Removal of copper from an electroplating industrial effluent using the native and modified spirogyra

The availability of clean water is a global concern. In the recent past, much attention has been paid to the contamination of the aquatic environment, causing severe threat to aquatic life and human health. The industrial effluents, not limited, belong to the mining, metals, electroplating, fertilizer, leather, machining, boiler, etc. that contain heavy metals are primarily damaging the aquatic environment (Nayak & Pal 2017). The most adverse characteristics of heavy metals over other pollutants in water are the non-biodegradation and their unfavorable accumulation in living organisms. Notably, copper (used in this study as the contaminant metal in wastewater) is one of the vital used metals in demand, specifically as interconnecting material via surface electroplating, thus inevitably generating a large volume discharge by the electroplating industry (Jun et al. 2016). The rinse water generated after plating contains a significant amount of copper (up to 500 mg·L⁻¹ Cu²⁺) that cannot be directly discharged without treatment. Interestingly, the hazard quotient of copper is higher than the toxic metal, cadmium, and observed as: As > Cr > Pb > Cu > Cd (Yu et al. 2014). Due to this, the World Health Organization has restricted the maximum of 1.5 mg·L⁻¹ copper in drinking water, or else it may cause severe damages of hepatic, renal and the central nervous systems, and necrotic changes in kidney and liver (Jun et al. 2016). A proper remedial action before discharging such effluent streams is therefore of great importance and often needed.

To eliminate the heavy metal contaminants, various physical, chemical and biological methods can be used, and some of them are already in practice (Volesky & Holan 1995). Such methodologies include flocculation, coagulation, precipitation, micro-filtration, resin ion-exchange, chemical/carbon adsorption, reverse osmosis, solvent extraction and electrochemical separation processes (Volesky & Holan 1995; Fu & Wang 2011; Jun et al. 2016). Nevertheless, each of them have their own constraints and may have proven unviable, especially when dealing with relatively less (<100 mg·L⁻¹) of the contaminant metals (Volesky & Holan 1995). The high operating and reagent cost, and difficulty in treatment of the generated solid waste by the physical and chemical means, is problematic. Additionally, the proven toxicity of heavy metals exposure to living microbes is retardant to biological strategy in wastewater treatment (Fu & Wang 2011). In recent times, the sorption of heavy metals from waste streams onto microbial biomass has considered a new cost-effective technique of industrial relevance for removal and recovery of heavy metals, as well (Azizi et al. 2016). Previous studies revealed that metal uptake capacity by microorganisms usually follows passive adsorption of metal ions via binding with several functional groups (Chatterjee et al. 2010). To exhibit an efficient adsorption by biomass from a contaminant stream, several factors viz. pH, temperature, solubility of metal ions, their concentration and aqueous speciation have been identified to play key roles in the biosorption process (Aravindhan et al. 2009; Lesmana et al. 2009; Wang & Chen 2009; Chatterjee et al. 2010; Azizi et al. 2016). Using biomass of bacteria, fungi, algae and yeasts, the biosorption study for removal of various heavy metals such as cadmium, lead, chromium, uranium, arsenic, copper, nickel, iron, zinc, etc. from the divergent effluent streams has been performed (Lesmana et al. 2009; Bayramoglu et al. 2016). The ubiquitous eukaryotic organisms, containing chlorophyll and carrying oxygenic photosynthesis, named green algae (Wang & Chen 2009), fulfilling these criteria have been extensively studied for heavy metals removal or their recovery from waste effluent streams (Aravindhan et al. 2009). The presence of large amounts of proteins, lipids, and polysaccharides on the cell walls along with containing a number of functional groups (such as hydroxyl, amino, carboxyl, sulfate, and phosphate) can potentially bind the metal ions with algal biomass (Bayramoglu et al. 2018). Additionally, the studies on modification of adsorbents surface and functional groups have shown significant improvement in their performance and sorption capacity (Bayramoglu et al. 2015).

Therefore, in the present study, the treatment of a typical electroplating rinse water discharge containing 194 mg·L⁻¹ copper in sulfate medium was investigated using biosorption technique. In which, the biomass of green filamentous algae, spirogyra as the potential sorbent was employed with aiming to assess the removal efficiency of copper from the effluent discharge. The sorption behaviour exhibited by spirogyra in its native and modified form as a function of parametric variation viz. effluent pH, contact time, adsorbent dosage, and temperature are being revealed and also used to determine the isotherm, kinetics and thermodynamics of the biosorption process. The present study can also be used for other metal-contaminant systems by employing the native and modified biomass of spirogyra.

The copper bearing rinse water effluent, generated by a local electroplating industry, was used in the study as the sorbate feed solution. The effluent contains 194 mg·L⁻¹ copper and also contains a lesser amount of <5 mg·L⁻¹ nickel and zinc in sulfate medium. The characteristics of the industrial electroplating solution as supplied were: pH, 2.4; total dissolved solid, 56,800 mg·L⁻¹; conductivity, 4.16 μS·cm⁻¹; and chemical oxygen demand, 1,540 mg·L⁻¹. When needed to vary the concentration of copper in solution, the synthetic solutions of desired copper content were prepared by dissolving the calculated amount of CuSO₄·5H₂O (supplied by Sigma Aldrich) in distilled water. The pH of the solution was maintained by 5 vol.% H₂SO₄ and NaOH (supplied by Riedel-deHaën and Lab Scan, respectively).

Spirogyra belonging to the group of green filamentous algae and chlorophyceae family was used as the biosorbent in this study. The algal biomass was collected from the fish farm located at the University of Agriculture Faisalabad campus. The collected algae were water washed to remove the sand and other unwanted materials and squeezed to remove the water. Further, the biomass was sun-dried for 7 days and crushed to get 300 μm size of the biosorbent in its native form. In order to prepare the modified biomass as cross-linked gel, a 500 g algal sample was treated overnight with 1 L conc. H₂SO₄ (as the cross-linking agent) at 100 °C. It should be noted that the algal cell wall is basically composed of two layers: outer pectose layer and inner cellulose layer. By the treatment of native biomass with conc. H₂SO₄ (under the specific condition), cross-linked algal gel was obtained that have structural modification on cell wall that can improve the tendency of metal adsorption with the modified biomass. Subsequently, the filtered mass was neutralized with sodium bicarbonate and washed with distilled water until neutral pH was achieved. There after the biomass was dried at 50 °C for 3 h and crushed to 300 μm size that was used as the cross-linked modified spirogyra for biosorption tests of copper. The scheme of sample preparation and difference between their structures are presented in Figure 1. Further, the specific surface area of the native and modified spirogyra was determined by N₂ adsorption using the Brunauer–Emmett–Teller (BET) method, which was obtained to be 1.88 cm² and 3.42 cm², respectively.

After the optimization of batch sorption study, desorption study of only modified algal biomass was carried out by contacting the metal-loaded biomass with 1.0 mol·L⁻¹ HCl solutions. For this, the metal-loaded biomass separated after filtration was air dried overnight at room temperature and taken for desorption study. The biomass was introduced to acid solution (pre-heated at 50 °C) at a solid-to-liquid ratio of 1:4, and contacted for 60 min under a constant rotation speed of 200 rpm in an orbital shaker. After that the samples were centrifuged at 5,000 rpm for 10 min and the supernatant liquid was separated for analysis of the amount of copper eluted from the biomass, and the regeneration efficiency was determined. Four consecutive cycles of such sorption–desorption were performed with the same conditions; however, a washing with 10% H₂SO₄ was given each time of biomass recycling for adsorption.

By in-large the metal speciation and the active group attached on the cell wall sites primarily depends on pH of the sorption system (Hossain et al. 2014). Hence, the influence of pH on sorption behavior of copper onto the biomass of native and modified spirogyra was investigated in the range of 2.0 to 8.0. The other parameters viz. the biomass dosage 4 g/100 mL solution containing 194 mg·L⁻¹ Cu²⁺ and contact time 30 min at room temperature 20 °C were invariably maintained. The results presented in Figure 2 shows a significant change in sorption capacity with respect to the sorption pH. The percentage sorption of copper was 30.1% and 32.5% by native and modified biomass, respectively, which surpasses above 50% at pH 4.0 and reached its maximum levels of 74.8% and 84.7% at a pH 6.0. A further increase in pH could decrease the sorption, declined to 64% and 72% at pH 8.0 using the biomass in native and modified form, respectively. Such results can directly be corroborated to the surface adsorption sites of the functional group attached with cell wall of biomass (Gupta & Rastogi 2008; Hossain et al. 2014; Celekli et al. 2016), hence a comparative Fourier transform infrared (FTIR) spectra of the biomasses before and after contacting the effluent were acquired and analyzed.

The spectra of native and modified spirogyra were acquired by FTIR photometer within the IR range and shown in Figure 3, which clearly depicts the presence of carboxyl, amino, hydroxyl and carbonyl groups in the biomasses. In both types of biomass, the band displayed at 3,639, 3,542, and stretching to 2,889 cm⁻¹ can be attributed to –OH, –COOH, and –CH and –COOH groups, respectively (Gupta & Rastogi 2008; Celekli et al. 2016; Bayramoglu et al. 2018). The other common bands of the spectra could be assigned as: –N–H bending at 1,462 cm⁻¹; –CH₃ stretching at 1,379 cm⁻¹, overlapping of the stretching vibrational band for –SO₃ that represents the sulfonic acids of polysaccharides and NOO stretch at 1,274 cm⁻¹; >C═N; and >C═C stretching vibrations at 1,027 cm⁻¹ (Gupta & Rastogi 2008; Celekli et al. 2016). However, the band appeared at 1,642 cm⁻¹ (in spectra 2a and 2b) attributing the –NH₂ or, –C–N amide groups for the native spirogyra was found to be disappeared with the modified spirogyra (as shown in spectra 2c and 2d). A decrease in intensity and peak stretching at 2,889 cm⁻¹ was also observed with modified spirogyra. This difference is certainly due to the treatment of native spirogyra with conc. sulfuric acid that could take out the amides via complexation. When comparing the spectra of before and after adsorption of Cu²⁺ ions with native and modified spirogyra, very significant changes in the band regions attributed to –OH, –COOH, and –CH groups are direct evidence for the adsorption of metal cation takes place via the binding with these active groups onto the cell walls (Bayramoglu et al. 2015).

Thus, the sorption behavior exhibited as a function of pH suggests that at lower pH binding sites of the biomass cell becomes protonated, resulting in restriction for the metal cation Cu²⁺ to approach sorption sites (Jayakumar et al. 2015). With an increase in pH, more ligands with negative charges on active groups are expected to be exposed to attract the metal cation for its binding (Gupta & Rastogi 2008; Jayakumar et al. 2015; Celekli et al. 2016). However, the subsequent decline in sorption with pH above 6.0 can be attributed to the reduced solubility of Cu²⁺ and precipitation of metal hydroxide.

The effect of biosorbent amounts on the removal efficiency of copper from the effluent stream was investigated under the range of 1−10 g biosorbent dosage added to a 100 mL solution. The other parameters viz. metal concentration 194 mg·L⁻¹ Cu²⁺ in solution of pH 6.0, contact time 30 min, and temperature 20 °C were invariably maintained. The results presented in Figure 4 show that the sorption of copper increased with respect to the increasing amount of biosorbent up to 6 g/100 mL and further remains constant. The maximum sorption efficiency with native spirogyra could be achieved from 56% to ∼83%; whereas it yielded 63% to more than 96% with the modified spirogyra when biosorbent dosage was increased from 1 g to ≥6 g. The behavior exhibited by both types of sorbent can be described by the increase of available sorption sites with increasing the amount of sorbent (Srivastava et al. 2012); however, at higher population of the biomass (>6 g) in the pulp may result in aggregation of biomass to decrease the surface area of interaction and thus no more improvement in metal adsorption (Jayakumar et al. 2015). At the chosen dosage of 6 g/100 mL, the sorption of copper with modified biomass was approximately 15% higher than that with the native spirogyra (82.8%). A higher biosorption with modified spirogyra can be attributed to the difference in its surface area with the native form, mainly due to oxidation of the green algae with H₂SO₄ which induces the reaction of phenolic –OH group present on the surface of spirogyra.

The batch sorption efficiency of the biomasses at different contact times was investigated from 2 to 30 min by exposing 100 mL effluent (194 mg·L⁻¹ Cu²⁺) to a 6 g sorbent at pH 6.0 and temperature of 20 °C. Figure 5 displays the copper sorption rate obtained using both native and modified biomasses of spirogyra. It can be seen that the removal of copper from effluent started to increase rapidly at the initial stage of the sorption process until 15 min, reaching near to 80% metal adsorption. Subsequently, it increased with a slow rate to attain equilibrium after a 20 min contact with native and 25 min contact time with the modified biomass that could remove approximately 82% and 96% of copper, respectively. For the engineering design of any sorption system, knowing the sorption rate is an important factor, hence the kinetics analysis of the copper removal rate was also investigated.

All the calculated thermodynamic parameters are given in Table S1 (available online). The negative values of the thermodynamic properties (ΔG°, ΔH° and ΔS°) indicate the feasibility of a spontaneous process, exhibiting the exothermic biosorption with structural changes on the cell wall surface of the biomasses. The enthalpy of physical and chemical adsorption falls into a range of 2.1–20.9 kJ·mol⁻¹ and 80−200 kJ·mol⁻¹, respectively; therefore, the adsorption of copper in this study can be attributed to the physico-chemical adsorption process instead of being only physical or chemical adsorption.

Using the experimental data, graphs were plotted for both models and shown in Figure S2a and S2b (available online). The analysis of plots indicate that the native biomass better fitted to Freundlich isotherm (R² > 0.99), revealing the heterogeneity of adsorption (Pathak 2017), whereas the modified biomass well fitted to the Langmuir model, indicating the homogeneous characteristics of the sorption process with cross-link structure of the modified spirogyra to exhibit the monolayer adsorption of copper. The values of the coefficients calculated for both isotherms are given in Table 2. Also, the obtained adsorption capacity in this study has been compared with the earlier reported different biomasses capacity (for sorption of Cu²⁺ from various solutions) and presented in Table S2 (available online). It indicates that the sorption capacity obtained with modified biomass in this study is the highest of all.

The batch sorption study established the higher sorption efficiency with modified algal biomass but in order to determine the reusability of this biomass further, the elution of adsorbed metal ion was evaluated in four consecutive cycles of sorption–desorption. The sorption condition was maintained as 6 g modified sorbent in 100 mL electroplating effluent, time 30 min, pH 6.0, and temperature 20 °C. Subsequently, the separated metal-loaded biomass was introduced to 1.0 mol·L⁻¹ HCl solution (preheated at 50 °C) by maintaining a solid-to-liquid ratio of 1:4 and contacted for 60 min. The elution temperature (50 °C) was chosen because of the minimum adsorption observed at this temperature (as given in Figure 7(a)). By contacting the HCl solution, the coordination of Cu²⁺ ions with the active group of biomass gets disrupted to release the metal ions from the adsorbent surface (Bayramoglu et al. 2015). The prominent results (>98.5%) for elution of Cu²⁺ ions from the biomass was obtained with the fresh biomass, which was further used for the next three sorption–desorption cycles. In the second use of biomass, the sorption efficiency was slightly lower than fresh (∼93.2%) but thereafter until the fourth cycle of study, the regeneration efficiency was good enough with ∼90% efficiency.

In this study, the potential use of green algae spirogyra has been investigated for copper removal from an effluent generated by electroplating industry. A comparison between the biosorption capacity of biomasses in the native and modified forms after a sulfuric acid treatment revealed the attainment of cross-linked structural change could enhance the sorption efficiency as compared to the native spirogyra. The study showed a maximum sorption at pH 6.0, either increase or decrease in solution pH depicted a significant decline in adsorption of Cu²⁺ was directly affected by the surface adsorption sites of the functional group attached with the cell wall of biomass, which was also confirmed by FTIR spectral analyses. A 6 g sorbent mass treated to 100 mL effluent for 30 min at pH 6.0 and temperature of 20 °C were optimized to yield the maximum 82.8% and 96.4% adsorptive removal of copper from the effluent by the native and modified spirogyra, respectively. The sorption data exhibited pseudo-first-order kinetic for multilayer sorption via external surface diffusion onto the native biomass followed the Freundlich isotherm; whereas the modified spirogyra showed pseudo-second-order kinetics for monolayer intra particle diffusion followed the Langmuir model. The calculated ΔH° values (−41.29 kJ·mol⁻¹ and −75.06 kJ·mol⁻¹ for native and modified spirogyra, respectively) was attributed to the physico-chemical nature of the spontaneous adsorption with negative values of ΔG °. The sorbed metal was successfully eluted using 1.0 mol·L⁻¹ HCl solution at solid-to-liquid ratio of 1:4, 50 °C, and 60 min, and good regeneration efficiency (>90%) was obtained after three stages of recycling.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors declare no competing financial interest.