Combined neutralization–adsorption system for the disposal of hydrothermally altered excavated rock producing acidic leachate with hazardous elements

Abstract

Hydrothermally altered rock excavated in a tunnel project produces acidic leachate containing hazardous elements that include arsenic (As), lead (Pb), copper (Cu) and zinc (Zn). To mitigate this problem, this paper evaluated a combined neutralization-adsorption system that used readily available and cheap reagents like calcium carbonate (CaCO 3 ) and partly-weathered volcanic ash. Batch neutralization experiments showed that CaCO 3 was effective in raising the pH of the leachate around neutral while the batch adsorption experiments illustrated that the volcanic ash sample collected near the tunnel project area was highly capable of adsorbing arsenate (As[V]), Pb, Cu and Zn. Under column conditions, the amount of hazardous elements released from the rock increased by several folds and their breakthrough curves had flushing-out trends. The mechanisms of As and heavy metals release probably include the dissolution of soluble phases and pyrite oxidation. Addition of CaCO 3 in the column experiments based on estimates from the batch results underestimated the amount of neutralizer needed to adjust the effluent pH to around 8, resulting only in slight increase of the pH. Nevertheless, the presence of CaCO 3 drastically reduced the amount of hazardous elements released from the altered rock especially during the initial stages of the column experiments. Combining neutralization and adsorption effectively reduced the amount of As and heavy metals in the effluent throughout the duration of the column experiments, which is attributed to the slight neutralizing effect of volcanic ash that raised the pH around circumneutral as well as its rich Al and Fe oxyhydroxide/oxide contents. The combined system immobilized the hazardous elements through a combination of co-precipitation and adsorption reactions and showed potential as an alternative method for the disposal of altered rocks producing acidic leachate.

Comparing Schwertmannite and Hydrobasaluminite Dissolution in Ammonium Oxalate (pH 3.0): Implications for Metal Speciation Studies by Sequential Extraction

“…The implications of our study are relevant because they could reveal specific metal-mineral associations and dissipate doubts about metal transport and storage in AMD-affected soils and sediments. The relevance of our results can also expand to other environments where amorphous Al-Fe phases play an important role in metal mobility, such as naturally contaminated rocks, aquifers and soils [23][24][25][26][27][28].…”

Solid-phase partitioning and release-retention mechanisms of copper, lead, zinc and arsenic in soils impacted by artisanal and small-scale gold mining (ASGM) activities

“…Igarashi et al (2020), for example, noted that the removal of Cu 2þ and Zn 2þ in AMDs occurred via their coprecipitation with HFOs rather than as distinct and separate Cuand Zn-oxide minerals. Similarly, the bulk of Cu in the strongly adsorbed fraction was most probably associated with HFOs like goethite, which are well-known for their strong heavy metal sorption capacities above around pH 7.5 (Park et al, 2019;Tabelin et al, 2014c;Tatsuhara et al, 2012;Violante et al, 2003;Veiga et al, 1991). According to Hua et al (2012), adsorbed Cu 2þ on goethite are not readily mobilised because it forms inner sphere complexes that have intrinsic adsorption rate constant about two orders of magnitude higher than the intrinsic desorption rate constant.…”

Suppression of the release of arsenic from arsenopyrite by carrier-microencapsulation using Ti-catechol complex

“…Mining, mineral processing and tunnel/underground construction for roads and railways often generate large amounts of wastes that contain sulfide minerals such as pyrite (FeS 2 ) and arsenopyrite (FeAsS), which produce acid mine drainage (AMD) when exposed to the environment [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. AMD is very acidic and could further extract heavy metals (e.g., Fe, Cu, Zn, and Pb) from other minerals found in the wastes.…”