Introductory Chapter: An Overview of Phosphate Mineral and Electrochemical Detection of Phosphate for Environmental Remediation

1. Introduction

A class of inorganic salts, derived from sustainable phosphoric acid, is known as phosphate minerals. Over 200 different phosphate mineral classes have been identified to date, and all of them feature isolated (PO₄) tetrahedral units in their structure. Tetrahedrally coordinated phosphate (PO₄³⁻), along with occasionally substituting arsenate (AsO₄³⁻) and vanadate (VO₄³⁻), chloride (Cl⁻), fluoride (F⁻), and hydroxide (OH⁻) that also fit into the crystal structure, is present in phosphate minerals. Although the phosphate class of minerals is a sizable and varied group, only a few species are comparatively widespread. Phosphates can be divided into three categories: (i) primary phosphates, which have formed from a liquid; (ii) secondary phosphates, which have emerged as a result of the repeated occurrence of primary phosphates; and (iii) fine-grained rock phosphates. These phosphates have mostly developed from the sea containing phosphorus-bearing organic material at low temperatures.

Approximately 15–20% of the world’s phosphate resources are thought to come from volcanic and weathered deposits, with the remaining 75% originating from sedimentary, marine rock formations. Aqueous fluids produced during the latter phases of crystallization are often where primary phosphates form. The granitic pegmatites are the common examples of the primary phosphates such as apatite [Ca₅(F,Cl,OH)(PO₄)₃], triphylite [LiFePO₄], lithiophilite [LiMnPO₄], and the rare-earth phosphates monazite [(LaCe)(PO₄)] and xenotime [Y(PO₄)]. Carbonatites and nepheline syenites are examples of ultramafic rocks, which are very low in silica and frequently include primary phosphates. Both impure limestones and calc-silicate rocks contain metamorphic apatite. The formation of secondary phosphates in different oxidation states can occur in water at low temperatures. Iron and manganese are typically present in both their divalent and trivalent oxidation forms, which results in vibrant hues. The phosphates such as strengite [Fe(PO₄)(H₂O)₂] and vivianite [Fe₃(PO₄)₂(H₂O)₈] are two typical species. There are various varieties of phosphate minerals, as follows:

1.1 Pseudomorph mineral

Pseudomorph minerals are created when another substance undergoes chemical or structural change while preserving its original outward shape. The majority of pseudomorphs are granular and waxy on the inside, lack a regular cleavage, and appear to be crystalline, but they really exhibit optical properties that are distinct from those needed for their outer appearance. Pseudomorphs can be produced by putting the crystals of one mineral on top of the crystals of another. Alteration pseudomorphs can be created in a variety of ways, (i) through a modification in internal structure of the crystal without any modification in chemical composition (these pseudomorphs are known as paramorphs, e.g., aragonite changes to calcite and brookite changes to rutile), (ii) via the removal of a component from the original mixture (e.g., cuprite loses oxygen to form copper), and (iii) through the introduction of an ingredient to the (e.g., feldspar loses potassium silicate and gains water to become kaolinite).

1.2 Triplite mineral

It is a phosphate mineral comprising of Mn, Fe, Mg, and Ca phosphate [(Mn, Fe, Mg, Ca)₂PO₄(F,OH)], named as Triplite mineral. This mineral normally occurs in several parts of globe, for example, Bavaria, Ger.; Kimito, Fin.; Karibib, Namibia; and Maine, Connecticut, and Colorado in the United States, and notably, it is present in granite pegmatites as brightly colored (brown, salmon, flesh-red) masses.

1.3 Fluorapatite mineral

The fluorapatite mineral, also known as Ca₅(PO₄)₃F, is a common phosphate mineral. It can be found in many igneous rocks as tiny, frequently green, glassy crystals as well as magnetite deposits, hot hydrothermal veins, and metamorphic rocks. Additionally, the collophane is found in marine deposits.

1.4 Borate mineral

Borate mineral is a naturally occurring boron and oxygen combination. Borate minerals are generally rare; however, some can be found in significant deposits that can be mined for profit. The BO₃ triangle or BO₄ tetrahedron wherein oxygen or hydroxyl species are located at the triangle vertices or at the tetrahedron corners with a central boron atom, respectively, is incorporated into the structures of borate minerals. There may be both kinds of units in a single construction. Extended boron-oxygen networks can be formed by vertices sharing an oxygen atom, or they can contain a hydroxyl group if they are bound to another metal ion. Any given mineral’s boron-oxygen complex shrinks in size as the temperature and pressure at which it forms rises and falls, respectively.

1.5 Tributyl phosphate

Tributyl phosphate is an organic liquid solvent used as a heat-exchange medium, a solvent for nitrocellulose, and the extraction of uranium and plutonium salts from reactor effluents. A phosphorus-containing substance with the chemical formula (C₄H₉)₃PO₄ is created when butyl alcohol and phosphorus oxychloride combine. Tributyl phosphate irritates the mucous membranes and corrodes the skin.

1.6 Amblygonite mineral

Amblygonite comprising of Li, Na, and Al phosphate [(Li,Na)AlPO₄(F,OH)] is phosphate mineral, which is extracted from ore of Li. It is often obtained from phosphate of lithium, that is, phosphate-rich granitic pegmatites having a very large crystal, white in color, and translucent masses. It has been mined at Keystone, South Dakota, as well as in a number of other nations, such as Zimbabwe and South Africa.

1.7 Cellophane mineral

Massive cryptocrystalline apatite, often fluorapatite or fluorian hydroxylapatite, makes up the majority of the fossil bone and phosphate rock known as collophane. It is typical to find horn-shaped concretions that are grayish-white, yellowish, or brown in hue.

1.8 Vanadate mineral

Vanadate is a naturally occurring mineral composed of vanadium (V), oxygen (O), and other metals. The majority of mentioned minerals are unusual and crystallized under highly specific circumstances, making them rare. Even though carnotite and vanadinite are occasionally mined as uranium and vanadium ore, respectively, most vanadates are of minimal economic significance; yet, mineral collectors esteem them for their vivid hues.

1.9 Sulfide mineral

Any member of the sulfur family-based compounds with one or more metals is referred to as a sulfide mineral. The majority of sulfides have straightforward structural characteristics, great crystallographic symmetry, and numerous metal-like characteristics, such as cluster of metals and electrical conductivity. They usually have high specific gravities, vivid hues, and low hardness. The general chemical formula A_mS_n, where A represents a metal, S ascribes to sulfur, can be used to indicate the composition of sulfide minerals. This formula yields the stoichiometries A₂S, AS, A₃S₄, and AS₂. Fe, Cu, Ni, Pb, Co, Ag, and Zn are the metals that are most frequently found in sulfides, while roughly 15 other metals can also enter sulfide structures.

1.10 Electrochemical detection of phosphate

The management of phosphorus nutrients is currently seen as a highly important societal task with significant implications for the economy, the environment, health, and industry, as a part of phospholipids, nucleic acids, or adenosine triphosphate, which are connected to cell membranes, genetic information storage and retrieval, and energy sources for cells, respectively. Phosphorus is in fact a crucial chemical element in live cells. Inorganic phosphate is produced in large quantities (82%) for use as fertilizers in agricultural fields, where the majority is lost to the environment [1, 2].

To solve environmental, financial, and health issues linked to phosphate processing, it is evident that there is a significant demand for quick, dependable, and affordable detection systems for continuous measurement. A variety of analytical techniques, including ion chromatography [3, 4], luminescence/fluorescence sensing [5, 6, 7], biosensing [8], and electro-analytical techniques [9, 10, 11], have been developed. Here, we give a brief summary of recent advancements in the design of nanomaterials to meet the needs of selectivity and sensitivity for potentiometric and amperometric sensors, or biosensors, for phosphate measurement in actual waters.

2. Metal-based electrodes

Xiao et al. [12] introduced the unique cobalt-linked electrode for phosphate sensor. Due to particular interactions with the thin CoO layer generated at the electrode surface, solid-state Co-electrodes demonstrated a potentiometric response to H₂PO₄. This specific reactivity of the cobalt oxide surface and phosphate anions was recently validated by Ogata et al. [13]. Cobalt wires with a diameter of 1 mm were recently used to optimize a Co-based microsensor that can be used in lake water and soil samples with a few millimeters of spatial resolution [14].

3. Polymer-based sensors

In response to the electrochemical detection, despite the difficulties brought on by the hydrophilic nature of phosphate, ion selective membranes have been employed for phosphate detection [15]. The polyaniline film was doped with 0.5 M phosphonic acid and electrodeposited on a gold electrode [16]. According to Satoh et al. [17], an ionophore-doped polyvinyl chloride (PVC) membrane based on bis (dibromophenylstannyl) methane responds primarily to HPO₄²⁻ among different PO₄ species. The limitations of this sensor are the interference with OH⁻ and its short life-time (< 5 h). A new PVC membrane recently developed by Topcu et al. [18] was doped with a chitosan-clay combination. The as-prepared electrode after conditioning in Cr (III) solution expresses an anionic response, being particularly sensitive and selective toward HPO₄²⁻.

4. Metal complex-based sensors

Applying some metal complexes including copper phthalocyanine (CuPc) [19, 20, 21] or M-2,6-bis(bis(2-pyridylmethyl)amino methyl)-4-methylphenol (M-BPMP, M = Zn and Cu) [22], uranyl salophene III [23] have already being used for the detection of phosphates.