1 Korean J. Chem. Eng., 32(4), 1-10 (2015) DOI: 10.1007/s11814-016-0078-6 pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: doctornadh@yahoo.co.in Copyright by The Korean Institute of Chemical Engineers. Kinetic, thermodynamic and equilibrium studies on removal of hexavalent chromium from aqueous solutions using agro-waste biomaterials, casuarina equisetifolia L. and sorghum bicolor Mokkapati Ramya Prasanthi * , Mokkapati Jayasravanthi ** , and Ratnakaram Venkata Nadh *** ,† *Department of Chemistry, ANUCET, Acharya Nagarjuna University, Guntur - 522510, India **Department of Biotechnology, Acharya Nagarjuna University, Guntur - 522510, India ***GITAM University - Bengaluru Campus, Karnataka - 561 203, India (Received 30 August 2015 • accepted 15 March 2016) Abstract-Removal of Cr(VI) by biosorption on two agro waste materials, casuarinas fruit powder (CFP) and sor- ghum stem powder (SSP), has been investigated. The prepared adsorbent materials were characterized by SEM, EDX, FTIR and BET. These biomaterials effectively removed Cr(VI) with a maximum removal of 93.35% and 63.75% using 15 gL -1 and 5 gL -1 of CFP and SSP, respectively, at 60 o C with 20 mgL -1 initial Cr(VI) concentration in solution. In both cases of adsorbents, kinetic data of adsorption fitted well in pseudo-second-order in terms of correlation coefficient (R 2 ). This helps in proposing the process of adsorption as chemical coordination, which is correlated with the thermo- dynamic study results conducted at different values of temperature. Langmuir, Freundlich and D-R models were evalu- ated for description of metal sorption isotherms. Values of coefficients of intra-particle diffusion and mass transfer have also been determined at different values of temperature. Keywords: Casuarinas Fruit Powder, Sorghum Stem Powder, Removal, Chromium, Adsorption INTRODUCTION Chromium is a type of heavy metal involved in a number of industrial applications like textile dyeing, pigment, leather tanning, electroplating, oil refineries, fertilizers, production of ferrochrome, steel and cement, fungicides, and metal finishing, causing water pollution, which is a major environmental concern [1]. Chromium exists in two stable oxidation states: relatively innocuous and im- mobile Cr(III) and a readily movable and strong oxidizing agent Cr(VI). This common aquatic pollutant in its hexavalent state Cr(VI) released by industrial activities into natural waters has det- rimental effects on both the living organisms and the ecosystems [2,3]. The permissible limit of Cr(VI) into the water bodies given by WHO is 0.05-1 mgL -1 ; however, a report by WHO, 2004 says that a much higher concentration around 100 mgL -1 of Cr(VI) would be present in the untreated effluent of an electroplating indus- try [4]. Therefore, to bring down the concentration of Cr(VI) to acceptable levels, almost all the industries must treat their efflu- ents before disposal. Hexavalent chromium is capable of being absorbed through the skin, which makes it highly toxic to living organisms [5]. It has very adverse effects if exposed strongly causing skin irritation to lung cancer, kidney, liver and gastric damage, epigastric pain, nausea, vomiting, severe diarrhea and hemorrhage; and due to its muta- genicity and carcinogenicity to human beings, it belongs to group “A” human carcinogen [6,7]. The downside of many conventional techniques for the removal of Cr(VI) from aqueous solutions such as chemical reduction, pre- cipitation, ion-exchange, and adsorption (using commercial char- coal) is that they are expensive for large-scale treatment of industrial effluents, which is making these procedures unaffordable for devel- oping nations [8]. Emphasizing on this aspect, many researchers have used unconventional materials as adsorbents for the removal of Cr(VI) from aqueous solutions and industrial effluents [9-23]. To potentially remove the toxic heavy metals from water streams, biosorption is an attractive alternate to traditional industrial efflu- ent treatment processes as it utilizes the biological materials as ad- sorbents which have high surface area and electric charges to accu- mulate heavy metals [24-32]. Polar functional moieties (contain- ing heteroatoms) adsorb Cr(III) because these heteroatoms donate their lone pairs of electrons to Cr(VI), and in the process reduced Cr(VI) to Cr(III) [33] . The brown macroalga Pelvetia canaliculata was used as a natural electron donor for the reduction of Cr(VI) to Cr(III) at acidic pH followed by removal of other metal ions present in the mixture by ion-exchange [34]. In addition, removal of Cr(III) by different biosorbents was studied recently [35-39]. It has been reported in the literature that, in comparison to the crude ones, the chemically modified natural adsorbents are much more advantageous in their pollutant removal efficiency from aqueous environment [8]. Moreover, due to the capability of lowering heavy metal concen- tration to mgL -1 level, wide range of availability and ecofriendly nature, the usage of biopolymers from agricultural wastes as adsor- bents is attracting many industries [40]. Hence, in the present work, the chemically modified powders of two abundantly available agri- cultural wastes (casuarinas fruit and sorghum stem) have been used