|Title:||Study of magnetite-immobilized bacterial cell system for removal and recovery of heavy metals from electroplating effluent|
|Subject:||Heavy metals -- Absorption and adsorption|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Structural Engineering|
|Pages:||xvi, 315 leaves : ill. ; 30 cm|
|Abstract:||Cu2+ and Ni2+ are the major heavy metal ions in electroplating wastewater of Hong Kong. Unlike organic pollutants, which in most cases can eventually be destroyed, heavy metal ions such as Cu2+ and Ni2+ discharged into environment tend to persist indefinitely, circularly and eventually throughout the food chain thus causing a series of threats to human. Therefore, the Cu2+ and Ni2+ must be removed from electroplating effluent before discharging into water bodies. The most common methods for removing heavy metals from wastewater, such as chemical precipitation, ion exchange, electrochemical treatment and evaporative recovery have significant disadvantages, namely, removing metal incompletely, high reagent requirement, generation of toxic sludge or very expensive when the contaminant concentrations are less than 100 mg L-1. During the past two decades, biosorption, as an alternative technology, using bacteria, algae and fungi as biosorbent for removing heavy metal has received a great deal of attention. Bacteria are of particular interest because their surface: volume ratio is often relatively high. The present study was thus conducted to investigate the removal and recovery of Cu2+ and Ni2+ from electroplating wastewater by bacterial cell biomass. A gram-negative bacterium Pseudomonas putida with high Cu2+ and Ni2+ accumulating capability was isolated from local electroplating effluent. P. putida 5-x cells cultured in sulfate limiting medium was found of obviously high Cu2+ and Ni2+ adsorption capacity compared with those cultured in other media. Higher incubation temperature had a negative effect on Cu2+ and Ni2+ adsorption of P. putida 5-x cell, with effect particularly obvious at the logarithmic growth phase. The bacterial cells harvested in 34~38h and 28-30h had maximum Cu2+ and Ni2+ adsorption capacity, respectively. Both TEM and X-ray analysis confirmed the above phenomenon. Pretreating the bacterial cells with diluted HCl could increase their Cu2+ adsorption capacity by 24% and Ni2+ adsorption capacity by 31%, respectively, mainly due to the degradation of a loose superficial layer outside the fresh cell-capsule. The adsorption process of Cu2+ and Ni2+ by fresh cell consisted of two phases, namely a rapid, metabolism-independent metal ions adsorption phase (biosorption) followed by a slow metabolism-dependent bioaccumulation phase, while pretreated cell only consisted of a rapid, metabolism-independent adsorption phase. About 80% of the total Cu2+ and Ni2+ taken up by the fresh bacterial cells were removed within the rapid adsorption phase. The adsorption capacity of P. putida 5-x cell was obviously affected by changes of pH. The high the pH, the high the Cu2+ and Ni2+ adsorption capacities. Other cations such as Pb2+ and Zn2+ might act as competitors of Cu2+ and Ni2+ to inhibit their removal. The affinity of P. putida 5-x cell biomass to Pb2+, Cu2+, Zn2+ and Ni2+ was in order of Pb2+> Cu2+> Zn2+>Ni2+. The presence of Cu2+ at high concentration inhibited Ni2+ adsorption, but when the ratio of mole concentration of Cu2+: Ni2+ in wastewater reduced to 0.1 below, the inhibition of Cu2+ to Ni2+ adsorption of P. putida 5-x cell disappeared basically. Anions such as Cl-, SO42- and NO3- hardly inhibited the Cu2+ and Ni2+ uptake. The Cu2+ and Ni2+ binding by P. putida 5-x cell could be considered as adsorption process, their adsorption process obeyed Freundlich isotherms in higher pH solution, while obeyed Langmuir isotherms in lower pH solution. Adsorption isotherms showed that pretreated cell was a better biosorbent either for Cu2+ or for Ni2+ than fresh cell. Under suitable conditions, 0.1-0.3 M HCl could effective recover Cu2+ and Ni2+ from loaded cell biomass with less biomass loss rate. The desorption process was quite rapid, with 95% Cu2+ and 99% Ni2+ being desorbed in the first 5 minutes. P. putida 5-x cell as biosorbent could be effectively regenerated and reused at least five cycles for removing and recovering Cu2+ or Ni2+. The adsorption capacity of magnetite immobilized P. putida 5-x cell to Cu2+ and Ni2+ appeared to be much higher than using magnetite alone. The Cu2+ and Ni2+ adsorption by both magnetite alone and magnetite immobilized cells obeyed the Freundlich isotherms of QM-Cu = 2.1 Ce-Cu0.68, Qlm-Cu = 11.9 Ce-Cu0.74 and QM-Ni = 0.78 Ce-Ni0.79, QM-Ni = 9.7, Ce-Ni0.21. A two-stage semi-continuous stirred reactor with immobilized P. putida 5-x cell as biosorbent was developed to sequentially remove and recover Cu2+ and Ni2+ from wastewater. In suitable operation conditions, the two-stage system could effectively remove and recover sequentially from synthetic wastewater at least five cycles. Removal efficiency as high as 96% and 97% for Cu2+ and Ni2+ respectively could be attained, while the concentrations in treated effluents reduced from 3Omg/L to around 1 mg/L for Cu2+ and to 0.9 mg/L below for Ni2+. The Cu2+ and Ni2+ recovery rate from loaded biosorbent could reach 95% and 98% above, respectively. The treatment efficiency of the two-stage biosorption system to real electroplating wastewater was slightly lower than that to synthetic wastewater due to the interfering of other cations and anions in real wastewater. However, the Cu2+ and Ni2+ concentration in the treated effluent were also reduced to a concentration that meeting the effluent discharge standard formulated by Environmental Protection Department of Hong Kong, SAR. The capsules outside the fresh cell of P. putida 5-x reduced the adsorption capacity to Cu2+ due to the Cu2+-bridging formed by binding divalent Cu2+ on electronegative groups in the capsule, which might induce a conformation change within the capsule, and thus resulted in some metal-binding sites on the cell outer membrane or PEG layer becoming inaccessible for Cu2+ binding. Chemical and physical treatment during the separation course of cell envelopes would liberate more metal-binding sites on cell envelope which were heavy metal accessible. Therefore the separated cell envelope increased 4 times more Cu2+ adsorption capacity than that of fresh intact cell. Separated PEG layer, outer membrane and inner membrane all played roles on Cu2+ adsorption by cell envelope of P. putida 5-x. The total contribution of PEG layer, outer membrane and inner membrane to Cu2+ adsorption of the cell envelope was in order of outer membrane > inner membrane > PEG layer materials. The variation of Cu2+ adsorption capacity of cell envelope in different cell growth phases was due to the variation of PEG layer content, outer and inner membrane content in cell envelope, and the variation of Cu2+ adsorption capacity of outer membrane and inner membrane in different cell growth phases. The variation of both phospholipids and lipopolysaccharides content in outer membrane was found attributed to the variation of Cu2+ adsorption capacity of the outer membrane in different cell growth phases, nevertheless, the variation of only phospholipids content in inner membrane might result in the variation of its Cu2+ adsorption capacity in different cell growth phases.|
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