Transform hazardous metals from sludge to spinels: examples of nickel and copper

Abstract

A feasibility of stabilizing nickel and copper laden sludges with commonly available ceramic precursors was investigated. High metal incorporation efficiency was achieved via spinels (NiAl2O4, NiFe2O4, CuAl2O4) formation through the reactions with aluminum-rich and iron-rich raw materials commonly used in construction ceramics. The NiAl2O4 was the immobilization phase produced when mixtures of NiO were sintered (> 800oC) with -Al2O3, corundum, kaolinite, or mullite, as aluminum-rich precursors. Analogously, NiFe2O4 was the stable phase produced by firing (> 600oC) NiO with hematite as an iron-rich precursor. The CuAl2O4 was found in sintering (> 700oC) with alumina and kaolinite precursors, but cuprous aluminate (CuAlO2) was only found in sintering (> 1000oC) with alumina. Quantitatively X-ray Diffraction (XRD) analysis was further applied in nickel system to reveal different metal incorporation mechanisms. Prolonged leach tests of potential products of nickel phases were carried out in acidic environments similar to TCLP to evaluate the durability of sintered products. Both aluminate and ferrite spinels proved superior to nickel oxide for immobilization of nickel. The leaching behavior of NiAl2O4 was found to be incongruent dissolution with no significant reprecipitation, while the leachates composition of NiFe2O4 demonstrated explicable iron reprecipitation as ferric hydroxide (Fe(OH)3). While current cement solidification/stabilization technologyies are not generally successful in preventing metal mobilization in acidic environments (i.e., pH < 4.0), this study has demonstrated the success of forming spinels to stabilize metal-laden sludge from a wide-range of precursors. These results suggest a preferred incorporation mechanism by forming spinel(s) at higher, but still attainable, sintering temperatures for more effective stabilization of metal sludge and ash. With current sludge incineration strategy utilized in many cities, the mechanisms reported this study are also expected to develop an environmentally sustainable way to safely immobilize the hazardous metals in the ash incinerated from sludge, and reduce the need of its final landfill disposal by blending into marketable construction materials.