Recycling of lead-contaminated EDTA wastewater.
Ethylene diaminetetraacetic acid (EDTA) is one of the chelating agents used in the soil washing process for the decontamination of lead-contaminated soil. Lead-EDTA complexes in the wastewater from the soil washing process must be removed before the wastewater can be safely discharged. This study outlines a method to recycle Pb-EDTA wastewater by substituting the Pb complexed with EDTA with Fe(III) ions at low pH, followed by precipitation of Pb ions with phosphate or sulfate ions. Fe(III) ions complexed with EDTA were then precipitated at high pH using sodium hydroxide. The resulting solution (Fe-precipitated solution) was tested on three lead-contaminated soils. The Fe-precipitated EDTA solution was found to have similar extraction capabilities as fresh EDTA solution. Experimental results showed that the recycled EDTA solution may be recycled several times without losing its extractive power. Recycled EDTA wastewater with phosphate precipitation was found to be slightly more effective than recycled EDTA solution using sulfate precipitation. The recycling procedure may be applied to wastewater generated during soil washing of lead-contaminated soil, resulting in a reduction in wastewater generated and savings in the amount of EDTA used.
Hydroxide precipitation of complexed metals
Abstract In this study, theoretical analysis of the complexed metal treatment by conventional hydroxide precipitation is made. The analysis is aimed to evaluate the mechanism of ligand-sharing effect of metals which are added to wastewater to ensure effective removal of complexed heavy metals. Theoretical expression of the analysis is made using phase diagrams. Cadmium, copper and nickel are selected as heavy metals. EDTA, NTA and succinic acid are selected as organic complex formers of different strengths. Calcium is used as the ligand binding agent. Theoretical solutions are made for, single metal-single ligand system and several metal and ligand combinations in the closed system and single metal-single ligand in the open system. Theoretical solutions are interpreted considering the solubility, stability and ionization constants and their relative magnitudes. Experimental studies are conducted to check the theoretical predictions. Result indicate that high pH precipitation is applicable to the cases where organic ligand can be effectively bound by calcium or any other coagulant or pH adjustment agent thus freeing the heavy metal to form hydroxide or carbonate solids. Theoretical diagrams can be used to predict the favorable conditions for precipitation and as a guide to determine the operating conditions.
Techno-economic analysis of chemical looping combustion with humid air turbine power cycle
Abstract Power generation from fossil fuel-fired power plant is the largest single source of CO2 emission. CO2 emission contributes to climate change. On the other hand, renewable energy is hindered by complex constraints in dealing with large scale application and high price. Power generation from fossil fuels with CO2 capture is therefore necessary to meet the increasing energy demand, and reduce the emission of CO2. This paper presents a process simulation and economic analysis of the chemical looping combustion (CLC) integrated with humid air turbine (HAT) cycle for natural gas-fired power plant with CO2 capture. The study shows that the CLC–HAT including CO2 capture has a thermal efficiency of 57% at oxidizing temperature of 1200 °C and reducer inlet temperature of 530 °C. The economic evaluation shows that the 50 MWth plant with a projected lifetime of 30 years will have a payback period of 7 years and 6 years for conventional HAT and CLC–HAT cycles respectively. The analysis indicates that CLC–HAT process has a high potential to be commercialised.