Peel, also known as skin, is the outer protective layer of a fruit or vegetable, currently gaining wide attention as adsorbent in water treatment. Peels of different fruits such as, orange, banana, watermelon, cassava, mango, etc. can be effectively used as adsorbents for wastewater treatment .
Formosa papaya (Carica papaya L.) seed powder (FPSP), a solid by-product from agricultural activities, was investigated by Pavan et al.  as an alternative adsorbent for the removal of crystal violet (CV) from aqueous solutions. The FPSP was characterized by specific surface area (BET), scanning electron microscopy (SEM), infrared spectroscopy (FT - IR), thermal analysis (TGA) and Boehm titration techniques. The effects of initial pH of solution, adsorbent dosage, contact time and initial dye concentration on CV adsorption were studied using batch contact mode at 25oC. The pseudo-second order model agreed very well with the kinetic data. The equilibrium adsorption was analyzed by Langmuir, Freundlich and Redlich-Peterson isotherms. The adsorption of CV onto FPSP was well fitted using Langmuir isotherm with a maximum adsorption capacity of 85.99 mg g-1.
Asgher and Bhatti  compared raw, immobilized and acetic acid-treated citrus waste to remove reactive blue 19 and reactive blue 49 from aqueous solution. Very excellent performance was achieved for both dyes. The physio-sorption was the predominant mechanism of removing reactive blue 19 and reactive blue 49 biosorption using citrus waste. The biosorption process of both dyes adequately followed all Langmuir, Freundlich and Temkin isotherms.
Acetone-treated capsicum seeds were used for reactive blue 49 uptakes in batch and continuous column systems. This process well described by Langmuir model in both batch and continuous modes. In batch biosorption experiments, ionic strength increase made a slight decrease in dye removal efficiency, while there were no significant changes obtained by increasing initial dye concentration .
Akar et al.  studied RR198 biosorption onto olive pomace in synthetic and real wastewater treatment. They found this process was spontaneous and endothermic in nature by calculating the thermodynamic parameters and well fitted by Langmuir isotherms models. RR198 biosorption onto olive pomace was independent of ionic strength in the concentration range of 0.01-0.15 M, whereas, it decreased in the ionic strength over 0.15 M. Besides, there was no tangible decrease in biosorption capacity of olive pomace when it was utilized for treating real wastewater containing several interfering species.
Garlic peel was investigated by Hameed and Ahmad  for the removal of Methylene blue from aqueous solution. Equilibrium isotherms were determined and analyzed using the Langmuir, Freundlich and Temkin isotherms. The maximum monolayer adsorption capacities were found to be 82.64, 123.45 and 142.86 mg g-1 at 303, 313 and 323 K, respectively. For Methylene blue, author observed that the adsorption capacity was higher due to the presence of polar functional group.
Dogan et al. studied hazelnut shell, an agricultural waste, without any pretreatment for the removal of Methylene blue with respect to the initial dye concentration, pH, ionic strength, particle size and temperature. Kinetic studies showed that the kinetic data were well described by the pseudo-second-order kinetic model. Significant increases in initial adsorption rate were observed with the increase in temperature followed by pH and initial MB concentration.
Jack fruit peel has been investigated as adsorbents for removal of Methylene blue by Hameed . The effect of different system variables like adsorbent dose, initial dye concentration, contact time and pH were evaluated and found that as the amount of adsorbent increased, the percentage of dye removal increased accordingly. Low concentrations of Methylene blue favored high adsorption percentages and the optimum pH value for dye adsorption was found to be 4.0. Among four different types of linearized Langmuir isotherm, the Freundlich isotherm and the Temkin isotherm, the equilibrium biosorption data were best fitted with the type 2 Langmuir model. The sorption capacity of Methylene blue on jack fruit peel was found to be 285.713 mg g-1.
Pavan et al.  reported that the use of yellow passion fruit (YPFW), a powdered solid waste, was tested as biosorbent for the removal of a cationic dye MB from aqueous solution. Adsorption of MB onto this low-cost natural adsorbent was studied by batch adsorption at 25°C. The effects of shaking time, biosorbent dosage, and pH on adsorption capacities were studied. In alkaline pH region, the adsorption of MB is favourable. The adsorption kinetics was better fitted to pseudo-first-order and ion exchange kinetic models. The equilibrium data was fitted to Langmuir, Freundlich, Sips, and Redlich-Peterson isotherm models. The maximum amount of MB is absorbed by YPFW biosorbent was 44.70 mg g '.
Banana stalks were studied as adsorbents by Hameed et al.  for basic dyes in aqueous solutions with equilibrium isotherms and kinetic adsorptions. High adsorption capacity of 243.90 mg g-1 was observed and authors suggested that banana stalks consists of cellulose and lignin; it is the polyol structure of cellulose-based materials that has relatively strong chemical adsorption of cations such as metal ions and organic bases as well as physical adsorption of other material such as acidic and anionic compounds.
Orange peels have been investigated as an adsorbent by Sivaraj et al.  for the removal of an acid dye: acid violet17. The adsorption capacity was 19.88 mg g 1 at initial pH of 6.3. The equilibrium time was found to be 80 min for 10, 20, 30, and 40 mg L 1 dye concentration, respectively. A maximum removal of 87% was obtained at pH 2.0 for an adsorbent dose of 600 mg 50 mL 1 of 10 mg L 1 dye concentration.