  Thermonuclear X-ray bursts on the surface of neutron stars (NSs) can enrich
photosphere with metals, which may imprint the photoionization edges on the
burst spectra. We report here the discovery of absorption edges in the spectra
of the type I X-ray burst from the NS low-mass X-ray binary GRS 1747$-$312 in
Terzan 6 during observations by the Rossi X-ray Timing Explorer. We find that
the edge energy evolves from $9.45\pm0.51$ keV to $\sim6$ keV and then back to
$9.44\pm0.40$ keV during the photospheric radius expansion phase and remains at
$8.06\pm0.66$ keV in the cooling tail. The photoionization absorption edges of
hydrogen-like Ni, Fe or Fe/Ni mixture and the bound-bound transitions of metals
may be responsible for the observed spectral features. The ratio of the
measured absorption edge energy in the cooling tail to the laboratory value of
hydrogen-like Ni(Fe) edge energy allows us to estimate the gravitational
redshift factor $1+z=1.34\pm0.11$($1+z=1.15\pm0.09$). The evolution of the
spectral parameters during the cooling tail are well-described by the
metal-rich atmosphere models. The combined constraints on the NS mass and
radius from the direct cooling method and the tidal deformability strongly
suggest very high atmospheric abundance of the iron group elements and limit
the distance to the source to $11\pm1$ kpc.
