Iron mining tailing toxicity is increased by lower pH affecting lettuce seed germination, seedling early growth, and leaf anatomy.

Abstract

Solid iron mining tailings are some of the most relevant pollutants because dam failures release huge amounts of such materials into the environment. These pollutants contain several potentially toxic elements, which may cause high toxicity to plants depending on factors such as their availability, preventing restoration programs. This work aimed to investigate the effect of the modification in the pH from iron mining tailings to Fe and Al toxicity accessed by lettuce biotests, a known sensitive species. We conducted two experiments: 1) comparing the intrinsic toxicity from iron mining tailings against sand and 2) investigating the effects of pH values: 4, 5, 6, and 7 on tailing's toxicity. These materials were placed in gerbox pots and lettuce seeds were sown and kept in a growth chamber. The composition of the iron mining tailings was evaluated. Seed germination and seedling early growth as well as their cotyledon anatomy were accessed. The iron mining tailing pH was 6 and comprises P, Mg, K, Ca, Fe, Mn, Zn, Cu, Na, Al, Cr, Cd, and P; about 97.5% are Ca, Fe, and Al. Iron mining tailings at pH 6 did not significantly change seed germination and shoot growth but promoted positive anatomical changes compared with sand. Iron mining tailings at pH 6 reduced root growth compared with sand. The reduction of the pH to 4 and 5 strongly increased the toxicity of the tailing, preventing lettuce germination; however, seeds germinated equally in pH 6 and 7. The pH 7 promoted the reduction of biometric parameters such as the seedling shoot length and diameter and the number and root length. No significant modifications were caused by the pH 7 to the seedling's fresh and dry masses. The pH 7 promoted no noteworthy changes in the anatomy of the photosynthetic cotyledons. Thus, the lower pH values of 4 and 5 increase the toxicity of the iron mining waste but its elevation to 7 promoted no evident damage to lettuce plants.