In this study, the effectiveness of stabilization and solidification to reduce the leachability of chromium contamination was studied using the complete block leaching method. In conclusion, OPC effectively reduced chromium leaching but did not meet the Malaysian regulatory standard of 0.05 ppm.
Background
Due to the extensive use of heavy metals in industries and agricultural applications, exposure to heavy metals has increased significantly over the years. S/S is one of those famous recovery technologies that limit the mobility of heavy metals in the ecosystem by encapsulating it in the S/S matrix.
Fundamentals of Leaching
Categories of Leaching
Static methods can be further classified into crushed block leaching method, which includes TCLP, SCLP and deionized water leaching procedure (DWLP), whereas monolithic leaching test is represented by whole block leaching method. As a whole, 16 types of leaching methods are classified and implemented based on climate and local environment.
Leachate Regulatory Level
Two approaches have been developed to quantify and evaluate the legal limit in milligrams of heavy metal per liter of leach water or milligrams of heavy metal per kilogram of dry matter. Evaluation based on milligrams of contaminant per liter of leachate base does not take into account the effect of the total mass of waste, is considered safe because the allowable leaching concentration is below the limit.
Problem Statement
Ordinary Portland Cement (OPC) capable of forming calcium silicate hydrates (C-S-H) gel which is effective in reducing heavy metal leachability. This study therefore aimed to reveal the effectiveness in remediation of heavy metal leachability by applying RHA as pozzolanic additive together with OPC as coagulant.
Aim and Objectives
Migration of Heavy Metal in the Natural Environment
Overview of Chromium in the Environment
- Application of Chromium
- Exposure Pathway
- Fundamentals of Chromium Chemistry
Trivalent chromium and hexavalent chromium are the most common oxidation states available on the Earth's surface. On the other hand, low pH and other reducing condition of aquifer reduces hexavalent chromium to trivalent chromium.
Concept of Treatment Technologies
- Toxicity Reduction Methods
- Destruction and Removal Methods
- Containment Methods and Fundamentals of Solidification/Stabilization (S/S) Technology
Mitigating the health effects of chromium can be achieved by reducing hexavalent chromium to trivalent chromium, which is biologically unavailable. Thus, the concentration of chromium in the leachate plays an important role in determining the efficiency of S/S treatment.
Fundamentals of Cement Chemistry
Hydration of Cement
The main compounds of Portland cement consist of tricalcium silicate C3S, dicalcium silicate C2S, tricalcium aluminate C3A, tetracalcium aluminoferrite C4AF and gypsum CSH2. The ratio of gypsum to tricalcium aluminate determines the ettringtie conversion that occurs at an early stage.
Pozzolanic Reaction
Rice husk is one of the major agricultural wastes in Malaysia traditionally burnt openly in the field or left the field by truck and dumped. Burning rice husks at a temperature between 500 ˚C and 700 ˚C for more than 12 hours produces ash without crystalline substances and high reactivity. The reaction between rice husk ash and OPC and its effectiveness to restore heavy metal leachability still remain unclear (Ghassan and Hilmi, 2010).
Leachability of Chromium from Municipal Solid Waste Incinerator Bottom Ash
Bottom ash samples from both MSWIs were collected by the Hua Zhang research team to study the flow behavior and factors affecting the flowability of heavy metals from MSWI bottom ash. The leakage of heavy metals from bottom ash has been verified to be relatively low compared to the limit values for hazardous waste. In summary, although the amount of heavy metal that constitutes the bottom ash of MSWI is considerable, but cementation using the S/S technique can reduce its fluidity to a safe level.
Relationship between Chromium Concentration and pH of Leachate
Study of Leachate pH as a Function of Time
Leaching of all heavy metals is in accordance with the legal limit for the use of heavy metal-containing cement as a construction material in Belgium, concluded that it is considered safe to use MSWI bottom ash as a construction material. Thus, by replacing AEC bottom ash with RHA, which initially has no heavy metal and more silica, it is believed to have better performance in reducing chromium leachability. Significant heavy metal decomposition usually occurs at low pH (Valrie and Rudy, 2007).
Equilibrium of Chromium Species at Different pH
Hexavalent chromium prefers species of CrO42- when pH is greater than 6.5 and is the only form under highly alkaline conditions. The fraction of dissolved hexavalent chromium as high as 100% compared to negligible dissolved amount of trivalent chromium, 0.966%. In other words, the hexavalent form of chromium is highly mobilized in the environment, e.g. high concentration of hexavalent chromium in soil from road runoff has a high risk of contaminating ground and surface water areas.
Relationship between Chromium Concentration and Redox Potential (Eh) of Leachate
Further study continues by investigating the distribution of chromium species between precipitated, dissolved and sorbed phases. The aim of further study is to find out which dominant species that mainly dissolve play a decisive role in the quantification of toxicity of chromium in leachate.
General Procedure Description
Preparation of Cement Blocks
The cubes were prepared by mixing cement and distilled water according to the weight ratio of 7:3.3; while cubes with 10,000 ppm and 30,000 ppm chromium (VI) doping were prepared using sodium chromate-4-hydrate solution instead of distilled water. Calculation showed that 10k ppm chromium solution corresponding to 45.01 grams of sodium chromate hydrate dissolved in 1 liter of distilled water. The same calculation procedure was used to prepare the 30 k ppm solution, which required 135.04 grams of sodium chromate 4-hydrate in 1 liter of distilled water.
Preparation of RHA
However, sodium chromate hydrate was chosen as the source for chromium (VI) ions only because sodium salts are completely soluble in water, dissociating chromium ions responsible for reaction with cement. The solution consisting of chromium is then mixed with cement to give a paste according to ratio of cement to solution equal to 7:3.3. For blocks requiring RHA doping, 50 grams of RHA were pre-mixed with 1 kilogram of cement powder and worked into paste, followed by curing process.
Flow Chart for Experimental Procedure
Instrumental Analysis
- Leachate pH Determination
- X-Ray Diffraction (XRD) Analysis
- Scanning Electron Microscope (SEM) Analysis
- Inductively Coupled Plasma Optical Emission Spectrometry (ICO-OES) Analysis
On the other hand, powder diffraction technique is commonly used to identify unknown substances by comparing diffraction data against a database contained by the International Center Diffraction Data (ICDD), or formerly known as the Joint Committee on Powder Diffraction Standards (JCPDS). SEM is a scientific instrument that uses a focused beam of electrons to image the sample and obtain meaningful data such as topography, morphology, composition and crystallographic information, the Hitachi S-3400N serves this purpose, on the other hand, the SC7620 sputter was responsible for coating gold on the sample surface before SEM testing. Inductively coupled plasma optical emission spectrometry model Optima 7000 DV by PerkinElmer, capable of determining the concentration of metal in a given solution.
Identification of Leachate Concentration by Inductive Coupled Plasma Optical Emission Spectroscopy (ICP-OES)
Leachability of Chromium
Correction of chromium concentration could be done by subtraction of sample light intensity with background light intensity followed by substitution in calibration curve. To quantify the effectiveness of whole-block leaching for hazardous waste treatment, the mass and percentage of chromium leached are calculated. Total mass of leached chromium calculated by multiplying measured chromium concentration by 0.60 litter, which represented the volume of acetic acid in the beaker, shown in Table 4.2.
Leachability of Calcium
The calcium concentration tended to decrease, which resulted in a negative gradient for all samples after 1 day provided that the deposition rate was greater than the dissociation rate. On the other hand, samples doped with RHA reduced calcium flux, typical example shown in Figure 4.9. RHA is one of the good pozzolanic materials that is able to react with CH during cement hydration and form C-S-H, as described by equation 4.1.
Changes of pH over Contact Period
Calcium ions in excess reacted with carbonate ions forming calcium carbonate precipitate, leaving unreacted calcium ions. Due to the precipitation of calcium carbonate, calcium ions in lye water are no longer saturated, allowing calcium hydroxide to resume its decomposition. As a conclusion, pH development of leachate is largely influenced by equilibrium between calcium ions and carbonate ions species.
X-Ray Diffraction (XRD) Analysis
The peak pattern of the 10k ppm Cr - 100% OPC samples after 1 day immersion duration showed similarity to the chromium (III) oxide fingerprint, shown by Figure 4.17 (a). Additionally, the 7 day immersion duration match 10k ppm Cr - 100% OPC Sample Peak Data illustrated in Figure 4.17 (b) indicates the presence of additional chromium (III) compound, forming iron chromium oxide. The more chromium oxide of iron that is detected, the less chromium (VI) remains in the effluent.
Scanning Electron Microscope (SEM) Analysis
This finding indicates that the dissociated calcium ions eventually precipitate as calcium carbonate, reflecting the ICP-OES results which showed decreased calcium concentration after 1 day of immersion duration. This specimen had undergone an immersion duration of 28 days, the chromium (III) compound was precipitated in the matrix of the cement cube.
Energy Dispersive X-ray Spectroscopy (EDX) Analysis
Postulated Leaching Mechanism
Deposition of calcium carbonate caused two main phenomena, the concentration of calcium in leachate was reduced and the pH decreased to 9. When the rate of deposition of calcium carbonate faster than the rate of dissociation of CH, the net calcium ion concentration in the control volume would decrease, and thus the leaching ability of calcium reduced. For each mole of calcium ions deposited as calcium carbonate, it required 2 moles of hydroxide ions.
Conclusion
Chromium oxide and ferric chromium oxide were detected for samples with immersion duration of more than 1 day, which indicated that deposition of chromium (III) started 1 day after leaching started. It was found that the longer the immersion time, the more different types of chromium complexes such as sodium chromium oxide, magnesium aluminum chromium and chromium carbide were formed. EDX elemental qualitative analysis showed chromium species presence in sample cubes, XRD results revealed that chromium compound was actually chromium hydroxide and iron chromium oxide, consistent with postulated leaching mechanism.
Recommendations
With additional iron doping that can stimulate the reduction kinetics of chromium(VI), we hope to achieve rapid reduction, even at an early stage. Chromium(III) characterization conjugated to Chromium(VI) analysis allows a full assessment of the risk associated with exposure to chromium-containing contaminants by considering the effect of total chromium concentration. Study on methodological aspects related to limit values for pollutants in aggregates in the context of the possible development of end-of-waste criteria under the EU Waste Framework Directive.
Effects of chromium supplementation on the infrapopulations of Anacanthorus penilabiatus and Piscinoodinium pillulare parasites of Piaractus mesopotamicus. Ca–OH bonding in the C–S–H gel phase of tricalcium silicate and white portland cement paste measured by inelastic neutron spectroscopy.
