Arsenic immobilization in soil affected by mining waste using waste-derived functional hydrochar and iron-encapsulated materials

Arsenic (As) contamination is a widespread problem. Continued and concerted effort in exploring sustainable remediation strategies is required, with in situ immobilization emerging as a promising option. This work valorized a waste by-product from olive (Olea europaea L.) milling into functional hydrochar (HC). The HC was then transformed into iron oxide-encapsulated carbon with three different iron loading rates (10, 25, and 50% w/w of iron chloride hexahydrate added to the olive mill waste feedstock). The HC and the three iron oxide-encapsulated carbon materials were then tested in a pot trial using a 3% w/w application rate as a means to immobilize As in a mining-contaminated soil (2,580 +/- 110 mg kg(-1) As). After a 45-d incubation period, the effect of adding the amendments on As mobility and bioaccessibility compared with an untreated control was measured using a sequential extraction procedure and in vitro bioaccessibility, respectively. All four treatments resulted in a decrease in mobility and in vitro bioaccessibility as compared with the control. Specifically, As in the mobile phases was up to 35% less than the in control, whereas bioaccessibility was 21.8% in the control and ranged from 17.5 to 12.3% in the treatments. The efficiency of amendments to immobilize As increased with the iron content of the developed materials. This work positions HCs and iron oxide-encapsulated carbon materials produced from olive mill waste as promising options to immobilize As in situ.