In situ heavy metal attenuation in landfills under methanogenic conditions

Abstract

The purpose of this research was to determine the fate and behavior of heavy metals co-disposed with municipal waste under methanogenic conditions. Two landfill simulating reactors, one with leachate recirculation and the other without, were operated in a constant room temperature at 32 °C. These reactors were filled with shredded and compacted municipal solid waste having a typical solid waste composition of Istanbul region. After the onset of the methanogenic conditions, the selected heavy metals including iron, copper, nickel, cadmium and zinc were added according to the amounts suggested for co-disposal under the directives of the Turkish Hazardous Waste Control Regulations. The results of the experiments indicated that about 90% of all heavy metals were precipitated from the reactors within the first 10 days due to the establishment of highly reducing environment and the formation of sulfide from sulfate reduction which provided heavy metal precipitation. No inhibition to the biological stabilization was observed.

Introduction

The generation of both municipal and industrial solid wastes has increased in parallel to rapid industrialization. Effective management of these wastes has become a major social and environmental concern. One of the important waste management strategies is co-disposal, which is a technique for the controlled disposal of industrial wastes together with municipal solid wastes. The result of the comingling of wastes is a decrease in the cost of waste disposal [1]. However, prior to co-disposal, attention must be given to the determination of the types of waste to be accepted, the loading rates and the design of the sites to provide containment for proper management of gaseous and liquid emissions [2], [3].

The major sources of heavy metals in landfills are the co-disposed industrial wastes, incinerator ashes, mine wastes and household hazardous substances such as batteries, paints, dyes, inks, etc. [4]. The most common heavy metals in landfills are iron, cadmium, copper, zinc and nickel [5]. The solubility of metals in leachate depends on the pH, the redox potential, and the solubility of the deposited metal species, concentration of complexing agents (NH₃/NH₄⁺, humic acids) and ion strength [4]. Metal solubilities in the leachate increase as pH decreases. The highest metal concentrations are observed during the acid formation phase of waste stabilization when pH values are low. Therefore, methanogenic conditions and neutral pH must be established within the landfill site to form insoluble metals in the reducing atmosphere before the co-disposal commences [1], [2]. Under methanogenic conditions, soluble metals precipitate as insoluble sulfides, carbonates, hydroxides and possibly phosphates in landfills [6]. However, in the presence of sulfides, most of the heavy metals except chromium form extremely insoluble sulfide salts [7].

Sulfides can be formed during the anaerobic decomposition of solid waste either from sulfur-containing amino acids or by reduction of inorganic sulfur compounds [8]. Dissimilatory microbial sulfate reduction is a process in which certain bacteria use sulfate as the electron acceptor in the oxidation of organic matter. Desulfovibrio and Desulfotomaculum are two genera of sulfate-reducing bacteria [9]. It is known that sulfate reduction and methane production can occur in the same environment. Sulfate reducing bacteria (SRB) have a thermodynamic advantage over the methane producing consortia. Therefore, SRB out-compete the methane-producing consortia for available substrates and sulfide toxicity will be more severe for methane producers [10]. On the other hand, they play an important role in the removal of heavy metals in anaerobic systems.

When organic sulfur compounds are decomposed by bacteria, the initial sulfur product is generally H₂S. Although a fraction of sulfide escapes in anaerobic systems in the biogas, the majority of sulfide remains dissolved in solution as either H₂S_(aq) or HS⁻ [11]. H₂S_(aq) is in equilibrium with H₂S_(g) and when pH increases, H₂S_(aq) is converted to HS⁻. The dissolution of H₂S in water forms the following equilibrium.

$$\text{H}{}_2\text{S}\text{↔}\text{H}{}^{\mathrm{+}}\text{+}\text{HS}{}^{\mathrm{-}}\text{↔2}\text{H}{}^{\mathrm{+}}\text{+}\text{S}{}^{\mathrm{2-}}$$

Depending on the pH, the percentage of unionized H₂S drops from 90% at pH 6.0 to 50% at pH 7.0 and to 10% at pH 8.0 [12]. Total dissolved sulfide concentrations (H₂S+HS⁻+S²⁻) of 145–200 mg S/L cause inhibition of sulfate reducing bacteria (SRB) and methane producing bacteria (MPB) in anaerobic systems [13]. Metal-sulfide precipitation as indicated in Eq. (2) is the major factor controlling biological inhibition [14].

$$\text{Me}{}^{\mathrm{2+}}\text{+}\text{S}{}^{\mathrm{2-}}\text{→}\text{MeS}$$

where Me is taken as the symbol for a metal.

In this research, metal-sulfide precipitation as one of the major attenuation mechanisms of heavy metals was examined during the methanogenic phase of solid waste decomposition to better understand the attenuation capacity of co-disposal landfills. The effect of various leachate recirculation regimes on the attenuation of heavy metals was also investigated.