Abstract
The hydrometallurgical treatment of waste printed circuit boards for the recovery of precious metals generates acidic wastewater containing nitrate, chloride and residual base metals. The scope of this work is the study of a biological treatment process for the concurrent metal sequestering, nitrate reduction and wastewater neutralization. A pilot-scale packed-bed biofilm reactor was set up, inoculated with the strain H. denitrificans and experimentally monitored. The range of operating parameters examined included: (a) nitrate concentration 750–5750 mg/L NO3−; (b) pH 3–8; (c) Cu, Ni, Zn and Fe at 50 mg/L and 100 mg/L; and (d) chloride concentration 5%–10% as NaCl. The presence of metals did not affect denitrification at the concentrations examined. H. denitrificans completely reduced nitrate and the intermediately produced nitrite at elevated chloride levels. Denitrification shifted pH towards circumneutral to alkaline values, where iron, zinc, copper and nickel were sequestered quantitatively from solution via bioprecipitation. The proposed simple, robust and low-cost biological treatment unit is advantageous compared to the conventional wastewater treatment, where metal precipitation is based on chemical neutralization and the problem of nitrate removal remains unresolved.
Highlights
Waste Electrical and Electronic Equipment (WEEE) ranks among the fastest growing waste streams in the world
Complete nitrate removal is attained within 4 h and 6 h for the case of 750 mg/L, 2,750 mg/L NO3, respectively, whereas at 5,750 mg/L NO3, complete denitrification is observed within 25 hours
The results confirm the high denitrification capacity of the biofilm reactor achieved by the strain Halomonas denitrificans
Summary
Waste Electrical and Electronic Equipment (WEEE) ranks among the fastest growing waste streams in the world. Hydrometallurgical processing can be established in local WEEE recycling industries permitting the decentralized “green” production of pure metals (Tuncuk et al 2012; Tunsu and Retegan 2016). These processes involve essentially two steps: (a) leaching of metals, which is generally achieved by means of strong acids such as nitric, hydrochloric and their mixtures (aqua regia) and (b) separation/selective extraction of the metals of interest from the leachate. The effluents from the separation and recovery stages contain residual anions such as NO3−, Cl− as well as residual metal ions which have not been recovered Neutralization of these streams resolves the problem of acidity, removes part of the soluble metal ions as hydroxides when Ca(OH) is used as neutralizing agent. The biological action is stepwise and is completed via four enzymatic steps (reactions (1)-(4)) which are catalyzed by specific enzymes; molybdenum containing nitrate reductase, cytochrome cd or copper containing enzymes, nitric oxide reductase and nitrous oxide reductase, respectively (Tavares et al 2006)
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