Article pubs.acs.org/est
Palladium Recovery in a H2‑Based Membrane Biofilm Reactor: Formation of Pd(0) Nanoparticles through Enzymatic and Autocatalytic Reductions Chen Zhou,*,† Aura Ontiveros-Valencia,† Zhaocheng Wang,†,‡ Juan Maldonado,† He-Ping Zhao,§ Rosa Krajmalnik-Brown,† and Bruce E. Rittmann† †
Swette Center for Environmental Biotechnology, Biodesign Institute, Arizona State University, Tempe, Arizona 85287, United States Department of Water Engineering and Science, College of Civil Engineering, Hunan University, Changsha, China § Department of Environmental Engineering, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou, China ‡
S Supporting Information *
ABSTRACT: Recovering palladium (Pd) from waste streams opens up the possibility of augmenting the supply of this important catalyst. We evaluated Pd reduction and recovery as a novel application of a H2-based membrane biofilm reactor (MBfR). At steady states, over 99% of the input soluble Pd(II) was reduced through concomitant enzymatic and autocatalytic processes at acidic or near neutral pHs. Nanoparticulate Pd(0), at an average crystallite size of 10 nm, was recovered with minimal leaching and heterogeneously associated with microbial cells and extracellular polymeric substances in the biofilm. The dominant phylotypes potentially responsible for Pd(II) reduction at circumneutral pH were denitrifying β-proteobacteria mainly consisting of the family Rhodocyclaceae. Though greatly shifted by acidic pH, the biofilm microbial community largely bounced back when the pH was returned to 7 within 2 weeks. These discoveries infer that the biofilm was capable of rapid adaptive evolution to stressed environmental change, and facilitated Pd recovery in versatile ways. This study demonstrates the promise of effective microbially driven Pd recovery in a single MBfR system that could be applied for the treatment of the waste streams, and it documents the role of biofilms in this reduction and recovery process.
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INTRODUCTION Palladium (Pd), which belongs to the precious platinum-group metals (PGM), is widely applied in industry, especially in automotive catalytic converters (∼65% of its consumption).1,2 As the most industrially exploited precious metal, Pd is in short supply due to growing global demand to produce gasolinepowered vehicles meeting tighter emission standards. Efficient recovery of Pd from its major waste streams, mining, metalrefining, and catalytic-converter industries, is needed for a sustainable means to recycle Pd, lowering its price and reducing its environmental impacts.3 Conventional physical and chemical processes for Pd recovery are costly and introduce contamination into the environment.4−6 Alternatively, microorganisms7 or plant extracts8 can biologically recover soluble Pd(II) by reducing it to insoluble Pd(0) (i.e., metallic palladium). Compared to chemical reduction, microbial reduction of Pd(II) yields more controllable nanoparticulate Pd(0) having large specific surface area and, therefore, higher surface activity for catalytic applications.7,9 A wide range of microorganisms have proven capable of enzymatic reduction of Pd(II) coupled with oxidation of various electron donors.10−15 Compared to organic electron © XXXX American Chemical Society
donors, H2 is an advantageous electron donor due to its nontoxicity, lower biomass yield, lower cost, and negative CO2 release.16,17 As adding organic substrates to stimulate microbial reduction can lead to organic residuals in the effluent, using H2 is particularly promising when recovering palladium from mining waste streams containing high levels of soluble Pd (>100 mg/L), but minimal organics (99% Pd reductions for the steady states of stages 1 and 2, despite large pH differences, suggest that the
but it rapidly rebounded to >99% once the H2 pressure was back to 1.6 atm. In the beginning of stage 2 (days 18−29), fluctuating Pd(II) recovery occurred, probably because the microorganisms had been disturbed by biofilm sampling (day 16) and because the new medium with more than doubled phosphate led to a higher pH (discussed below). Only at a higher H2 pressure of 2.1 atm (day 27), the Pd concentration in the effluent steadily dropped to 99% removal) and was maintained at this level even when we decreased the H2 partial pressure back to 1.6 atm (the same pressure of the steady state for the lower-phosphate medium, days 11−16). In summary, the performance for Pd recovery correlated to the H2 delivery capacity established by D
DOI: 10.1021/acs.est.5b05318 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 4. Phylogenetic profiling of the biofilms at the levels of order (vertical bars in middle) and family (horizontal bars at the bottom) and clustering based on the unweighted UniFrac analyses (branch lines on the top). Abundance values of the substantial (>5%) orders are labeled. Unknown or
