Geochemical behaviour of heavy metals in sludge effluents and solid deposits on the Zambian Copperbelt: Implication for effluent treatment and sludge reuse

Cryton Phiri; Daiki Shimazui; Tsubasa Otake; Ryosuke Kikuchi; Isaac Chintu; Meki Chirwa; Lawrence Kalaba; Imasiku Nyambe; Tsutomu Sato

doi:10.1016/j.scitotenv.2020.144342

Geochemical behaviour of heavy metals in sludge effluents and solid deposits on the Zambian Copperbelt: Implication for effluent treatment and sludge reuse

Sci Total Environ. 2021 May 15:769:144342. doi: 10.1016/j.scitotenv.2020.144342. Epub 2020 Dec 29.

Authors

Cryton Phiri¹, Daiki Shimazui², Tsubasa Otake², Ryosuke Kikuchi², Isaac Chintu³, Meki Chirwa⁴, Lawrence Kalaba⁵, Imasiku Nyambe⁴, Tsutomu Sato²

Affiliations

¹ Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan; University of Zambia, School of Mines, Geology Department, BOX, 32379, Lusaka, Zambia. Electronic address: cryton.phiri@unza.zm.
² Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan.
³ Konkola Copper Mines (KCM), Plc, Safety, Health Environment and Quality Department, P.O Box 2000, Chingola, Zambia.
⁴ University of Zambia, School of Mines, Geology Department, BOX, 32379, Lusaka, Zambia.
⁵ Zambia Ministry of Mines, Geological Survey Department, Lusaka, Zambia.

PMID: 33477054
DOI: 10.1016/j.scitotenv.2020.144342

Abstract

Sludge effluents and solid deposits generated from the conventional lime treatment processes on the Zambian Copperbelt have led to reports of copper (Cu) and cobalt (Co) contamination into the nearby water bodies. To better understand the behaviour of the metals; partitioning, adsorption and their specific binding forms were studied through sequential extraction, batch adsorption experiments and surface complexation modeling (SCM). Results of mineral composition analyses indicated that micas, kaolinite, quartz and feldspar are abundant with hydrous ferric oxide (HFO) precipitates that formed as a result of the weathering of biotite grains existing as grain surface coating. Sequential extractionrevealed that Cu and Co metals are partitioned in the order of: exchangeable (F1: 600-1500 mg/kg Cu; 100-200 mg/kg Co), acid-soluble (F2: 2200-5500 mg/kg Cu; 190-220 mg/kg Co) and reducible fraction (F3: 2200-5500 mg/kg Cu; 260-300 mg/kg Co). Metals in F1 are hosted by kaolinite, F2 by both kaolinite and HFO whereas in F3 by dominantly HFO. Equal Cu concentration between F2 and F3 is due to both the limited amount of HFO (i.e. 5-10 g/kg) and desorption of loosely adsorbed Cu and Co metals to HFO surfaces. Batch adsorption experiments revealed adsorption as the dominant metal retention mechanism. According to modeling predictions, HFO sites are the dominant metal adsorption sites. At HFO site; >(s)FeOCo+, Co showed adsorption decrease from 40% in single system to 25% in binary system between pH 7 - 7.5 due to metal competition for adsorption sites. The high Cu concentration (i.e. 0.5-1.1% Cu) displaced low Co (i.e. 0.03-0.07% Co) concentration from the adsorption sites present in sludge, thus rendering Co mobile into the environment. To keep the adsorbed metals stable from release, optimal pH of 7.5 is suggested during treatment with lime. At this optimal pH, metals are decreased to below the regulation standard values and with less generation of voluminous sludge. Adsorbed Cu and Co can be recoverable from sludge through acid treatment at pH <3 based on sequential extraction results. The resultant metal-free sludge material has potential of been used as aggregate in construction.

Keywords: Adsorption; Cobalt; Copper; Metal partitioning; Surface complexation modeling.