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Developing novel water purification technologies with Molecular Dynamics Simulations. A study of adsorbing organic matter with carbon nanotubes.

  • Sacyr Agua


Carbon nanotubes (CNTs) are increasingly being used in the field of microelectronics, energy harvesting and upcoming green energy technologies. CNTs are composed of fused carbon rings with high electron angular momentum, attributing them unique surface electrostatic properties and strong adsorptive properties towards organic compounds. Removal of impurities from drinking water with current technologies includes the approach of filtration using conventional ultra filters with limited use-time until fouling and clogging. Used water purification filters represent a waste problem. CNTs in the micrometer length class are tested as active ingredients in water purification systems, through their adsorption of organic matter. Initial studies using molecular dynamics simulations (MSDs) show that CNTs bind organic water pollutants, such as the polycyclic aromatic hydrocarbon (PAH) phenanthrene, selectively in water solutions. The binding is maintained, requiring boiling for the removal of the PAH molecules. This unique feature of CNTs makes them candidate materials for novel and recyclable battery-like ultra filters for organic matter removal, acting potentially in suspensions.
Dr. Otto Andersen
Western Norway Research Institute
Developing novel water purification technologies with
Molecular Dynamics Simulations
A study of adsorbing organic matter with carbon nanotubes.
Sergio Manzetti1, Cristian Garcia2, Elena Campos3, and Otto Andersen4*
1. Fjordforsk A.S. Institute for Science and Technology. Midtun, 6894 Vangsnes. Norway.
2. Incotec. C/ Princesa 25. Planta 2. Oficina 1. 28008 Madrid, Spain.
3. Valoriza Agua, Departamento de I+D, Paseo de la Castellana - CP 28046, Madrid, Spain.
4. Western Norway Research Institute. Fosshaugane Campus. Sogndal, Norway.
Molecular Dynamics Simulations (1)
- Apply the Newtonian equation of motion, F = ma, on molecules and
particles to simulate their behavior in the microscopic world of
- Atoms are presented in Cartesian coordinates.
- Bonds are simulated with harmonic potentials of bond stretching
and contraction.
-Coulomb-integrals are used to determine the electrostatic
interactions between charged atoms.
- Van der Waal forces between neutral atoms are estimated using
various mathematical models, such as the Lennard-Jones potential.
Molecular Dynamics Simulations (2)
- Molecular dynamics simulations can be used to simulate biological
systems (proteins, enzymes, membranes), nanomaterials (carbon
nanotubes, graphene, fullerenes etc), gases and liquids, ions,
solvents and nanoparticles.
- The advantage of MDS lies in the predictive potential of the
simulations in anticipating effects of physical and nature, material
properties, interactions and effects at the molecular level.
- MDS is a powerful tool to design, predict and calculate the
properties of nanomaterials, the functions of membranes and the
effects of specific materials in water purification systems.
- Currently, MDS is being used to develop a new rationale for a
technology for removing organic matter from water by the use of
carbon nanotubes (CNTs).
Potential filtering materials: Carbon nanotubes
- Carbon nanotubes (CNTs) are composed of hexagonal carbon moieties (benzene
groups) arranged in contiguous tubular structures and can be arranged in one or several
tubular structures on top of each other (single-walled or multi-walled).
- CNTs have extraordinary physical strength, tensile strength and high resilience and
comprise normally diameters ranging from 1.5nm – 2.5nm.
- CNTs can be synthesized at industrial or medical grade purity using various methods
such as arc-discharge, aerosol methods, or other methods.
Potential filtering materials: Carbon nanotubes
-Carbon nanotubes have aromatic surfaces, defined by their conjugated π-
chemistry, and have properties that are normally associated with organic
solvents such as naphthalene, and anthracene.
-The chemical properties of CNTs are unique, because of their special angle of
curvature across their tubular ensembles.
-The carbon atoms are in a chemical configuration which lies between the
purely aromatic (e.g. benzene) and the aliphatic class (such as alkanes), given
the physical strain that is exerted on the π-orbitals of the carbon atoms by the
curved shape of the surface.
- Increasing diameters, giving a lower curvature give therefore also more
aromatic properties of the surfaces compared to narrow tubular structures.
Molecular dynamics analysis of CNTs as
sorptive materials for organic matter.
lThree molecules representing organic
matter where selected: Retene (A),
perylene (B) and cholesterol (C).
l125 copies of each were inserted in
virtual boxes of 1000nm³ along with 8
CNTs of different diameters (1nm->2nm).
lThe mixtures of thousands of atoms and hundreds of
molecules were simulated for 10ns at 300K.
lInteractions between CNT and each of the three molecular
types were studied at nanometer resolution.
lEnergies of binding and energies of the formed complexes
were calculated.
Methods -simulation
lThe GROMACS package was used for all calculations.
lAll systems were equilibrated from 0 Kelvin and 1 atm. to
300K with PME method of electrostatics and Cut-off for
van der Waal forces for 100 picoseconds.
lLINCS algorithms used for simulating all bonds. Parrinello-
Rahman scheme for pressure coupling in the equilibration
lFull simulation of 10 nanoseconds run in vacuum, at 0
atm, 300K to observe particle and molecular behavior
without influence from other factors (eg. solvent, high
temperatures, pressure etc).
Methods -energies
Total energies were calculated with g_energy in the
GROMACS package.
Lennard-Jones potentials calculated to quantify the overall
energy of the formed complexes between CNTs and the
three molecules individually.
Binding energies were calculated using the formality:
Ebind = Ecomplex (ECNT + EMOL)
(where EMOL is the total energy of the retene, perylene or cholesterol molecules and ECNT the total energy of the
Graph: The energy of the
formed complex must be
lower than the sum of its
components in order for the
complex to form
lAll molecules bound strongly to the CNTs.
lThe CNTs with large diameters bound the aromatic
molecules perylene and retene more tightly, based on the
effects of π-electrons.
Figure: Retene bound and encapsulated by CNT of 2nm diameter.
lThe effects from π-electrons dominated all bonding to CNTs
from retene and perylene, particularly from 1.5nm and up to
2nm in diameter of the CNTs.
lCNTs with diameter larger than 1.2 nm were able to encapsulate
retene molecules inside their hollow structures. These bound
also through π-stacking.
lCholesterol bound through pure hydrophobic forces (non-planar
and flexibly) and formed the most dense clusters with all CNT
types (Figure), based on its more flexible geometry.
Left: CNT20+Cholesterol, Middle CNT10+Cholesterol, Right: CNT12+Cholesterol
CNTs and each of the molecules were also analyzed
individually and show to form dense clusters with high very
stable hydrophobic and aromatic packing effects.
Left: 125 mols. Retene, Right: Eigth CNTs, 1nm diam.
All hydrophobic energies were in favorable configurations and
the larger the diameter the more relaxed and equilibrated are
the interactions.
Lennard-Jones potentials for CNTs and Perylene mixtures. All
energies are negative and in favorable states. Larger diameters
give more favorable states.
Lennard-Jones potentials for CNTs and Retene mixtures.
All energies are negative and in favorable states.
Lennard-Jones potentials for CNTs and Cholesterol mixtures. All
energies are negative and in favorable states. Larger diameters
give more favorable states. Larger diameters give more
favorable states.
Physical and chemical properties quantities of the formed nanoclusters.
CNT10, 12, 15 and 20 designate respectively their diameters in Å.
lPerylene, which is large and planar aromatic binds with
strongest energy to the CNTs.
lCholesterol which is flexible and hydrophobic clusters in more
dense arrangements, with highest packing density. This
generates also the smallest particles of all molecules.
lCNTs with large diameters generally bind all molecules better,
because of encapsulation inside and good packing on the
surface, independently of molecule.
lCNTs can be used as sorptive materials to purify water,
however the binding to the test compounds is so strong that
eventual reuse of CNTs may require rigorous treatment.
lCNTs bind up to 10 molecules of cholesterol pr cylindrical unit
of 2 Å. This means that for CNTs of length of 2.6nm (as studied)
can bind up to 130 molecules. For CNTs of such dimensions,
this implies a sorption ratio of 1 : 130 molar ratio.
Further studies
lParticle collisions in water phase.
lSimulations of CNT and molecules in water at
various temperatures.
lEstimation of interaction profiles in water.
lSimulations with other molecules, such as
lEstimation of best CNT propertiers for general
purification approaches.
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Institute for Science and Technology. Midtun, 6894 Vangsnes
  • A S Fjordforsk
Fjordforsk A.S. Institute for Science and Technology. Midtun, 6894 Vangsnes. Norway.
Departamento de I+D, Paseo de la Castellana -CP 28046
  • Valoriza Agua
Valoriza Agua, Departamento de I+D, Paseo de la Castellana -CP 28046, Madrid, Spain.