Growth of naturally occurring microbial isolates in metal-citrate medium and bioremediation of metal-citrate wastes

Abstract

The use of citrate as a chelating agent in decontamination operations is of environmental concern as it can mobilize toxic heavy metals if discharged into the environment. Many heavy metalcitrate complexes are recalcitrant to biodegradation. Citrate-utilizing strains of Pseudomonas aeruginosa and Pseudomonas putida were isolated from a mixed culture which had been maintained with EDTA as the carbon source for 2 years. Citrate (5 mM) was used as the sole carbon source in medium supplemented with 5 mM Cd, Zn, Cu, Fe, Co, or Ni. Removal of the metals from the medium was promoted by the incorporation of inorganic phosphate as a precipitant, with formation of nickel and cobalt phosphates con®rmed by X-ray powder diffraction analysis. The potential of P putida to biodegrade citrate in a nickel±citrate secondary waste was illustrated using a ®ll-and-draw reactor supplied with ef¯uent from a bioinorganic ion exchange column that had been used previously to concentrate nickel from aqueous solution.

Simultaneous biodegradation of Ni–citrate complexes and removal of nickel from solutions by Pseudomonas alcaliphila

“…Above pH 8, the complex exists in a tridentate form involving the hydroxyl group of citrate, and above pH 9, it exists in a polymeric form [Ni 4 (OH)Cit 3 ] 5À (Still and Wikberg, 1980;Strouse et al, 1977 complex can be biodegraded by bacteria; however, the tridentate and polymeric Ni-citrate complexes are recalcitrant to biodegradation (Francis et al, 1996). In a medium containing Ni and citrate in a ratio of 1:1, 95 and 100% of the citrate was utilized by Pseudomonas aeruginosa and P. putida, respectively, and the addition of inorganic phosphate as a precipitant promoted Ni removal effectively (Thomas et al, 2000). Francis et al (1996) observed that 70% of the Ni-citrate complexes were degraded by P. fluorescens in medium with a 1:1 Ni:citrate content and buffered at pH 6.1, and that the addition of Fe 3+ significantly promoted the degradation of Ni-citrate complex and removal of Ni from solution.…”

Sorption and Distribution of Copper in Unsaturated Pseudomonas putida CZ1 Biofilms as Determined by X-Ray Fluorescence Microscopy

“…At that point in time, the concentration of copper in the biofilm was already nearly 14 times higher than that of the medium (2.82 mg g Ϫ1 compared with 0.192 mg g Ϫ1 ). Citrate likely played an important role in the quick penetration of Cu into the biofilm, since metal-citrate transport systems have been identified in many bacteria (21) and P. putida has been found to use metalcitrate complexes as carbon sources (33). The high percentage (52.6 to 61.4%) of citrate-Cu-like species in the biofilm confirms that copper was primarily complexed with citrate in P. putida CZ1 biofilms.…”

“…Degradation of the citrate metal complex would have resulted in the release of copper ion, which would have then been free to interact with phosphate, thereby causing precipitation and a reduction in the toxicity of copper (1). P. putida has been found to precipitate Co and Ni as metal phosphates when utilizing Co/Nicitrate complexes (33). Protection of biofilm cells by precipitation has also been reported in mixed sulfate-reducing bacteria biofilms, where Cu was precipitated at the biofilm surface or in the liquid phase as Cu-sulfide (37).…”

Presence of Fe3+ and Zn2+ promoted biotransformation of Cd–citrate complex and removal of metals from solutions

“…The mechanism is postulated to proceed according to the following reactions for the complete citrate utilization in the 1:1:2 Fe:Cd:citrate (1-3) and Zn:Cd:citrate media (4-5). It can be seen from Table 1 that the formation constants of tridentate Fe(III)-citrate and bidentate Zn-citrate complexes were all relatively greater than that of Cd-citrate complex [7,19] …”