Kirstie Trup and Katie Gee, both 18, are recovering after they were targeted by two men on a moped on August 7 during a volunteering holiday.
Miss Trup says she has many happy memories of Zanzibar where the friends worked with underprivileged children.
The student, who gave local police a "very detailed" description of the attack, has questioned why authorities have not caught the culprits in Stone Town, the old part of the island's main city.
"Stone Town is too small for it to be this hard and I fear they will never be caught," she said.
The teenagers, from north London, were nearing the end of a month-long stint teaching English when they were attacked after leaving a restaurant.
Miss Trup suffered severe chemical burns to her shoulder and back from the sulphuric acid.
Police in Zanzibar have interviewed several people, including eyewitnesses, and are believed to have identified a possible culprit.
The girls were flown to London for treatment at Chelsea and Westminster Hospital.
Miss Trup was discharged after three days. Miss Gee, the more seriously injured of the two, is still believed to be in hospital.