Removal of divalent heavy-metal ions from aqueous solutions by adsorption process with titanium dioxide nanowires Snežana Maletić, Jelena Tričković, Marijana Kragulj Isakovski, Sr đ an Rončević, Jasmina Agbaba, Ákos Kukovecz, Božo Dalmacija

 Snežana Maletić  Associate Professor at the chair of Chemical Technology and Environmental Protection.  University of Novi Sad Faculty of Sciences  Department of Chemistry. Biochemistry and Environmental Protection  Republic of Serbia 

Heavy metal pollution  Heavy metal pollution is a serious threat to aquatic ecosystems and cause harm to animals and humans via ingestion of soil and/or dust, food, and water; inhalation of polluted air; and absorption via the skin from polluted soil, water and air.  Heavy metals are not biodegradable and tend to accumulate in living organisms and represent a special risk for the local biota.  Copper are known to be essential to life.  Other heavy metal ions, such as Cd (II) and Pb(II), are not essential to life.

TiO 2  Adsorption is considered to be an economical and effective method for removal of heavy metals from aqueous solutions.  Titanium dioxide has been intensively investigated as potentially sorbent due to its high chemical stability in the pH range 2–14.  Nano-size titanium dioxide possesses many unique features:  high surface area,  more surface atoms,  high surface reactivity,  unique catalytic activity and  high suspension stability when compared to larger size particles.

Objective of study:  The key objective of this work was to investigate kinetics and adsorption capacities of divalent metal ions (Cu 2+, Pb 2+, Cd 2+ ) from water on TiO 2 nanowires at pH 3 and 7.

2 Procedure of batch adsorption experiments ADSORPTION OF Cd 2+, Cu 2+ AND Pb 2+ BY TiO 2 NANO-WIRES 1.ADSORPTION KINETICS 2.ADSORPTION ISOTHERMS ADSORPTION OF Cd 2+, Cu 2+ AND Pb 2+ BY TiO 2 NANO-WIRES 1.ADSORPTION KINETICS 2.ADSORPTION ISOTHERMS BECKGROUND SOLUTION 0.01 M CaCl 2 SOLUTION BECKGROUND SOLUTION 0.01 M CaCl 2 SOLUTION pH=3 pH=7 ADSORBENT TiO 2 NANO-WIRES ADSORBENT TiO 2 NANO-WIRES AGITATION FILTRATION (0.22 µm) AAS ANALYSIS ADSORBENT CHARACTERISATION 1.SPECIFIC SURFACE AREA 2.PORE ANALYSIS 3.pH pzc ADSORBENT CHARACTERISATION 1.SPECIFIC SURFACE AREA 2.PORE ANALYSIS 3.pH pzc Cd 2+, Cu 2+ AND Pb 2+ ( mg/l)

Characterization of TiO 2 3  According to the average pore radius – mesoporous adsorbent.  Literature has shown that the pH pzc of TiO 2 nanoparticles range from 4.8 to 6.2.  When pH is lower than pH pzc surface of TiO 2 nanowires is positively charged (TiOH 2 + ). Specific surface area - BET (m 2 /g) Average pore radius - BET (nm) Micropore t-test (cm 3 /g) Total pore volume - BJH (cm 3 /g) pHpzc

The effect of contact time on the divalent metals adsorption by TiO 2 nano-wires. at pH 3 and 7  Maximum 20 hours was enough to achieve a state of equilibrium.  Therefore, 24 hours was chosen as the contact time in all subsequent experiments.

Parameters of pseudo-first order. pseudo second order. Elowich and Weber-Morris kinetic models for the adsorption of divalent metals by TiO 2 nano-wires. at pH 3 and 7 Metal Pseudo-first orderPseudo-second orderElowichWeber-Moris R2R2 k 1 (min -1 ) R2R2 k 2 (g mg - 1 min -1 ) R2R2 α (mg g -1 min -1 ) β (mg -1 g) R2R2 k i (mg g -1 min -0.5 ) Cd Cu Pb x Metal Pseudo-first orderPseudo-second orderElowichWeber-Moris R2R2 k 1 (min -1 ) R2R2 k 2 (g mg - 1 min -1 ) R2R2 α (mg g -1 min -1 ) β (mg -1 g) R2R2 k i (mg g -1 min -0.5 ) Cd x x Cu Pb x pH 3 pH 7 k 2 : Cd>Cu>Pb k 2 : Pb>Cd>Cu

Comparison of experimentally determined and theoretically calculated q e values Metal pH = 3pH = 7 Eksperimental determined q e values (mg/g) Theoretically calculated q e values (mg/g) Eksperimental determined q e values (mg/g) Theoretically calculated q e values (mg/g) Cd Cu Pb  Good agreement of experimentally determined and theoretically calculated q e values.  Additional indicators of the suitability of pseudo second order model to describe divalent metal adsorption on applied adsorbent.  Pseudo-second order model - most responsible process for the removal of divalent metals was so called chemical adsorption.

Weber-Morris intra-particle diffusion model for the adsorption of divalent metals by TiO 2 nano-wires. at pH 3 and 7 Average pore radius - BET (nm) 11.75

Parameters of Freundlich and Langmuir isotherm models for the adsorption of divalent metals by TiO 2 nano-wires. at pH 3 and 7 FreundlichLangmuir K F (µg/g)/(dm 3 /µg) n 1/nR2R2 K L (dm 3 /g) α L (dm 3 /µg) R2R2 q max (mg/g) RLRL AdsorptionCd (pH=3) on TiO Adsorption Cu (pH=3) on TiO Adsorption Pb (pH=3) on TiO Adsorption Cd (pH=7) on TiO Adsorption Cu (pH=7) on TiO Adsorption Pb (pH=7) on TiO

Comparasion of q max for divalent metals at pH 3 and 7  q max value was much higher at pH 7 than at pH 3 for Cd and Cu,  hence for Pb q max was slightly higher at pH 3.

pH < 4.8 Ti 4+ − OH + H + → Ti 4+ − OH 2 + pH > 4.8 Ti 4+ − OH + OH − → Ti 4+ − O − + H 2 O pH < 4.8 Ti 4+ − OH + H + → Ti 4+ − OH 2 + pH > 4.8 Ti 4+ − OH + OH − → Ti 4+ − O − + H 2 O Surface charge of TiO 2 nano-wires as a function of pH When pH is lower than pHpzc surface of TiO 2 nanowires is positively charged (TiOH 2+ ).

Mechanism of Cd 2+, Cu 2+ and Pb 2+ adsorption by TiO 2 nano-wires included: 1.Chemical interactions between divalent metal ions and the surface of TiO 2 nano-wires; 2.Diffusion of divalent metal ions inside the pores of TiO 2 nano-wires. Cd 2+, Cu 2+ and Pb 2+ adsorption capacity of TiO 2 nano-wires was highly influenced by pH value. Adsorption decreasing in the folowing order: Pb pH3 >Cd pH7 >Pb pH7 >Cd pH3 >Cu pH7 >Cu pH3. Conclusions Controled adsorption kineticks

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