Boehm titration explain the active surface of acidic and basic sites such as carboxylic, carbonyl and phenolic 42. Total acidic and basic sites were detected by back titration using 0.1 mol/L HCl and NaOH solutions. Carboxylic, lactonic and phenolic groups were evaluated by using titration with 0.1 mol/L NaHCO3, Na2CO3, and NaOH solutions. The acidic and basic sites of OB were (1.8 and 0.9 mmol/g), COB was (1.6 and 0.9 mmol/g), and OOBAR was (2.6 and 1.3 mmol/g). It is clear that the amount surface of total acidic sites was greater than basic sites in OB, COB, and OOBAR which have mainly acidic character. OB has 0.7 mmol/g carboxylic sites, 0.6 mmol/g lactonic sites and 0.5 mmol/g phenolic sites. The quantity of lactonic and phenolic sites was decreased from 1.1 mmol/g in OB to 0.6 mmol/g of COB while the number of carboxylic sites of COB was increased to 1.0 mmol/g due to an oxidation process. The amount of carboxylic and carbonyl groups of OOBAR is higher than COB due to the addition of phthalic acid.
The values of iodine number (I2 No) of OB, COB, and OOBAR were evaluated using back titration with 0.05 mol/L I2 and Na2S2O3 solutions. The I2 No of OB, COB, and OOBAR were 406.1, 320.1 and 291.9 mg/g (3.2, 2.5 and 2.3 mmol/g). The I2 No of OOBAR were decreased than OB and COB due to the coupling the biosorbent with phthalic acid. Methylene blue value (MB) of OB, COB and OOBAR were 176.97, 216.32 and 220.7 mg/g (0.59, 0.68 and 0.69 mmol/g). The increasing of MB value of OOBAR due to additional of functional groups. The results obtained indicated that the I2 No depends on the adsorption process while the MB value depends on the cation exchange process (the amount surface of total acidic sites is greater than basic sites in OB and COB). The I2 No of OOBAR was 291.9 mg/g (2.3 mmol/g) and methylene blue was 220.7 mg/g (0.69 mmol/L), these values are greater than those reported for other biosorbents 43-55.