Development of the technology of separated treatment of galvanic bath waste streams with subsequent heavy metals precipitation

Heavy metals belong to widely spread and highly toxic contaminants. They are extensively applied in various fields of industry. However, despite decontamination arrangements, the contents of heavy metals compounds in industrial waste streams are substantial. Their ability to accumulate in the environment and living organisms and to be transferred along the food chain results in disruption of biochemical processes in a human organism and inevitably makes them potentially hazardous.

At present, the reagent method is the most extensively used one in decontamination of industrial waste streams. Lime and limestone are the most often used precipitators in a majority of countries due to their accessibility and low cost (Mirbagheri 2005; Aziz 2008). Nevertheless, chemical precipitation requires application of large amounts of chemicals to reduce metals concentrations down to acceptable levels. Other disadvantages include slow precipitation (for instance, for heavy metals hydroxides), precipitate aggregation, and huge amounts of formed sludge that requires further disposal. The method of ion exchange allows attainment of virtually complete removal of heavy metals ions with initial concentrations of 100 mg/l from waste streams (Singru 2010; Sayee Kannan et al. 2011). However, the main disadvantage of the method of decontamination by ion-exchange resins is related to their selectivity. In the case of concentrated industrial wastes, ion-exchangers are easily contaminated by organics and other solid particles present in the waste, while ion-exchange processes are highly sensitive to the solution pH.

Recently, the method of flotation treatment has found a wider application in decontamination of waste streams with relatively low contents of heavy metals. However, disadvantages of this very method include poor accessibility and high cost of a majority of high-efficiency flotation agents. The exclusion here is the example when an aqueous solution (60%) of household detergent (soap) is used as a precipitator and a flotation agent for heavy metals ions (Skrylev et al. 1990).

In view of the above, it appears to be of importance to develop physical-chemical methods of decontamination of highly concentrated galvanic baths waste streams with the possibility of separation of metals for further use.

The element analysis of the initial vermiculite and its modified forms was carried out using an EDX-800-HS energy-dispersive X-ray fluorescence diffractometer (Shimadzu, Japan) (Table 2).

The carbon content was determined by wet ashing in a mixture of concentrated sulfuric acid and CrO₃.

The metals ions concentrations were determined using an АА-6601F atomic-absorption spectrophotometer (Shimadzu, Japan).

The organics contents in wastewaters were determined by the chemical oxygen demand (COD) method.

The samples IR spectra were recorded using a Spectrum-1000 spectrometer (Perkin Elmer, USA) on KBr pellets.

To prepare the detergent solution, a commercial soap of a concentration of 60% was used. 15 g of soap was dissolved at heating in 300 ml of distilled water. The obtained solution of a yellow color was added dropwise to galvanic wastes containing nickel, copper, and zinc cations with concentrations equal to 186.7, 218.37, and 187.92 g/l, respectively. The formed flaky precipitates were filtered, and their weights were equal to 25, 37, and 15 g, respectively. The values of the solutions pH changed only as a result of salts hydrolysis from 6.0 down to 3.0 in dependence on the metal ion concentration.

Adsorption under dynamic conditions was measured in a laboratory column of a height of 15 cm and a diameter of 1.3 cm. Low-concentration solutions containing nickel and copper ions were fed through a stationary adsorbent layer at a constant rate. Samples were taken at each 5^th feed.

Vermiculite from Kovdorskoe deposit (Karelia) of the composition (MgFe_0.8)Cа_0.9Al_0.4Si₃O_11.7H₂O was used as a layered silicate.

Preliminary acidic vermiculite modification: 100 g of vermiculite was heated at 120–150 °C until constant weight. Thereafter, it was ground; the fraction of a particle size of 0.10–0.05 mm was poured with 200 ml of 12.5% solution of hydrochloric acid and stirred for 2 days. Then the suspension was filtered and washed with distilled water until neutral reaction. The obtained vermiculite was dried until constant weight (sample 1) and analyzed for contents of main elements.

The vermiculite modification by paper cellulose was performed by dispersing in a cavitator at a frequency of 100 Hz. Acid-pretreated vermiculite (100 g) was placed into a glass of a volume of 800 ml, and, then, 9 g of dry cellulose was added (ash-free filter) and dispersed for 30 min. Finally, vermiculite was filtered and dried at 100 °С until constant weight (sample 2).

Vermiculite modified by cellulose was annealed at 600–700 °C for 3 hours (sample 3).

Galvanic production waste steams to be decontaminated are highly concentrated with the metal concentrations as follows: Ni – 186.7 g/l, Cu – 218.4 g/l, Zn – 187.9 g/l. At first stage of decontamination using detergent solution coagulation (Skrylev et al. 1990), the metal contents in the filtrate decreased down to 10.5 mg/l, 3.5 mg/l, and 6.5 mg/l, respectively. Upon annealing of precipitates produced by coagulation, metals were removed separately, which constitutes a clear advantage of this method.

Despite high degree of decontamination of galvanic wastes upon the coagulation stage, heavy metals ions concentrations did not attain the maximum permissible concentration (MPC). Thus, the second decontamination stage was applied to reduce concentrations of organics (filtrate CODs were from 28.5 mg O₂/l for the solution containing Ni ions to 577.5 mg O₂/l for that containing Cu ions Cu) and heavy metal ions down to MPC as well as to disinfect wastewaters.

At the second stage, the obtained low-concentration solutions underwent electrochemical treatment for 30 min with NaCl addition and using ruthenium oxide coated titanium anode and cathode. The decontamination results are shown in Table 1.

As seen from the obtained data, the zinc ions content in wastewaters attained the MPC value, and they do not require extra decontamination.

The third stage of adsorption decontamination under dynamic conditions was added for nickel- and copper-containing galvanic waste streams.

In this extra decontamination, the following sorbents were used (Shapkin et al. 2016): (a) vermiculite treated by 12% hydrochloric acid (sample 1); (b) vermiculite treated by 12% hydrochloric acid and cellulose (sample 2); (c) vermiculite treated by 12% hydrochloric acid and cellulose and annealed at 600–700 °C (sample 3).

The bands corresponding to Si-O(Si) and Si-O(Al) bonds vibrations are in the range 1,200–400 cm⁻¹. However, one can state that the band at 1,000–1,007 cm⁻¹ undergoes a shift into the higher-frequency range at vermiculite activation by hydrochloric acid. Positions of the bands in the range 800–400 cm⁻¹ also undergo shifts due to deformation and disruption of four- and six-membered cycles (SiO₄ and AlO₄) in tetrahedra.

At the vermiculite modification by cellulose (sample 2), low-intensity bands corresponding to C–H bond vibrations in the polysaccharide emerge in the IR spectrum in the range 2,980–2,870 cm⁻¹ (Figure 1(b)). These bands intensities decrease to a minimum upon annealing (Figure 1(c)). However, upon the composite annealing, the carbon content reduces by 55 % (Table 2). The residual carbon (45 %) and the changed composite color allow concluding on an unusual form of the residual cellulose interacting with the silicate surface.

The vermiculite mesopores filling by cellulose in the absence of heating proceeds simultaneously with interaction of hydroxyl groups by silicon and aluminum atoms with cellulose alcohol groups. The latter is corroborated by disappearance of absorption bands at 961 cm⁻¹, which must correspond to bending vibrations of the – Si-O-H and Al-O-H bonds and by reduction of the intensities of bands in the range 3,600–3,400 cm⁻¹ corresponding to Si-O-H and Al-O-H bonds stretching vibrations.

Extra decontamination of waste streams from nickel and copper ions was carried out under dynamic conditions. Based on the obtained data, dynamic curves for adsorption of nickel and coper ions from solutions by the sorbents under study were built.

1. At the stage of coagulation by detergent solution, the degree of waste streams decontamination from nickel, copper, and zinc was about 99.99%.

2. Electrochemical treatment using ruthenium oxide coated titanium electrodes with subsequent adsorption decontamination on vermiculite treated by 12% hydrochloric acid and cellulose (sample 2) for nickel ions and on vermiculite treated by cellulose and then annealed (sample 3) enable one to decontaminate wastewaters down to MPC values contents.

3. The suggested method of decontamination of highly concentrated galvanic waste streams allows separated removal of metals from galvanic baths waste streams upon annealing of precipitates produced through coagulation by detergent solution in view of metals further use.

This work was supported by a grant from The Ministry of Education and Science of the RUSSIAN FEDERATION, Application No. 4.8063.2017/БЧ.