Table 2.

A summary of common chemical and enzymatic treatment methods for producing (L)CNMs

Methods		Chemical	Chemical recovery, impact 1	Raw materials	CNM type	Surface groups	Refs.
Concentrated mineral acid hydrolysis		Sulfuric acid	−	Bleached wood pulp	CNCs	[HSO3]	[21, 112]
		Hydrochloric acid	−	Bleached softwood pulp	CNCs	None	[149]
		Phosphoric acid	−	Whatman paper	CNCs	[PO4]	[150]
		Sulfuric acid	−	Bleached wood pulp	CNCs + CNFs	[HSO3]	[126, 142]
		Sulfuric acid	−	Poplar wood	LCNCs	[HSO3]	[120]
Concentrated dicarboxylic acid hydrolysis		Oxalic acid or Maleic acid	+	Bleached wood pulp	CNCs + CNFs	[COOH]	[143, 148]
		Oxalic acid	+	Whatman paper	CNCs + CNFs	[COOH]	[151]
		Maleic acid	+	Unbleached hardwood pulp	LCNCs + LCNFs	[COOH]	[123, 124]
Acid hydrotrope		p-TsOH (aromatic sulfonic acid)	0	Undelignified birch fibers Wheat straw	LCNFs LCNFs	None None	[72] [44]
		Maleic acid	+	Poplar, birch wood Switchgrass	LCNFs LCNFs	[COOH] [COOH]	[45, 121] [152]
Dilute acid		Oxalic acid	0	Bleached wood pulp	CNFs	None	[117]
Oxidation		Ammonia persulfate	−	Variety cellulosic materials	CNCs	[COOH]	[130]
		Ammonia persulfate	−	Bleached birch pulp	CNFs	[COOH]	[153]
		Periodate + chlorite	−	Bleached wood pulp	CNFs	[COOH]	[129]
		TEMPO + NaBr + NaClO	0	Bleached wood pulp	CNFs	[COOH]	[140]
		TEMPO + NaBr + NaClO	−	Softwood mechanical pulp	LCNFs	[COOH]	[154]
Solvent	DES	Choline chloride + urea	0	Bleached birch pulp	CNFs	None	[132]
	GVL	GVL	0	Unbleached GVL pulp	LCNFs	None	[128]
	Organosolv	Ethanol + SO2	0	Wood	LCNCs, LCNFs	None	[127]
	Ionic Liquid	[BMIMCI]	0	Cellulose powder	CNFs	None	[155]
		[BMIM][HSO4]	0	Bleached wood pulp; microcrystalline cellulose	CNCs	None	[156]
		[EMIM][OAc]	0	Wood	LCNCs	None	[157]
Enzymes		Endoglucanase	~	Bleached wood pulp	CNFs	None	[118, 119, 134]
		Xylanase	~	Bleached wood pulp	CNFs	None	[135, 136]
		Complex enzymes	~	Bleached wood pulp	CNFs	None	[137]

¹ − : difficult and negative impact; + : relatively easy and less impact; 0: moderate; ~ : benign and low dosage no need for recovery