Adsorption isotherms and kinetic studies for the defluoridation from aqueous solution using eco-friendly raw marine green algae, Ulva lactuca Ghada F. El-Said & Manal M. El-Sadaawy & Mohamed A. Aly-Eldeen Received: 10 May 2017 /Accepted: 5 December 2017 # Springer International Publishing AG, part of Springer Nature 2017 Abstract The defluoridation from aquatic medium by raw marine algae, Ulva lactuca, was investigated. The element components and adsorbent’ s properties were determined. The influence of pH, contact period, and Ulva sp. weight on the deflouridation was achieved. The complete defluoridation of 10 mg F - /l solution was obtained within 10 min at definite pHs. Fourteen ad- sorption isotherms and four kinetic models (pseudo-first order and pseudo-second order, Elovich, and intraparticle diffusivity) were studied. The most ade- quate model was performed by using five error func- tions. The application of chi-squared test indicated that the nonlinear isotherm models (two and three parame- ters) were more applicable than the linear ones. The adsorption kinetic proceeded by pseudo-first-order model. The adsorption mechanism was very complex; thus, the solute distribution step was not the only dominant one during the process. The adsorption seemed to be a physical negative binding cooperative type with activation energy and free energy of 3.16 and - 1.672 kJ/mol, respectively. Thus, raw marine algae, Ulva lactuca, can be used as an adsorbent in solving the fluoridation problem in aquatic mediums throughout the world due to their fast procedure, high efficiency, low effort, and high economic value. Keywords Raw Ulva lactuca . Element components . Adsorbent’ s properties . Adsorption and kinetic isotherms . Error function analyses Introduction Many fluoride compounds containing other elements are widely spread around the world. The low fluoride concentrations are extremely important for many phys- iological processes in human beings (Tang et al. 2009; Umlong et al. 2012). Its concentrations of 0.5–1.0 mg/l are useful for a healthy body and provide considerable prevention against tooth decay. Fluoride operates a re- markable role in the composition, and in preserving the safety of bones (Li et al. 2017). The best fluoride con- centration for the drinking water in hot and cold weathers must be within 1–1.2 mg/l (WHO 2011; Valencia-Leal et al. 2012). The fluoride amount in po- table water of 4 mg/l was estimated as the maximum contaminant level (US EPA 2003). Fluoride causes nu- merous health risks that lead to many environmental problems (Venkata Mohan et al. 2007; Karthikeyan Environ Monit Assess (2018) 190:14 https://doi.org/10.1007/s10661-017-6392-6 G. F. El-Said (*) : M. M. El-Sadaawy Marine Pollution Laboratory, Marine Environment Division, National Institute of Oceanography and Fisheries, Kayet Bay, El-Anfoushy, Alexandria, Egypt e-mail: ghadafarouk25@yahoo.com e-mail: gfarouk66@hotmail.com M. M. El-Sadaawy e-mail: manal_dn@yahoo.com M. A. Aly-Eldeen Marine Chemistry Laboratory, Marine Environment Division, National Institute of Oceanography and Fisheries, Kayet Bay, El-Anfoushy, Alexandria, Egypt e-mail: m_niof@yahoo.com