Ionic Crystals: Synthesis of modified structures and their study for environmental remediation
To state that the environment is in crisis is to state the obvious. The Earth will continue to revolve around the Sun but whether humans will be ever-present to enjoy the sunrise is another story. Our fate depends on the air we breathe and the water we drink and we have excelled at polluting both with compounds toxic to life. Our research has been the design and development of materials – organic polymers – that are used to remove toxic metals from water. Along with the fundamental knowledge we have published in the open literature, we have patented and produced three polymers that are used commercially to remove metal ions from water.
Our research has been based on polystyrene as the organic polymer. Though it has the advantage of chemical stability, it is a petroleum-based product and thus neither biocompatible nor sustainable. More recently, we have begun developing polymers for environmental remediation that are prepared from biocompatible materials. These polymers fall under the general category of inorganic polymers of which clays, silica, and zeolites are examples. Ionic crystals are a type of inorganic polymer. Some, such as sodium chloride, are water-soluble and some, such as calcium phosphate, are water-insoluble. It is the latter that are of interest to this research.
Ionic crystals have a low but finite affinity for metal ions and compounds in water. Their possible application as adsorbents is precluded by their low capacities. They are, however, biocompatible; e.g., calcium phosphate (hydroxyapatite, HAP) is a constituent in bone and can be made from waste eggshells (300 million pounds produced annually in the U.S. alone). If HAP can be made into a useful adsorbent, it valorizes what is normally landfilled. Our research is now focused on transforming HAP into an adsorbent with a high capacity for toxic compounds such as dyes, and their removal from wastewater in effluent from the textile industry. The problem is pervasive in developing countries without stringent environmental controls. Moreover, the chemical transformation reaction which we study to transform HAP utilizes biocompatible bisphosphonates as reagents to give hybrid polymers.
Generally, the ionic crystal M+X- is reacted with a modifier A+B- and, in an ion exchange reaction, M+ binds B- to yield a net solid-to-solid conversion. The polarity of the crystal can be varied further by incorporating transition metal ions. The hybrids are characterized by techniques that include FTIR, solid state NMR, electron microscopy, and X-ray diffraction.
Adsorption isotherms show the hybrid to have a high affinity for a dye: Modification of HAP with a bisphosphonate increases its affinity for methylene blue, a toxic dye contaminant in water from 10 mg MB /g HAP to 893 mg MB /g. Since dyes are pollutants in rivers and other water supplies, the high capacity allows application to environmental remediation. That it is biocompatible gives it an important advantage over organic polymers.
Biocompatibility combined with high capacity makes the modified HAP attractive for drug delivery applications. Results with the dye imply high loading is possible with antibiotics. The modified HAP can be implanted and slowly release the antibiotic into the body.
The bisphosphonate below is a modifier we are studying in the reaction with HAP (Ca3(PO4)2) to form the modified HAP, shown as Ca(HEDP):
The adsorption isotherm of HAP with methylene blue is shown below. It follows the Langmuir equation. A capacity of 10mg /g is consistent with literature values.
Modified HAP has a far greater affinity for the dye. Linearization shows a loading capacity of 893mg/ g.
Characterization of HAP by FTIR:
Characterization of the modified HAP shows binding of the Ca2+ to the phosphonate ligand as well as unbound P=O and P-OH bonds:
Characterization by Phosphorus solid-state NMR shows that HAP has one peak as expected from the literature. The SSNMR of the modified HAP is surprising in showing two peaks at about 20ppm. Their significance is being investigated.
