Calcite, the most stable calcium carbonate in soil, is a major part of soil solid phase in arid and semiarid regions. This mineral significantly affects the chemical behavior of ions including oxyanions and transition metal ions in the soil. Arsenate and phosphate are two important oxyanions in natural systems like soil and because of similar ionic properties strongly compete for the colloids surface charge via ADSORPTION and desorption reactions. Because of the importance of this reaction in controlling the equilibrium concentrations of arsenate and phosphate in soil, in this research, arsenate ADSORPTION on calcite was measured and modeled as a function of arsenate concentration and ionic strength and also in competition with phosphate. In addition, charging behavior of calcite was measured by acid-base titration at various ionic strength. Based on the titration data, calcite point of zero charge (PZC) was obtained at pH=8. 2. ADSORPTION isotherms showed that arsenate ADSORPTION is affected by the ionic strength and the initial concentration of arsenate. ADSORPTION of arsenate was high at low ionic strength and was decreased with increasing the ionic strength. Arsenate ADSORPTION was also decreased with increasing phosphate concentration, but arsenate had no effect on phosphate ADSORPTION indicating phosphate is adsorbed stronger than arsenate on calcite. The experimental data were successfully (R2=0. 998) described with a single set of parameters by CD-MUSIC model, considering two inner sphere surface complexes ≡ Ca2O2AsO2 and ≡ Ca2O2PO2. Overall, the experimental data and model parameters implied that the stronger interaction of phosphate with calcite increases bioavailability and mobility of arsenate in calcareous soils.

Activated Montmorillonite (AM) reveals as a low-cost and efficient adsorbent for the ADSORPTION of nicotine and pyridine from aqueous solutions. In this study, the influence of several operation conditions (initial compounds concentration, volumetric flow rate, and height of bed) on the shape of breakthrough curves and the mass transfer resistance was evaluated. ADSORPTION experiments were developed to determine the ADSORPTION isotherm of the system, then the ADSORPTION of pyridine and nicotine onto activated Montmorillonite in single and binary systems has been studied using fixed-bed ADSORPTION column. In continuous ADSORPTION, Results show that the maximum nicotine uptake 0. 68 mmol/g of AM was achieved through electrostatic attraction and hydrogen bond at a pH = 6. 3, a flow rate of 1 ml/min and a height of bed equal to 12 mm. In binary mixtures, zeolite ADSORPTION is governed primarily by the size of pollutants present in water. Thus, the bigger compound (in this case, Nicotine), was adsorbed more easily than the pyridine present in the mixture. Experimental data were fitted according to Fowler Guggenheim for the isotherms and Wolborska model for the breakthroughs. AM was regenerated by ethanol and the results show that about 94% of the ADSORPTION capacity is maintained after three times of cyclic ADSORPTIONdesorption process.

The elemental composition of termite mound was determined by XRF which revealed K, Ti and Mn as minor constituents while Ca and Fe as major constituents. The dominant functional groups of termite mound are Fe-OH, Fe-O and O-H. The Pb (II) and Zn (II) ADSORPTION capacities (mg g-1) are in order of Pb(II) (13.07) > Zn(II) (12.40) > Pb(Pb/Zn) (11.72) > Zn(Pb/Zn) (7.62). The Langmuir ADSORPTION isotherm fitted the ADSORPTION data better than Freundlich isotherm. The ADSORPTION process was best described by pseudo-second-order kinetic model for single and binary solutions, and the rate constants k 2 (g mg-1 min-1) are 0.036, 0.016, 0.024 and 0.015, and the calculated value of q e (mg g-1) is 12.33, 12.25, 11.52 and 7.84 for Pb(II), Zn(II), Pb (Pb/Zn) and Zn (Pb/Zn), respectively. The regression coefficient, R 2 values, for these solutions ranged between 0.9966 and 0.9978. The DH (kJ mol-1) values were positive for single and binary solutions in the order Pb (II) (32.0) > Pb (Pb/Zn) (30.8) > Zn (Pb/Zn) (28.0) > Zn (II) (19.0), while DS (kJ mol-1 K-1) are in the order of Pb (II) (0.103) > Pb (Pb/Zn) (0.097) > Zn (Pb/Zn) (0.082) > Zn (II) (0.06). The DG value for Zn (II) is positive in both single and binary systems, while that for Pb (II) was positive between 313-333 and 323–333 K for single and binary systems, respectively. The data show that the use of neglected termite mound for Pb (II) and Zn (II) removal from aqueous solutions is economically significant in wastewater treatment.

The ADSORPTION behavior of four anionic dyes and one disperse dye in single solution and binary solutions on fly ash was investigated in order to elucidate the effect of COMPETITIVE ADSORPTION on the kinetics. The experimental findings showed that ADSORPTION equilibriums of four anionic dyes were reached within 50 min either in the single solution or in binary mixtures. COMPETITIVE ADSORPTION increased the time of attaining equilibrium of disperse dye. Desorption of dyes suggested the predominant ADSORPTION mechanisms, that is, chemisorption for anionic dyes and physisorption for disperse dye. For the binary mixtures, the anionic dyes could be adsorbed preferentially on fly ash at the first stage. Second-order kinetic models fitted better to the equilibrium data of all dyes in the single solution as well as in the binary mixtures. The maximum rate constant of intraparticle diffusion and the minimum external mass transfer coefficient was found for disperse dye both in single and in binary solutions. The intraparticle diffusion constants and external mass transfer coefficients of the four anionic dyes in binary solution are similar to those obtained in single solution. The Biot number confirmed that the intraparticle diffusion was the rate-limiting step in the dye sorption process.

Zinc (Zn) is one of the micronutrients that is toxic in high concentrations. One of the best ways for removing heavy metals (HMs) from water and waste water is the ADSORPTION of HMs by using different adsorbents. The main component of Gel-E-Sarshour (GES) is expandable minerals. Therefore it can be used for removal of HMs such as Zn from surface water. A trial with different concentrations of Zn in the absence or the presence of humic acid (HA) was conducted to study the COMPETITIVE and non-COMPETITIVE retention characteristics of Zn onto GES. The Freundlich model (R2=0. 98-0/99 and SE= 0. 04-0. 07), followed by Langmuir (R2=0. 85-0. 90 and SE= 0. 002-0. 006) and Redlich-Peterson (R2=0. 98-0. 99 and SE= 0. 10-0. 17) models the best fitted to COMPETITIVE and non-COMPETITIVE zinc ADSORPTION data onto GES in presence and absence of copper (Cu) and HA. The fit of two surfaces Langmuir was more desirable as compared to conventional form of the Langmuir to Zn ADSORPTION data. In all treatments, the equation constant regarding to bonding energy of the first ADSORPTION site (KL′ ) was several times greater than bonding energy of the second ADSORPTION site (KL′ ′ ), while, ADSORPTION maxima for the second site (b′ ′ ) were higher than that of the first site (b′ ). This means that although, the first site had a much greater tendency for Zn ADSORPTION, it was rapidly saturated with Zn. The amount of Zn ADSORPTION capacity in non-COMPETITIVE state were 7. 69, 73. 6, and 100. 9 percent more than in the presence of HA and Cu alone and together, respectively. The presence of HA and Cu as a competitor reduced 7. 14 and 42. 14 percent Zn ADSORPTION capacity of GES, respectively. Since, GES has a low price, being natural and abundant in Iran and due to its high ADSORPTION capacity for the removal of Zn, application of this substance is promising for the removal of HMs.

Background: Removal of heavy metals by bioadsorbents is one of the effective and inexpensive methods for water and wastewater treatment. The aim of this study was to investigate the ability of tea pulp in order to remove Cu +2 and Pb +2 metals in two states of ADSORPTION (single element and two elements) from aqueous solutions. Methods: Experiments were performed on synthetic and real samples at ambient temperature. The effect of solution pH, adsorbent dose, contact time, and initial concentration on single and COMPETITIVE removal of copper metals and lead was studied. ADSORPTION kinetics and ADSORPTION isotherms were analyzed by pseudo-second-order kinetic equations, Elovich model and intraparticle diffusion, Freundlich and Langmuir equations, respectively. Results: The maximum ADSORPTION capacity for copper and lead was observed at pH = 5-8 and pH = 4-8, respectively. Maximum ADSORPTION capacity for copper and lead by tea pulp in single-element solution (single) was 37. 17 and 48. 54 mg/g and in two-element solution (COMPETITIVE), was 28. 41 and 43. 47 mg/g, respectively. The ADSORPTION reaction of heavy metals by tea pulp followed the Longmuir isotherm and pseudo-second-order kinetic models. Conclusion: Tea pulp as an inexpensive bioadsorbents is able to remove about one-third of the copper, and approximately, half of lead from aqueous solutions, so its use in the treatment of aqueous solutions will be beneficial.