Question: Mercury has a large iron what?
About the article "Solar System", section "Mercury".

Candidate passages:
Mercury is larger than the Moon, with a diameter of 4,879 km versus 3,476 km, and has a higher density due to its large iron core.
As a result, gravity on the surface of Mercury is 0.377 "g", more than twice that of the Moon (0.1654 g) and very close to the surface gravity on Mars.
Since there is evidence of human health problems associated with extended exposure to low gravity, from this point of view, Mercury might be more attractive for long-term human habitation than the Moon. 
 Mercury (0.4 AU from the Sun) is the closest planet to the Sun and the smallest planet in the Solar System (0.055 Earth masses).
Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history.
Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.
Its relatively large iron core and thin mantle have not yet been adequately explained.
Hypotheses include that its outer layers were stripped off by a giant impact; or, that it was prevented from fully accreting by the young Sun's energy.
The apparatus shown is divided into two types of cells separated by slate walls.
The first type, shown on the right and left of the diagram, uses an electrolyte of sodium chloride solution, a graphite anode (A), and a mercury cathode (M).
The other type of cell, shown in the center of the diagram, uses an electrolyte of sodium hydroxide solution, a mercury anode (M), and an iron cathode (D).
Note that the mercury electrode is shared between the three cells.
This is achieved by having the walls separating the cells dip below the level of the electrolytes but still allow the mercury to flow beneath them.
The reaction at anode (A) is: 2Cl– → Cl2 + 2e– The chlorine gas that results vents at the top of the outside cells where it is collected as a byproduct of the process.
The reaction at the mercury cathode in the outer cells is 2Na+ + 2e– → 2Na The sodium metal formed by this reaction dissolves in the mercury to form an amalgam.
The mercury conducts the current from the outside cells to the center cell.
In addition, a rocking mechanism (B shown by fulcrum on the left and rotating eccentric on the right) agitates the mercury to transport the dissolved sodium metal from the outside cells to the center cell.
The anode reaction in the center cell takes place at the interface between the mercury and the sodium hydroxide solution.
2Na (amalgam) → 2Na+ + 2e– Finally at the iron cathode (D) of the center cell the reaction is 2H2O + 2e– → 2OH– + H2 The net effect is that the concentration of sodium chloride in the outside cells decreases and the concentration of sodium hydroxide in the center cell increases.
As the process commences some sodium hydroxide solution is withdrawn from center cell as output product and is replaced with water.
Sodium chloride is added to the outside cells to replace what has been electrolyzed. 
The main chain of the Mercury Islands consists of the large Great Mercury Island (also known as "Ahuahu") to the west, Red Mercury Island (Whakau) to the east, and five much smaller islands between the two (Korapuki, Green, Atiu/Middle, Kawhitu/Stanley and Moturehu/Double Islands).
Only the main island is inhabited – the others form part of a nature reserve.
To the south of this chain numerous tiny islets lie to the north of the mouth of Mercury Bay.
One lone island, Cuvier Island, lies to the north of Great Mercury Island, although this island is not normally considered part of the Mercury Island group.
Great Mercury Island is what remains of a Pliocene rhyolitic volcano. 
KDF is known to kill algae and fungi, control bacteria growth, remove chlorine, pesticides, organic matter, rust, unpleasant taste and odour, hydrogen sulfide, iron, lead, nickel, chromium, cadmium, calcium, aluminium, mercury, arsenic, and other organic compounds.KDF has a wide spread of effectiveness against a large number of impurities in water.
KDF is optimised when used in conjunction with another filter media, especially any form of Activated carbon (whether in block OR granulated form).