Background:A novel biosorbent based on defatted Chlorella vulgaris (DCV) as a by-product of the biofuel industry was considered as an economical and inexpensive biosorbent in the form of magnetic modified defatted Chlorella vulgaris (MDCV/Fe3O4) for methylene blue (MB) removal. The lipid extraction was performed on raw Chlorella vulgaris (RCV). Phosphoric acid was selected as a DCV modifier. During acid modification, the variables affecting the biosorption capacity and the residual algae such as temperature (30-70 ), the contact time of DCV with acid (3-9 hr), the concentration of acid (2-6 mol/L), and the ratio of acid volume to DCV (30-70 mL/g) were investigated and optimized using Minitab-18 software. The modified defatted Chlorella vulgaris (MDCV) was prepared by acidification of DCV under optimal conditions. MDCV/ was prepared using the co-precipitation method for easy and low-cost separation of biosorbent. The XRD, FTIR, SEM, EDS, BET, and VSM analyses were performed to identify the structures and characteristics of RCV, DCV, MDCV, and MDCV/. Some experimentsere designed using Minitab-18 software to investigate the effects of temperature (5-45 ), contact time (30-90 min), biosorbent dosage (15-45 mg), initial concentration of MB (20-100 mg/L), and pH (5-9) on the biosorption capacity of MDCV/. The kinetic, isothermal and thermodynamic parameters were investigated on MDCV/.Results:The specific surface area of MDCV/ was 25.20 . RCV, DCV, and MDCV/ had a crystalline structure and MDCV had an amorphous structure. The data were most consistent with Pseudo-second-order and Freundlich models. The maximum biosorption capacity of MDCV/ was calculated in the amount of 32.44‌ mg/g. According to the positive values of and negative values of (-46.56 kJ/mol) and (-0.17 kJ/mol.K), the biosorption of MB on MDCV/ was non-spontaneous, exothermic with a decrease in irregularity.Conclusions:In this study, MDCV/ was applied as the MB biosorbent. Modifications such as lipid extraction, phosphoric acid modification and magnetization improved this biosorbent in its biosorption capacity and separation. Data were optimized in the acid modification and MB biosorption steps. Comparison of the characteristics of MDCV/ with unmodified species confirmed its high efficiency. Kinetic, isothermal and thermodynamic studies were also performed.