment and disposal of hazardous waste. This project researched a stabilization and solidification method involving CBPCs. This method reduces heavy metal mobility in soil and ash wastes by producing relatively inert waste forms with good structural properties. In addition, it reduces the final volume of the waste, leading to reduction in transportation and landfill disposal costs.wastes, and the leachability index was ~14. XRD patterns of waste forms indicated vermiculite in the ash waste was chemically incorporated into the CBPC matrix. SEM showed that waste forms are layered when compacted by uniaxial press and are homogeneous when compacted by harmonic press. INTRODUCTIONThe U.S. Department of Energy (DOE) has generated large volumes of low-level radioactive, hazardous, and mixed wastes as a result of its energy-and defense-related research over the last 60 years.1 Having acknowledged this problem, the DOE has funded several projects in its research laboratories in an effort to find an easy, cost-effective, and safe means of disposing this waste. Much of the DOE waste streams containing heavy metals and radionuclides require solidification/stabilization (S/S) before final disposal to reduce the mobility of contaminants into the environment. The most common practice at DOE and commercial facilities is to solidify waste using Portland cement.Although cement stabilization processes are inexpensive, readily available, and easy to process, there are limitations. In most commercial cement stabilization processes, contaminant metals are precipitated as insoluble hydroxides, which have minimum solubility between...

Low-grade magnesium oxide by-products for environmental solutions: Characterization and geochemical performance

“…Moreover, magnesium oxide as a source of magnesium has been used as a reagent to remove and recover ammonium and/or phosphates from wastewater; these precipitate in the form of struvite (MgNH 4 PO 4 · 6H 2 O), bobierrite (Mg 3 (PO 4 ) 2 · 8H 2 O) or newberyite (MgHPO 4· 3H 2 O) in the presence of magnesium . On the other hand, chemically bonded phosphate ceramics (CBPCs) are well suited for hazardous waste encapsulation because solidification occurs at low temperatures and within a wide pH range (Rao et al, 2000). CBPCs are mainly fabricated from an acid-base reaction between calcined MgO and monopotassium phosphate (KH 2 PO 4 ) in solution to form a hard and dense magnesium potassium phosphate hydrate (MgKPO 4 ·6H 2 O) ceramic.…”

Material Modifications and Related Materials

“…More recently, a biocompatible, radiopaque Ceramicrete-based dental/ bone material has been created by incorporating hydroxyapatite powder and cerium oxide radiopaque fi ller into the phosphosilicate ceramic. The Ceramicrete-based material has an initial setting time of 6 min and a fi nal setting time of 12 min, it can also be rolled into a sausage-like formation for easier manipulation with dental instruments and sets under water with minimal washout [ 143 ]. A modifi ed version of the material (Ceramicrete D) was introduced by mixing the powder with deionized water.…”

Encouraging microbial activity in cementitious systems: an emerging frontier in contaminated soil treatment

“…Subsequent efforts considered the utilisation of cement that might be much more benign to microbes in terms of providing a relatively lower pH closer to the optimum for bacterial survival (5.5–8.5) as opposed to using highly alkaline Portland cement (pH 12–13). Magnesium phosphate cements, which are formed at room temperature by rapid acid–base reaction between dead burned (or unreactive) magnesia (MgO) and an acid phosphate source, and has the potential to form mixes with different pH ranges, depending on the constituents, was considered an obvious choice for the subject matter. Magnesium oxide cement, which is composed of reactive MgO, has also been considered owing to its relatively lower pH of approximately 10.…”

“…However, it is documented that the phosphate reactions of magnesium phosphate cements convert hazardous contaminants into non‐leachable phosphate reaction products and the cement encapsulates these insoluble reaction products into a dense and durable matrix . Further, the phosphates of the contaminants have a much lower solubility than their oxides or other salts . Hence, magnesium phosphate cements provide a very effective chemical immobilisation of contaminants…”