Environ. Sci. Technol. 2010, 44, 3486–3492
Microbially Mediated Clinoptilolite Regeneration in a Multifunctional Permeable Reactive Barrier Used to Remove Ammonium from Landfill Leachate Contamination: Laboratory Column Evaluation THOMAS VAN NOOTEN, LUDO DIELS, AND LEEN BASTIAENS* Flemish Institute for Technological Research (VITO), Separation and Conversion Technologies, Boeretang 200, 2400 Mol, Belgium
Received December 21, 2009. Revised manuscript received March 25, 2010. Accepted March 31, 2010.
This study focuses on multifunctional permeable reactive barrier (multibarrier) technology, combining microbial degradation and abiotic ion exchange processes for removal of ammonium from landfill leachate contamination. The sequential multibarrier concept relies on the use of a clinoptilolite-filled buffer compartment to ensure a robust ammonium removal in case of temporary insufficient microbial activities. An innovative strategy was developed to allow in situ clinoptilolite regeneration. Laboratory-scale clinoptilolite-filled columns were first saturated with ammonium, using real landfill leachate as well as synthetic leachates as feed media. Other inorganic metal cations, typically present in landfill leachate, had a detrimental influence on the ammonium removal capacity by competing for clinoptilolite exchange sites. On the other hand, the metals had a highly favorable impact on regeneration of the saturated material. Feeding the columns with leachate deprived from ammonium (e.g., by microbial nitrification in an upgradient compartment), resulted in a complete release of the previously sorbed ammonium from the clinoptilolite, due to exchange withmetalcationspresentintheleachate.Thereleasedammonium is then available for microbial consumption in a downgradient compartment. The regeneration process resulted in a slightly increasedammoniumexchangecapacityafterward.Thedescribed strategy throws a new light on sustainable use of sorption materials for in situ groundwater remediation, by avoiding the need for material replacement and the use of external chemical regenerants.
Introduction Due to an ongoing increase in waste generation and a limited waste incineration capacity, landfilling is worldwide still the most common way of solid waste disposal (1). Extensive amounts of landfill leachate, generally enriched in organic matter, ammonium (NH4+), and inorganic ions, are generated due to rainwater infiltration and moisture release from the waste (2). Leachates can lead to large groundwater contamination plumes, covering several to hundreds of hectares, * Corresponding author phone: +3214335179; fax: +3214580523; e-mail:
[email protected]. 3486
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if they are not properly collected and treated (3). A sequential multifunctional permeable reactive barrier (multibarrier) was recently proposed by Van Nooten et al. (4) as an innovative and semipassive in situ technology for remediation of leachate-polluted groundwater and for direct leachate treatment during the aftercare period of old confined landfills (Supporting Information (SI) Figure S1). The multibarrier concept, combining different reactive materials and contaminant removal processes, was successfully demonstrated in a laboratory-scale column experiment for the removal of ammonium, adsorbable organic halogens, chemical oxygen demand, and toxicity from leachate originating from the 40year-old Belgian landfill site Hooge Maey. This paper focuses on the ammonium removal concept, involving a combination of microbial degradation (nitrification-denitrification) and abiotic ion exchange processes. Ammonium is microbially converted to nitrate and nitrite (together called NOx-) in a first sand-filled nitrification compartment, equipped with diffusive oxygen emitters and inoculated with diluted nitrifying sludge (SI Figure S1). A second compartment is filled with granular clinoptilolite to remove remaining ammonium concentrations by ion exchange. The material is of special interest due to its low cost, the relative simplicity of application and operation (5), and the suitability for use in PRBs (6). In this way, complete ammonium removal can be ensured by abiotic processes in case of insufficient microbial activity in the former compartment (e.g., during the start-up phase of the multibarrier, or as a consequence of ammonium shock loads, variations in toxicity levels, and seasonal temperature changes). The NOx- formed in the nitrification compartment is microbially reduced to harmless nitrogen in a downgradient sand-filled denitrification compartment, fed with an external carbon source and inoculated with diluted denitrifying sludge. To remain cost-efficient and competitive with conventional treatment technologies, a multibarrier must preserve its semipassive character, and rely on sustainable processes and materials with a high longevity. The major drawback of ion exchange materials such as clinoptilolite, however, is the need for periodical replacements of the material after saturation, which renders the multibarrier technology less passive and less economically favorable. In addition, other inorganic cations typically present in landfill leachate (e.g., K+, Na+, Ca2+, Mg2+) may compete with NH4+ for cation exchange sites and accelerate saturation, thereby reducing the ammonium removal capacity of the clinoptilolite (7, 8). Disposal of ammonium-saturated clinoptilolite in the landfill would create a vicious circle and would not lead to an effective removal of ammonium from the landfill. Chemical regeneration of saturated clinoptilolite (usually with a concentrated brine solution) involves high operation and reagent costs, and still requires proper treatment of the ammoniumconcentrated spent regenerant (9, 10). This study, however, describes a strategy which allows in situ regeneration of ammonium-saturated clinoptilolite mediated by nitrifying bacteria, thereby avoiding the need for clinoptilolite removal and chemical regenerants. To become accessible to the bacteria, ammonium must first desorb and diffuse from the nanoscale clinoptilolite pores to the particle surface or into solution (10). Ammonium desorption can occur due to shifts in exchange equilibria when ammonium-poor leachate is flowing through the saturated clinoptilolite, as a result of sufficiently high ammonium removal rates in the upgradient nitrification compartment (4). The released ammonium can subsequently be degraded in another downgradient nitrification compartment as presented in SI Figure S1. In this 10.1021/es9038616
2010 American Chemical Society
Published on Web 04/13/2010
TABLE 1. Chemical Composition of the Landfill Leachate and the Synthetic Leachates
NH4+ AOX DOC TOC TIC evaporation residue total hardness K Na Ca Mg Al Ba Fe Cu Pb Mn Si Ag Zn S a
mg N/L meq/Lb mg Cl/L mg/L mg/L mg/L mg/L mmol/L mg/L meq/L mg/L meq/L mg/L meq/L mg/L meq/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L µg/L
columns 1-3 landfill leachate
column 4 synthetic leachatea
column 5 synthetic leachatea
244 ( 9 17.4 ( 0.6 1.9 130 140 380 3900 6.6 260 6.7 840 36.5 160 8.0 110 9.1 51 350 2000 2.5 96.8% during the first 41 PVs, followed by a slow breakthrough. After 180 PVs, still 17.4% of the incoming ammonium could be removed, indicating an ammonium removal capacity of more than 17.40 mg N (>1.24 meq) per g clinoptilolite. In addition to ammonium concentrations, influent and effluent samples were regularly analyzed for concentrations of major metal cations (Figure 2). The metal cations included K+, Na+, Ca2+, and Mg2+, and were present in the landfill leachate and the metal-containing synthetic leachate at influent concentrations of approximately 7, 37, 8, and 9 meq L-1, respectively. After passage through the columns, both leachates showed decreased effluent concentrations of K+ and Na+, indicating a partial retention of these cations on the clinoptilolite material (Figure 2A and B). Similar to the ammonium, the removal of K+ from the leachates decreased in time. The Na+ effluent concentrations, however, did not show a clear trend. Roughly estimated, ∼0.20 meq K+ and ∼0.08-0.14 meq Na+ were removed per g clinoptilolite from both metal-containing leachates. On the other hand, the column effluents contained increased Ca2+ concentrations, indicating that the adsorbed cations (NH4+, K+, and Na+) were primarily exchanged with calcium cations present in the clinoptilolite crystal structure. As expected, the release of Ca2+ decreased with the decreasing adsorption of other cations in time. Mg2+ cations were neither retained nor released in the columns, as effluent concentrations remained
similar to influent concentrations. Column 5 was fed with a synthetic medium, containing only ammonium and no metal cations. Effluent samples contained primarily calcium cations (up to 13.5 meq/L) and to a lesser extent also the other metal cations (
