He Shushan (2nd L), deputy mayor of Tianjin, speaks at a press conference in Tianjin, north China, Aug. 17, 2015. Most of the sodium cyanide surrounding the Tianjin blast area will be collected and cleared by Monday evening, He said here Monday morning. The chemicals were scattered mostly within a 100,000-square-meter core area. Monitoring has shown air quality in the area and water in the nearby river are within safe standards. (Xinhua/Zhang Chenlin)

Most of the sodium cyanide surrounding the Tianjin blast area will be collected and cleared by Monday evening, He Shushan, deputy mayor of Tianjin, said at a press conference Monday morning.

The chemicals were scattered mostly within a 100,000 square meter core area. A "very complicated and difficult" clean-up, partially due to the presence of 16,500 empty shipping containers, was underway on Monday, he said.

Workers are now searching empty shipping containers for chemicals dispersed by the blast.

Sand and earth barriers were built around the core area to prevent any leakage of cyanide and other pollutants. Cofferdams will be reinforced to prevent the outflow of contaminated water after it rains.

Monitoring has shown air quality in the area and water in the nearby river are within safe standards.

Initial estimates put the amount of cyanide at the site at 700 tonnes, most of which was unaffected, said Shi Luze, chief of staff of the Beijing Military Area Command at a press conference on Sunday.

Sodium cyanide is a highly toxic white, water-soluble powder that prevents the body from using oxygen. It is fatal if inhaled or ingested.

It is mainly used in chemical manufacturing and to extract gold and other metals in mining.

When treated with acid, it releases the highly poisonous gas hydrogen cyanide.