Continuous bioremediation of electroplating effluent

Abstract

There are significant quantities of free cyanide (F-CN) and heavy metal contaminated effluent being discharged from electroplating operations globally. However, there is an overwhelming tendency in the industry to use physical and/or chemical treatment methods for cyanides (CNs) and heavy metals in effluent. Although these methods may be effective for certain CNs and heavy metals, they produce toxic by-products and also involve high operational and capital investment costs when compared to bioremediation methods. In this study, the design of a two-stage membrane bioreactor (MBR) system was conceptualised for the bioremediation of CNs and heavy metals in the effluent which was collected from an electroplating facility located in the Western Cape, South Africa. The design included a primary inactive bioremediation stage, to reduce the impact of contaminate concentration fluctuations, and a secondary active bioremediation stage, to remove the residual contaminants, in the effluent under alkaline pH conditions which typify most industrial effluent containing these contaminants. An analysis of the electroplating effluent revealed that the effluent contained an average of 149.11 (± 9.31) mg/L, 5.25 (± 0.64) mg/L, 8.12 (± 4.78) mg/L, 9.05 (± 5.26) mg/L and 45.19 (± 25.89) mg/L of total cyanide (T-CN), F-CN, weak acid dissociable cyanides (WAD-CNs), nickel (Ni), zinc (Zn) and copper (Cu), respectively. An Aspergillus sp., which displayed the characteristic black conidiophores of the Aspergillus section Nigri, was isolated from the electroplating facilities’ effluent discharge using a selective pectin agar (PA) and subcultured on 2% (v/v) antibiotic (10,000 units/L penicillin and 10 mg streptomycin/mL) potato dextrose agar (PDA). The isolate was tolerant to F-CN up to 430 mg F-CN/L on F-CN PDA plates which were incubated at 37 ˚C for 5 days. However, a significant decline in microbial growth was observed after 200 mg F-CN/L, thus indicating that the isolate was suitable for the bioremediation of the electroplating effluent. The identification of the isolate as Aspergillus awamori (A. awamori) was definitively determined using a multi-gene phylogenetic analysis, utilising ITS (internal transcribed spacer), -tubulin and calmodulin gene regions. Although an anomaly in the morphology of the conidia of the isolate was observed during the morphological analysis, indicating a possible morphological mutation in the isolate. A comparative study between “sweet orange” (Citrus sinensis (C. sinensis)) pomace, “apple” (Malus domestica (M. domestica)) pomace, “sweetcorn” (Zea mays (Z. mays)) cob and “potato” (Solanum tuberosum (S. tuberosum)) peel, i.e. waste materials considered to be agricultural residues, was conducted in order to assess their potential and as a sole carbon source supplement for A. awamori biomass development for the bioremediation of CNs and heavy metals.

The suitability of these agricultural residues for these activities were as follows: C. sinensis pomace ˃ M. domestica pomace ˃ Z. mays cob ˃ S. tuberosum peel. For purpose of the sensitivity analysis, a temperature range of 20 to 50 ˚C and an alkaline pH range of 7 to 12 showed that: (1) optimal conditions for the uptake of Ni, Zn and Cu occurred at pH 12 and a temperature of 37.91 and 39.78 ˚C using active and inactive A. awamori biomass and unhydrolysed and hydrolysed C. sinensis pomace, respectively; (2) F-CN conversion increased linearly with an increase in pH and temperature using unhydrolysed and hydrolysed C. sinensis pomace; and (3) optimal conditions for the F-CN conversion and the respective by-products and sugar metabolism using active A. awamori biomass occurred at 37.02 ˚C and pH 8.75 and at conditions inversely proportional to F-CN conversion, respectively. The heavy metal affinity was Ni > Zn > Cu for all the biomaterials used and with the heavy metal uptake capacity being inactive A. awamori biomass > active A. awamori biomass > hydrolysed C. sinensis pomace > unhydrolysed C. sinensis pomace, respectively. Hydrolysed C. sinensis pomace had a 3.86 fold higher conversion of F-CN compared to the unhydrolysed C. sinensis pomace. The use of C. sinensis pomace extract as a nutrient media, derived from the acid hydrolysis of C. sinensis pomace, showed potential as a rich carbon-based supplement and also that low concentrations, < 0.1% (v/v), were required for the bioremediation of CNs and heavy metals. The two-stage MBR system was operated at 40 ˚C since this temperature was conducive to the bioremediation of CN and heavy metals. The primary bioremediation stage contained hydrolysed C. sinensis pomace while the secondary bioremediation stage contained active A. awamori biomass, supplemented by the C. sinensis pomace extract. After the primary and secondary bioremediation stages, 76.37%, 95.37%, 93.26% and 94.76% (primary bioremediation stage) and 99.55%, 99.91%, 99.92% and 99.92% (secondary bioremediation stage) average bioremediation efficiencies for T-CN, Ni, Zn and Cu were achieved. Furthermore, the secondary bioremediation stage metabolised the CN conversion by-products with an efficiency of 99.81% and 99.75% for formate (CHOO-) and ammonium (NH4+), respectively. After the first, second and third acid regeneration cycles of the hydrolysed C. sinensis pomace, 99.13%, 99.12% and 99.04% (first regeneration cycle), 98.94%, 98.92% and 98.41% (second regeneration cycle) and 98.46%, 98.44% and 97.91% (third regeneration cycle) recovery efficiencies for Ni, Zn and Cu were achieved. However, the design only managed to treat the effluent for safe discharge and the use of a post-treatment stage, such as reverse osmosis, is recommended to remove the remainder of the trace contaminants and colour from the effluent to ensure that the effluent met the potable water standards for reuse. There was a relatively insignificant standard deviation (≤ 3.22%) detected in all the parameters measured in the continuous operation and this indicates the reproducibility of the bioremediation efficiency in this continuous system.