Table 1 Latest cyanide wastewater treatments
Treatment type | Method | Overview | Principle | Strengths | Limitations | Ref. |
|---|---|---|---|---|---|---|
Chemical | Precipitation | Ferrous sulfate-mediated cyanide precipitation | Cyanide is removed as a Prussian blue precipitate after addition of an aqueous ferrous sulfate solution. | Recovered cyanide as Prussian blue can be used for other purposes. Ferrous sulfate is a cost-effective precipitating agent. | Cyanide removal percentage of 58.74% after precipitation. Can increase to 92.42% after samples are filtered with activated charcoal. Low pH levels in samples after precipitation. | |
Combined oxidation with adsorption | Combined O3 and H2O2 cyanide oxidation with adsorption on activated charcoal | Cyanide is oxidized by hydroxyl radicals derived from O3 alkaline decomposition through H2O2 and activated charcoal’s basic surface groups. | H2O2 is a green, cost-effective, and commercially available oxidizer. Cyanide removal percentage as high as 99.98%. | Removal is only focused on free cyanide and not cyanide-metallic complexes. Requires an O3 gas-generating source. | ||
Catalytic oxidation | CuO-catalyzed cyanide oxidation with H2O2 | Hydroxyl and hydroperoxyl radicals, formed on the surface of CuO nanoparticles, oxidize cyanide in alkaline conditions. | CuO nanoparticles maintain catalyst activity up to four removal cycles. Reported cyanide removal up to 99.99%. | Cu2+ ions leach from CuO nanoparticles during cyanide removal at pH 11. | ||
CuSO4-catalyzed cyanide decomposition with H2O2 and Na2S2O5 | Cyanide decomposition is achieved through the Cu2+-catalyzed oxidizing action of H2O2 and Na2S2O5 in alkaline conditions. | Could be applied to cyanide found in gold leach tailings, tailings, or leachates. Synergetic effect between H2O2 and Na2S2O5 contributes to optimal cyanide oxidation | Cu2+ ions remain in treated water or leachates after cyanide removal. Cyanide removal is achieved through a seven-hour basis treatment. | |||
Photochemical | Photochemical oxidation | Cyanide oxidation through UV-LED/H2O2/Cu2+ system | Cyanide is oxidized by H2O2, and hydroxyl radicals derived from Fenton-like catalytic reactions of the latter and Cu2+ ions, under UV-LED (275 nm) irradiation. | Effective for free cyanide and cyanide complexes in presence of metallic ions such as Zn2+. Complete cyanide removal after 30 min treatment. | Removal rate is reduced in presence of Ni2+ and Cr6+. Removing cyanate by-products requires acidification. Once turned into ammonia, it may need further treatment to be mineralized. | |
Electrochemical | Electrochemical crystallization | Cyanide removal by electrochemical crystallization of Prussian and Turnbull’s blue | Cyanide is removed as crystalized Prussian and Turnbull’s blue via electrochemically generated Fe2+ ions, at a sacrificial iron electrode. | Cyanide content can be reduced further to < 1 mg L−1. Recovered Prussian and Turnbull’s blue can be reused. | Only acidic or neutral conditions favor cyanide removal. Prussian blue recovery is not favored at low or high dissolved oxygen levels. | |
Ozonation-assisted electrocoagulation | Cyanide removal through ozonation-assisted electrocoagulation using an Al anode | A hydroxyl radical, generated from ozone decomposition in water, initially oxidizes cyanide content. Later, to further lower concentration, remaining cyanide is oxidized on an Al electrode, where by-products are coagulated. | After 40 min of ozonation and 30 min of electrocoagulation, 94.7% of cyanide is removed. Electrocoagulation products are generated in concentrations below maximum allowable limits. | Suitable for removal processes with previous biological oxidation treatment. Combined treatment with ozonation and electrocoagulation is required to reduce cyanide content below maximum allowable limits. | ||
Persulfate-enhanced electrochemical oxidation | Persulfate-enhanced electrochemical cyanide oxidation using boron-doped diamond anode | Cyanide content, under acidic conditions, is jointly oxidized by persulfate ions and hydroxyl radical generated on electrode’s surface. | Cyanide removal percentage as high as 98.4% when initial concentration is 1280.15 mg L−1 and treatment time is fixed to 24 h. Addition of persulfate to the removal process reduces energy consumption. | Removal process is not optimized for alkaline conditions. Removal temperatures should not exceed 40 °C, as HCN quickly evolves from the acidic aqueous media. | ||
Electrolysis | Electrolysis-aided cyanide removal in hypochlorite media | Cyanide content is anodically oxidized on a graphite anode and further oxidized by hypochlorite and chlorate ions. | May be applied to remove cyanide from gold mining tailings. Applicable to both total and free cyanide. | Maximum allowable limits for Cl- by-product might be exceeded. TC4 cathode is required to fully apply the proposed cyanide removal method. | ||
Physicochemical | Adsorption on activated charcoal | Adsorption on acid-treated activated charcoal | At pH 7, cyanide is removed by adsorption on the acid-activated charcoal surface, where faujasite and ascroftine content have a positive charge. | 94.5% removal Is achieved under just 25 min of treatment and 30 g L−1 of activated charcoal. Complete cyanide removal is achieved when its initial concentration approaches 10 mg L−1. | Cyanide adsorption on activated charcoal limited when removal temperature > 30 °C. To ensure maximum cyanide removal, aqueous media must be stirred (120 rpm) continuously. | |
Cyanide catalytic oxidation by copper-loaded activated charcoal, enhanced by bimetallic synergy | Cu2+ ions loaded on activated charcoal, in the presence of Zn2+, lead to cyanide species complexation and further degradation, under oxidative conditions. | Maximum removal capacity found at 24.78 mg/g, under acidic or weakly alkaline reaction conditions. Applicable for low-concentration and bio- refractory total cyanide. | Zn2+ secondary pollution is a direct outcome of the catalytic oxidation enhanced by bimetallic synergy. | |||
Adsorption on inorganic adsorbents | Cyanide adsorption on raw and iron-modified synthetic zeolites | Iron-modified zeolites remove cyanide content by surface adsorption and cyanide complex formation, in alkaline media. | Cyanide has a greater affinity to iron-modified zeolite than solely zeolite. Iron-modified zeolite reaches adsorption equilibrium after 24 h of treatment, after which it can be regenerated. | Modifying zeolite with iron reduces its surface area, reducing its adsorption capacity. Removal is hindered under acidic conditions. pH must be 10.5–11. | ||
Alkaline oxidation of adsorbed cyanide on pyrite, with H2O2 | Cyanide is chemically adsorbed to the iron atoms in the pyrite crystals and subsequently desorbed with alkaline conditions and oxidized by H2O2. | May be applied to remove cyanide from mining tailings. Cyanide adsorption on pyrite is observed in a wide range of temperatures (25–85 °C) | Several treatment by-products: thiocyanate, hexacyanoferrate, and sulfate. | |||
Photocatalytic cyanide removal with green ZnO nanoparticles | Light-irradiated ZnO nanoparticles liberate electrons, which react with atmospheric oxygen to form H2O2. This later oxidizes cyanide. | ZnO nanoparticles are synthesized by green methods. 98% of cyanide is removed after 20 min treatment with 3 g L−1 of nanoparticles and UV-light irradiation. | Removal reaches only 45% when sunlight is used instead of UV light. Care must be taken when preparing nanoparticles with Eucalyptus Globulus extract, especially when experience with green nanoparticle synthesis is lacking. | |||
Adsorption on organic adsorbents | Cyanide adsorption on calcium alginate hydrogels | Cyanide removal is driven by adsorption on active sites of alginate sphere surface. | Applicable for free cyanide and metallic-cyanide complexes. No adsorption by-products. | Maximum cyanide removal of 85.02% achieved after two treatment cycles. Addition of activated charcoal or bioadsorbent to alginate spheres does not increase cyanide removal. | ||
Cyanide adsorption on coke breeze | Cyanide removal by adsorption is driven by cyanide’s nitrogen atom interactions with superficial carbonyl groups. | Coke breeze, a by-product of steel production, is reused Considered a cost-effective treatment as adsorbent is not purchased. | Removal temperatures can only vary between 25 °C–45 °C basis. Removal improved in acidic or neutral conditions. | |||
Cyanide adsorption on thermal and acid-treated kaolin | Cyanide removal achieved through its adsorption on the acid-treated kaolin, facilitated by presence of SiO2 and Al2O3 moieties. | Cost-effective removal method. Kaolin given new application beyond industrial uses. | Kaolin must be first converted into metakaolin via calcination (600 °C), before cyanide removal can start. During cyanide removal, aluminum ions leach from the kaolin. | |||
Cyanide adsorption on biochar | Cyanide removal driven by its adsorption on the biochar’s surface. | Cost-effective removal method. Applicable cyanide and heavy metal removal from gold wastewater. | Maximum cyanide removal of 75%. For maximum removal, a 12 h treatment required. | |||
Adsorption on bioadsorbents | Cyanide adsorption on rice husks | At pH 7, cyanide is removed by its adsorption on rice husks’ active surface sites. | Cyanide removal as high as 97%. An agricultural waste, rice husk is repurposed. | Removal process requires a pump system, and a column packed with the bioadsorbent. | ||
Cyanide adsorption on natural bitumen | Cyanide removed by surface adsorption on bitumen, facilitated by favorable interactions between cyanide and surface groups, e.g., phenol, carbonyl, amide, pyrrole. | Cost-effective removal method. Removal process is completed in just under 30 min. | Maximum cyanide removal reported as 61.64%. For optimal cyanide removal, gilsonite content in bitumen must be previously processed through flotation. | |||
Cyanide adsorption on pistachio shell waste | At pH 10, removal facilitated by cyanide chemical adsorption through intraparticle diffusion on pistachio shell. | Cost-effective removal method. Cyanide removal as high as 99%, after 60 min treatment. | Cyanide removal decreases beyond an initial cyanide concentration of 200 mg L−1. | |||
Cyanide adsorption on lemon peel | At pH 8, surface carboxylic group in lemon peel removes cyanide by adsorption. | Cost-effective removal method. Cyanide removal as high as 99% after 25 min treatment. | Removal process requires a pump system, and a column packed with the bioadsorbent. | |||
Biological | Phytoremediation | Water hyacinth (Eichhornia crassipes)-aided cyanide removal | Cyanide is adsorbed by the water hyacinth’s extensive root system and is bioaccumulated in vegetal tissues. | Cyanide removal is achieved without any chemical substances or specialized equipment. A 92.66% cyanide removal rate can be achieved from an initial cyanide concentration of 20 mg L−1. | 13-day period required for maximum cyanide removal. Removal process poisons the water hyacinth, destroying its photosynthetic system. | |
Mono and dicotyledon -aided cyanide removal | Mono and dicotyledon plants bioaccumulate cyanide in their roots, stems, and leaves. | Applicable for in situ treatment of gold mining tailings. Method can be further applied for Mn and As removal. | If monocotyledon plants are used, care must be taken as they are more susceptible to fungal infection, rendering them useless for phytoremediation. | |||
Microbial remediation | Cyanide biodegradation using Aerococcus viridians strain | Bacterial enzymes degrade cyanide into less or non-toxic products | High bacterial resistance to cyanide, up to a concentration of 550 mg L−1 Cyanide removal up to 84.1% and 86.7%, for initial free cyanide of 200 and 150 mg L−1 CN− (pH 7–8, 34 °C), respectively. | Carbon and nitrogen sources for bacterial metabolic activities are needed for subsequent cyanide biodegradation. Prolonged growth time, up to 72 h, needed for bacterial biodegradation. | ||
Cyanide biodegradation using indigenous bacterial strains | Cyanide removal achieved by bacterial metabolic activities using cyanide as nitrogen source | Highest cyanide biodegradation achieved at alkaline conditions (pH 9), thus preventing cyanide volatilization | Ammonia is released as by-product of bacterial bioremediation. Prolonged bacterial growth periods (i.e., 4 days). | |||
Enzymatic remediation | Nitrilase-mediated biodegradation of cyanide | Nitrilase enzyme isolated from Enterobacter zs (PTCC 1909) degrades cyanide into ammonia and carbon dioxide | Highest removal rate was 73% with 650 mg L−1 of initial free cyanide | Costly separation techniques are required to isolate the desired enzyme. Enzyme’s stability post-extraction not assessed. |
