BEIJING--China has returned a shipment of anthracite coal to North Korea because it failed to meet standards for mercury emissions, according to a local report on Saturday.

This appears to be the second rejection by China of the North Korean mineral this year.

The shipment arrived at the Longkou port of China's northern coastal province of Shandong late last month, but was returned as its quality did not satisfy China's environmental regulations, iQiru.com, a local Shandong Internet news site, reported, citing an unnamed Longkou port official. 

The report did not elaborate further or include the volume of the rejected North Korean coal. 

On March 4, another local media outlet reported that a shipment of North Korean anthracite coal was returned late February from the Rizhao port of Shandong for similar reasons. 

After three decades of rapid industrialization, China regularly sees hazardous air pollution with levels of particulate matter rising to nearly 40 times the limits set by the World Health Organization during the winter months. 

In September last year, China announced strict regulations against the sale and import of coal with high toxic pollutants, including mercury and sulfur, to improve the country's air and water quality. 

Anthracite coal accounted for 39.8 percent of North Korea's total exports to China last year. 

China's imports of North Korean coal plunged 53.2 percent from a year earlier to 16.78 million tons in January this year, according to Chinese customs data.(Yonhap)