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The third-generation bioethanol production depends on the utilization of algae as raw material and an excellent source of polysaccharides, which can be fermented to bioethanol. The microalga Chlorella vulgaris is suitable for bioethanol production due to its carbohydrate content. In this study, the chemical pretreatments of acidic and alkaline pretreatments of Chlorella vulgaris biomass showed that the maximum reducing sugars and total carbohydrate concentrations produced from acidic pretreatment of the algal biomass by using 2% sulfuric acid (H2SO4) at 120 °C for 30 min were (24.77 g/100 g dried biomass) and (16.08 g/100 g dried biomass), respectively. The bioethanol released from the biomass was increased gradually until the 5th day (14.32 g/100 g dried biomass). The maximum bioethanol produced after 2% (w/v) of sodium hydroxide (NaOH) at 120 °C for 30 min. alkaline pretreatment was (21.54 g/100 g dried biomass) reducing sugar, (15.72 g/100 g dried biomass) total carbohydrate and the produced bioethanol concentration on the 4th day (7.74 g/100 g dried biomass). At the same time, the physical pretreatments were done by microwaves and ultrasonication. The highest reducing sugars after ultrasonication, (18.97 g/100 g dried biomass) which fermented to (16.19 g/100 g dried biomass) bioethanol at the 4th fermentation day. While microwaves pretreatment reached (11.22 g/ 100 g dried biomass) reducing sugars, that synthesis (2.91 g/100 g dried biomass) at the 5th fermentation day. This work concluded that 2% H2SO4 was the best treatment method, which resulted in the highest concentrations of reducing sugars, total carbohydrate, and bioethanol.

Synthetic dyes are scattered in untreated or inappropriately treated effluents, and their dangerous items created during the halfway corruption are released into the water bodies that cause a horrendous smell, which prompts anomalous changes in the nature of water. In the present study, green algae and cyanobacteria are considered as a significant hotspot for decolorizing color and material gushing. Chlorococcum sp., Scenedesmus obliquus, Chlorella vulgaris, and Oscillatoria sp. were investigated for degradation and removal of some azo dyes [Reactive Orange 122 (Orange 2RL) and Reactive Red 194 (Reactive Red M-2BF)]. The results showed that the maximum decolorization was spotted at 20 ppm Reactive Orange 122 with Oscillatoria sp. mixed with S. obliquus (98.54%). 20 ppm Reactive Red 194 was decolorized by Oscillatoria sp. mixed with S. obliquus (97.58%) after 7 days of incubation. The decolorization was detected by spectroscopic analysis and Fourier transformed infrared (FTIR) spectroscopy. The suitable factors that accelerated the azo dye decolorization and enhanced the biological treatment methods to be more effective and speedier in decolorization were investigated. At 25 °C and continuous lighting, the highest percentage of the azo dye decolorization was obtained; BG11 was the suitable medium that gives a high percentage of the azo dye decolorization. However, relative to the effect pH on azo dye decolorization, results show pH 11 and pH 9 more effective on azo dye decolorization for Reactive Orange 122 and Reactive Red 194, respectively. A total of 6% of thiamine and ascorbic acid recorded maximum degradation activity at Reactive Orange 122 when treated with Oscillatoria sp. mixed with S. obliquus 79.13% and 77.18%, respectively.

Biosorption by algae is an effective process for the removal of heavy metals from the aqueous solutions. The present work deals with the biosorption of Cd(II) by Chlorella vulgaris in a batch system. The biosorption efficiency of Cd(II) removal was studied at different pH (3–8), initial metal concentrations (20–100 ppm), agitation time (5–120 min.), agitation speed (50–250 rpm), and biomass dosage (0.01–0.1 g/50 ml of metal solution). The optimum conditions for maximum biosorption capacity for C. vulgaris were at pH 6, initial Cd(II) concentration 75 mg/l, biomass dosage 0.08 g/50 ml metal solution, temperature 25 °C, agitation speed 250 rpm, and agitation time 30 min. The cadmium removal efficiency of the raw and pretreated algal biomass was studied under the optimum conditions. The results showed that pretreatment with acetic acid gave 99.346% as compared with raw biomass. Different algal weights (0.2, 0.15, 0.1, 0.05, and 0.025 g) were immobilized with 10 ml of 4% calcium alginate. The results showed that the highest cadmium biosorption efficiency was 76.448% for 0.025 g as compared with the control. The biosorption mechanisms were examined by Fourier-transform infrared analysis and scanning electron microscopy for raw and pretreated algal biomasses before and after cadmium biosorption. It was found that hydroxyl, amide with hydrogen bond, and carbonyl stretching in carboxyl groups played an important role in biosorption.